G22.2250: Operating Systems
2000-2001 Fall
Mon 5-7
Ciww 109

Allan Gottlieb
gottlieb@nyu.edu
http://allan.ultra.nyu.edu/~gottlieb
715 Broadway, Room 1001
212-998-3344
609-951-2707
email is best

Administrivia

Web Pages

There is a web page for the course. You can find it from my home page.

Textbook

Text is Tanenbaum, "Modern Operating Systems".

A Grade of ``Incomplete''

It is university policy that a student's request for an incomplete be granted only in exceptional circumstances and only if applied for in advance. Of course the application must be before the final exam.

Computer Accounts and Mailman mailing list

Homework and Labs

I make a distinction between homework and labs.

Labs are

Homeworks are

Upper left board for assignments and announcements.

Interlude on Linkers

Originally called linkage editors by IBM.

This is an example of a utility program included with an operating system distribution. Like a compiler, it is not part of the operating system per se, i.e. it does not run in supervisor mode. Unlike a compiler it is OS dependent (what object/load file format is used) and is not (normally) language dependent.

What does a Linker Do?

Link of course.

When the assembler has finished it produces an object module that is almost runnable. There are two primary problems that must be solved for the object module to be runnable. Both are involved with linking (that word, again) together multiple object modules.

  1. Relocating relative addresses.


  2. Resolving external references.


The output of a linker is called a load module because it is now ready to be loaded and run.

To see how a linker works lets consider the following example, which is the first dataset from lab #1. The description in lab1 is more detailed.

The target machine is word addressable and has a memory of 1000 words, each consisting of 4 decimal digits. The first (leftmost) digit is the opcode and the remaining three digits form an address.

Each object module contains three parts, a definition list, a use list, and the program text itself. Each definition is a pair (sym, loc). Each use is a pair (sym, loc). The address in loc points to the next use or is 999 to end the chain.

For those text entries that do not form part of a use chain a fifth (leftmost) digit is added. If it is 8, the address in the word is relocatable. If it is 0 (and hence omitted), the address is absolute.

Sample input
1 xy 2
1 z 4
5 10043 56781 29994 80023 70024
0
1 z 3
6 80013 19994 10014 30024 10023 10102
0
1 z 1
2 50013 49994
1 z 2
1 xy 2
3 80002 19994 20014

I will illustrate a two-pass approach: The first pass simply produces the symbol table giving the values for xy and z (2 and 15 respectively). The second pass does the real work (using the values in the symbol table).

It is faster (less I/O) to do a one pass approach, but is harder since you need ``fix-up code'' whenever a use occurs in a module that precedes the module with the definition.

xy=2
z=15

+0
0:       10043          1004+0=1004
1:       56781          5678
2: xy:   29994  ->z     2015
3:       80023          8002+0=8002
4: ->z   70024          7015
+5
0        80013          8001+5=8006
1        19994  ->z     1015
2        10014  ->z     1015
3 ->z    30024          3015
4        10023          1002+5=1007
5        10102          1010
+11
0        50013          5001+11=5012
1 ->z    49994          4015
+13
0        80002          8000
1        19994  ->xy    1002
2 z:->xy 20014          2002

The linker on unix is mistakenly called ld (for loader), which is unfortunate since it links but does not load.

Lab #1: Implement a linker. The specific assignment is detailed on the sheet handed out in in class and is due in two weeks, 2 October 2000 . The content of the handout is available on the web as well (see the class home page).

End of Interlude on Linkers

Chapter 1. Introduction

Homework: Read Chapter 1 (Introduction)

Levels of abstraction (virtual machines)

1.1: What is an operating system?

The kernel itself raises the level of abstraction and hides details. For example a user (of the kernel) can write to a file (a concept not present in hardware) and ignore whether the file resides on a floppy, a CD-ROM, or a hard magnetic disk

The kernel is a resource manager (so users don't conflict).

How is an OS fundamentally different from a compiler (say)?

Answer: Concurrency! Per Brinch Hansen in Operating Systems Principles (Prentice Hall, 1973) writes.

The main difficulty of multiprogramming is that concurrent activities can interact in a time-dependent manner, which makes it practically impossibly to locate programming errors by systematic testing. Perhaps, more than anything else, this explains the difficulty of making operating systems reliable.
Homework:1. (unless otherwise stated, problems numbers are from the end of the chapter in Tanenbaum.)

1.2 History of Operating Systems

  1. Single user (no OS).
  2. Batch, uniprogrammed, run to completion.
  3. Multiprogrammed
  4. Multiple computers
  5. Real time systems
Homework:2, 3, 5.

1.3: Operating System Concepts

This will be very brief. Much of the rest of the course will consist in ``filling in the details''.

1.3.1: Processes

A program in execution. If you run the same program twice, you have created two processes. For example if you have two editors running in two windows, each instance of the editor is a separate process.

Often one distinguishes the state or context (memory image, open files) from the thread of control. Then if one has many threads running in the same task, the result is a ``multithreaded processes''.

The OS keeps information about all processes in the process table. Indeed, the OS views the process as the entry. An example of an active entity being viewed as a data structure (cf. discrete event simulations). An observation made by Finkel in his (out of print) OS textbook.

The set of processes forms a tree via the fork system call. The forker is the parent of the forkee. If the parent stops running until the child finishes, the ``tree'' is quite simple, just a line. But the parent (in many OSes) is free to continue executing and in particular is free to fork again producing another child.

A process can send a signal to another process to cause the latter to execute a predefined function (the signal handler). This can be tricky to program since the programmer does not know when in his ``main'' program the signal handler will be invoked.

1.3.2: Files

Modern systems have a hierarchy of files. A file system tree.

Files and directories normally have permissions

Devices (mouse, tape drive, cdrom) are often view as ``special files''. In a unix system these are normally found in the /dev directory. Often utilities that are normally applied to (ordinary) files can be applied as well to some special files. For example, when you are accessing a unix system and do not have anything serious going on (e.g., right after you log in), type the following command

    cat /dev/mouse
and then move the mouse. You kill the cat by typing cntl-C. I tried this on my linux box and no damage occurred. Your mileage may vary.

Many systems have standard files that are automatically made available to a process upon startup. There (initial) file descriptors are fixed

A convenience offered by some command interpretors is a pipe or pipeline. The pipeline

  ls | wc
will give the number of files in the directory (plus other info).


================ Start Lecture #2 ================

Note: Lab 1 is assigned and due 2 October.

1.3.3: System Calls

System calls are the way a user (i.e. a program) directly interfaces with the OS. Some textbooks use the term envelope for the component of the OS responsible for fielding system calls and dispatching them. Here is a picture showing some of the OS components and the external events for which they are the interface.

Note that the OS serves two masters. The hardware (below) asynchronously sends interrupts and the user makes system calls and generates page faults.

What happens when a user executes a system call such as read()? We discuss this in much more detail later but briefly what happens is

  1. Normal function call (in C, ada, etc.).
  2. Library routine (in C).
  3. Small assembler routine.
    1. Move arguments to predefined place (perhaps registers).
    2. Poof (a trap instruction) and then the OS proper runs in supervisor mode.
    3. Fixup result (move to correct place).

Homework: 6

1.3.4: The shell

Assumed knowledge

Homework: 9.

1.4: OS Structure

I must note that tanenbaum is a big advocate of the so called microkernel approach in which as much as possible is moved out of the (protected) microkernel into separate processes.

In the early 90s this was popular. Digital Unix (now called True64) and Windows NT were examples. Digital Unix is based on Mach, a research OS from Carnegie Mellon university. Lately, the growing popularity of Linux has called into question the belief that ``all new operating systems will be microkernel based''.

1.4.1: Monolithic approach

The previous picture: one big program

The system switches from user mode to kernel mode during the poof and then back when the OS does a ``return''.

But of course we can structure the system better, which brings us to.

1.4.2: Layered Systems

Some systems have more layers and are more strictly structured.

An early layered system was ``THE'' operating system by Dijkstra. The layers were.

  1. The operator
  2. User programs
  3. I/O mgt
  4. Operator-process communication
  5. Memory and drum management

The layering was done by convention, i.e. there was no enforcement by hardware and the entire OS is linked together as one program. This is true of many modern OS systems as well (e.g., linux).

The multics system was layered in a more formal manner. The hardware provided several protection layers and the OS used them. That is, arbitrary code could not jump to or access data in a more protected layer.

1.4.3: Virtual machines

Use a ``hypervisor'' (beyond supervisor, i.e. beyond a normal OS) to switch between multiple Operating Systems


1.4.4: Client Server

When implemented on one computer, a client server OS is the microkernel approach in which the microkernel just supplies interprocess communication and the main OS functions are provided by a number of separate processes.

This does have advantages. For example an error in the file server cannot corrupt memory in the process server. This makes errors easier to track down.

But it does mean that when a (real) user process makes a system call there are more processes switches. These are not free.

A distributed system can be thought of as an extension of the client server concept where the servers are remote.

Homework: 11

Chapter 2: Process Management

Tanenbaum's chapter title is ``processes''. I prefer process management. The subject matter is processes, process scheduling, interrupt handling, and IPC (Interprocess communication--and coordination).

2.1: Processes

Definition: A process is a program in execution.

3.1.1: The Process Model

Even though in actuality there are many processes running at once, the OS gives each process the illusion that it is running alone.

Virtual time and virtual memory are examples of abstractions provided by the operating system to the user processes so that the latter ``sees'' a more pleasant virtual machine than actually exists.

Process Hierarchies


MS-DOS is not multiprogrammed so when one process starts another, the first process is blocked and waits until the second is finished.

Process states and transitions

The above diagram contains a great deal of information.

One can organize an OS around the scheduler.

2.1.3: Implementation of Processes

The OS organizes the data about each process in a table naturally called the process table. Each entry in this table is called a process table entry or PTE.

An aside on Interrupts

In a well defined location in memory (specified by the hardware) the OS stores an interrupt vector, which contains the address of the (first level) interrupt handler.

Assume a process P is running and a disk interrupt occurs for the completion of a disk read previously issued by process Q, which is currently blocked. Note that interrupts are unlikely to be for the currently running process (because the process waiting for the interrupt is likely blocked).

  1. The hardware stacks the program counter etc (possibly some registers)
  2. Hardware loads new program counter from the interrupt vector.
  3. Assembly language routine saves registers
  4. Assembly routine sets up new stack
  5. Assembly routine calls C procedure (tanenbaum forgot this one)
  6. C procedure does the real work
  7. The C procedure (that did the real work in the interrupt processing) continues and returns to the assembly code.
  8. Assembly language restores P's state (e.g., registers) and starts P at the point it was when the interrupt occurred.

2.2: Interprocess Communication (IPC) and Process Coordination and Synchronization

2.2.1: Race Conditions

A race condition occurs when two processes can interact and the outcome depends on the order in which the processes execute.

Homework: 2

2.2.2: Critical sections

We must prevent interleaving sections of code that need to be atomic with respect to each other. That is, the conflicting sections need mutual exclusion. If process A is executing its critical section, it excludes process B from executing its critical section. Conversely if process B is executing is critical section, it excludes process A from executing its critical section.

Requirements for a critical section implementation.

  1. No two processes may be simultaneously inside their critical section
  2. No assumption may be made about the speeds or the number of CPUs
  3. No process outside its critical section may block other processes
  4. No process should have to wait forever to enter its critical section

2.2.3 Mutual exclusion with busy waiting

The operating system can choose not to preempt itself. That is, no preemption for system processes (if the OS is client server) or for processes running in system mode (if the OS is self service). Forbidding preemption for system processes would prevent the problem above where x<--x+1 not being atomic crashed the printer spooler if the spooler is part of the OS.

But this is not adequate

Software solutions for two processes

Initially P1wants=P2wants=false

Code for P1                             Code for P2

Loop forever {                          Loop forever {
    P1wants <-- true         ENTRY          P2wants <-- true
    while (P2wants) {}       ENTRY          while (P1wants) {}
    critical-section                        critical-section
    P1wants <-- false        EXIT           P2wants <-- false
    non-critical-section }                  non-critical-section }

Explain why this works.

But it is wrong! Why?

Let's try again. The trouble was that setting want before the loop permitted us to get stuck. We had them in the wrong order!

Initially P1wants=P2wants=false

Code for P1                             Code for P2

Loop forever {                          Loop forever {
    while (P2wants) {}       ENTRY          while (P1wants) {}
    P1wants <-- true         ENTRY          P2wants <-- true
    critical-section                        critical-section
    P1wants <-- false        EXIT           P2wants <-- false
    non-critical-section }                  non-critical-section }

Explain why this works.

But it is wrong again! Why?

So let's be polite and really take turns. None of this wanting stuff.

Initially turn=1

Code for P1                      Code for P2

Loop forever {                   Loop forever {
    while (turn = 2) {}              while (turn = 1) {}
    critical-section                 critical-section
    turn <-- 2                       turn <-- 1
    non-critical-section }           non-critical-section }

This one forces alternation, so is not general enough. Specifically, it does not satisfy condition three, which requires that no process in its non-critical section can stop another process from entering its critical section. With alternation, if one process is in its non-critical section (NCS) then the other can enter the CS once but not again.

In fact, it took years (way back when) to find a correct solution. Many earlier ``solutions'' were found and several were published, but all were wrong The first true solution was found by dekker. It is very clever, but I am skipping it (I cover it when I teach OS II). Subsequently, algorithms with better fairness properties were found (e.g. no task has to wait for another task to enter the CS twice).


================ Start Lecture #3 ================

Note:
Jesse Wei-Yeh Chu  writes:

> The last program text in Module no. 4 of input set 2 has only 4 digits
> (4999). Is this a typo or an intended error for our linker to catch?

Typo!

> Also for input no. 2, there is a discrepancy between the pdf file and the
> text file. Module 5 in pdf indicates both definition and use list as 00,
> whereas the text version contains only one 00.

The text version is wrong.  A filter I used to save space squashed
duplicate lines (thinking they were blanks).  Alas it doesn't work
here.

I fixed both errors.

End of Note.

What follows is Peterson's solution. When it was published, it was a surprise to see such a simple soluntion. In fact Peterson gave a solution for any number of processes. A proof that the algorithm satisfies our properties (including a strong fairness condition) can be found in Operating Systems Review Jan 1990, pp. 18-22.

Initially P1wants=P2wants=false  and  turn=1

Code for P1                        Code for P2

Loop forever {                     Loop forever {
    P1wants <-- true                   P2wants <-- true
    turn <-- 2                         turn <-- 1
    while (P2wants and turn=2) {}      while (P1wants and turn=1) {}
    critical-section                   critical-section
    P1wants <-- false                  P2wants <-- false
    non-critical-section               non-critical-section

Hardware assist (test and set)

TAS(b) where b is a binary variable ATOMICALLY sets b<--true and returns the OLD value of b. Of course it would be silly to return the new value of b since we know the new value is true

Now implementing a critical section for any number of processes is trivial.

loop forever {
    while (TAS(s)) {}   ENTRY
    CS
    s<--false           EXIT
    NCS

P and V and Semaphores

Note: Tanenbaum does both busy waiting (like above) and blocking (process switching) solutions. We will only do busy waiting.

Homework: 3

The entry code is often called P and the exit code V (Tanenbaum only uses P and V for blocking, but we use it for busy waiting). So the critical section problem is to write P and V so that

loop forever
    P
    critical-section
    V
    non-critical-section
satisfies
  1. Mutual exclusion
  2. No speed assumptions
  3. No blocking by processes in NCS
  4. Forward progress (my weakened version of Tanenbaum's last condition

Note that I use indenting carefully and hence do not need (and sometimes omit) the braces {}

A binary semaphore abstracts the TAS solution we gave for the critical section problem.

The above code is not real, i.e., it is not an implementation of P. It is, instead, a definition of the effect P is to have.

To repeat: for any number of processes, the critical section problem can be solved by

loop forever
    P(S)
    CS
    V(S)
    NCS

The only specific solution we have seen for an arbitrary number of processes is the one just above with P(S) implemented via test and set.

Remark: Peterson's solution requires each process to know its processor number. The TAS soluton does not. Thus, strictly speaking Peterson did not provide an implementation of P and V. He did solve the critical section problem.

To solve other coordination problems we want to extend binary semaphores.

The solution to both of these shortcomings is to remove the restriction to a binary variable and define a generalized or counting semaphore.

These counting semaphores can solve what I call the semi-critical-section problem, where you premit up to k processes in the section. When k=1 we have the original critical-section problem.

initially S=k

loop forever
    P(S)
    SCS   <== semi-critical-section
    V(S)
    NCS

Producer-consumer problem

Initially e=k, f=0 (counting semaphore); b=open (binary semaphore)

Producer                         Consumer

loop forever                     loop forever
    produce-item                     P(f)
    P(e)                             P(b); take item from buf; V(b)
    P(b); add item to buf; V(b)      V(e)
    V(f)                             consume-item

Dining Philosophers

A classical problem from Dijkstra

What algorithm do you use for access to the shared resource (the forks)?

The purpose of mentioning the Dining Philosophers problem without giving the solution is to give a feel of what coordination problems are like. The book gives others as well. We are skipping these (again this material would be covered in a sequel course). If you are interested look, for example, at http://allan.ultra.nyu.edu/gottlieb/courses/1997-98-spring/os/class-notes.html.

Homework: 14,15 (these have short answers but are not easy).

Readers and writers

Quite useful in multiprocessor operating systems. The ``easy way out'' is to treat all processes as writers in which case the problem reduces to mutual exclusion (P and V). The disadvantage of the easy way out is that you give up reader concurrency. Again for more information see the web page referenced above.


================ Start Lecture #4 ================

Note: Announce grader assignments (show home page).

2.4: Process Scheduling

Scheduling the processor is often called ``process scheduling'' or simply ``scheduling''.

The objectives of a good scheduling policy include

Recall the basic diagram describing process states

For now we are discussing short-term scheduling, i.e., the arcs connecting running <--> ready.

Medium term scheduling is discussed later.

Preemption

It is important to distinguish preemptive from non-preemptive scheduling algorithms.

Deadline scheduling

This is used for real time systems. The objective of the scheduler is to find a schedule for all the tasks (there are a fixed set of tasks) so that each meets its deadline. The run time of each task is known in advance.

Actually it is more complicated.

We do not cover deadline scheduling in this course.

The name game

There is an amazing inconsistency in naming the different (short-term) scheduling algorithms. Over the years I have used primarily 4 books: In chronological order they are Finkel, Deitel, Silberschatz, and Tanenbaum. The table just below illustrates the name game for these four books. After the table we discuss each scheduling policy in turn.

Finkel  Deitel  Silbershatz Tanenbaum
-------------------------------------
FCFS    FIFO    FCFS        --    unnamed in tanenbaum
RR      RR      RR          RR
PS      **      PS          PS
SRR     **      SRR         **    not in tanenbaum
SPN     SJF     SJF         SJF
PSPN    SRT     PSJF/SRTF   --    unnamed in tanenbaum
HPRN    HRN     **          **    not in tanenbaum
**      **      MLQ         **    only in silbershatz
FB      MLFQ    MLFQ        MQ

First Come First Served (FCFS, FIFO, FCFS, --)

If the OS ``doesn't'' schedule, it still needs to store the PTEs somewhere. If it is a queue you get FCFS. If it is a stack (strange), you get LCFS. Perhaps you could get some sort of random policy as well.

Round Robbin (RR, RR, RR, RR)

Homework: 9, 19, 20, 21, and the following:

Consider the set of processes in the table below. When does each process finish if RR scheduling is used with q=1, if q=2, if q=3, if q=100. First assume (unrealistically) that context switch time is zero. Then assume it is .1. Each process performs no I/O (i.e., no process ever blocks). All times are in milliseconds. The CPU time is the total time required for the process (excluding context switch time). The creation time is the time when the process is created. So P1 is created when the problem begins and P2 is created 5 miliseconds later.

ProcessCPU TimeCreation Time
P1200
P233
P325

Processor Sharing (PS, **, PS, PS)

Merge the ready and running states and permit all ready jobs to be run at once. However, the processor slows down so that when n jobs are running at once each progresses at a speed 1/n as fast as it would if it were running alone.

Homework: 18.

Variants of Round Robbin

Priority Scheduling

Each job is assigned a priority (externally, perhaps by charging more for higher priority) and the highest priority ready job is run.

Priority aging

As a job is waiting, raise its priority so eventually it will have the maximum priority.

Homework: 22, 23

Selfish RR (SRR, **, SRR, **)

Shortest Job First (SPN, SJF, SJF, SJF)

Sort jobs by total execution time needed and run the shortest first.

Preemptive Shortest Job First (PSPN, SRT, PSJF/SRTF, --)

Preemptive version of above

Highest Penalty Ratio Next (HPRN, HRN, **, **)

Run the process that has been ``hurt'' the most.

Multilevel Queues (**, **, MLQ, **)

Put different classes of processs in different queues

Multilevel Feedback Queues (FB, MFQ, MLFBQ, MQ)

Many queues and processs move from queue to queue in an attempt to dynamically separate ``batch-like'' from interactive processs.

Theoretical Issues

Considerable theory has been developed.

Medium Term scheduling

Decisions made at a coarser time scale.

Long Term Scheduling


================ Start Lecture #5 ================

Chapter 3: Memory Management

Also called storage management or space management.

Memory management must deal with the storage hierarchy present in modern machines.

We will see in the next few lectures that there are three independent decision:

  1. Segmentation (or no segmentation)
  2. Paging (or no paging)
  3. Fetch on demand (or no fetching on demand)

Memory management implements address translation.

Homework: 7.

When is address translation performed?

  1. At compile time
    • Primitive.
    • Compiler generates physical addresses.
    • Requires knowledge of where the compilation unit will be loaded.
    • Rarely used (MSDOS .COM files).

  2. At link-edit time (the ``linker lab'')
    • Compiler
      • Generates relocatable addresses for each compilation unit.
      • References external addresses.
    • Linkage editor
      • Converts the relocatable addr to absolute.
      • Resolves external references.
      • Misnamed ld by unix.
      • Also converts virtual to physical addresses by knowing where the linked program will be loaded. Linker lab ``does'' this, but it is trivial since we assume the linked program will be loaded at 0.
    • Loader is simple.
    • Hardware requirements are small.
    • A program can be loaded only where specified and cannot move once loaded.
    • Not used much any more.

  3. At load time
    • Similar to at link-edit time, but do not fix the starting address.
    • Program can be loaded anywhere.
    • Program can move but cannot be split.
    • Need modest hardware: base/limit registers.
    • Loader sets the base/limit registers.

  4. At execution time
    • Addresses translated dynamically during execution.
    • Hardware needed to perform the virtual to physical address translation quickly.
    • Currently dominates.
    • Much more information later.

Extensions

Note: I will place ** before each memory management scheme.

3.1: Memory management without swapping or paging

Entire process remains in memory from start to finish.

The sum of the memory requirements of all jobs in the system cannot exceed the size of physical memory.

** 3.1.1: Monoprogramming without swapping or paging (Single User)

The ``good old days'' when everything was easy.

3.1.2: Multiprogramming and Memory Usage

Goal is to improve CPU utilization, by overlapping CPU and I/O

Homework: 1, 3.

3.1.3: Multiprogramming with fixed partitions

3.2: Swapping

Moving entire processes between disk and memory is called swapping.

3.2.1: Multiprogramming with variable partitions

Homework: 4

MVT Introduces the ``Placement Question'', which hole (partition) to choose

Homework: 2, 5.

MVT Also introduces the ``Replacement Question'', which victim to swap out

We will study this question more when we discuss demand paging

Considerations in choosing a victim

NOTEs:
  1. So far the schemes presented have had two properties:
    1. Each job is stored contiguously in memory. That is, the job is contiguous in physical addresses.
    2. Each job cannot use more memory than exists in the system. That is, the virtual addresses space cannot exceed the physical address space.

  2. Tanenbaum now attacks the second item. I wish to do both and start with the first

  3. Tanenbaum (and most of the world) uses the term ``paging'' to mean what I call demand paging. This is unfortunate as it mixes together two concepts
    1. Paging (dicing the address space) to solve the placement problem and essentially eliminate external fragmentation.
    2. Demand fetching, to permit the total memory requirements of all loaded jobs to exceed the size of physical memory.

  4. Tanenbaum (and most of the world) uses the term virtual memory as a synonym for demand paging. Again I consider this unfortunate.
    1. Demand paging is a fine term and is quite descriptive
    2. Virtual memory ``should'' be used in contrast with physical memory to describe any virtual to physical address translation.

** (non-demand) Paging

Simplest scheme to remove the requirement of contiguous physical memory.

Example: Assume a decimal machine with page size = frame size = 1000.
Assume PTE 3 contains 459.
Then virtual address 3372 corresponds to physical address 459372.

Properties of (non-demand) paging.

Homework: 13


================ Start Lecture #6 ================

Notes:

Lab 2 due 6 November, but lab3 will be handed out 30 October, so you may want to finish lab 2 early. Lab 2 is being handed out 16 October and is available on the web.

It is NOW DEFINITE that on monday 23 Oct, my office hours will have to move from 2:30--3:30 to 1:30-2:30 due to a departmental committee meeting.

End of Notes:

Address translation

Choice of page size is discuss below.

Homework: 8.

3.3: Virtual Memory (meaning fetch on demand)

Idea is that a program can execute even if only the active portion of its address space is memory resident. That is, swap in and swap out portions of a program. In a crude sense this can be called ``automatic overlays''.

Advantages

3.2.1: Paging (meaning demand paging)

Fetch pages from disk to memory when they are referenced, with a hope of getting the most actively used pages in memory.

Homework: 11.

3.3.2: Page tables

A discussion of page tables is also appropriate for (non-demand) paging, but the issues are more acute with demand paging since the tables can be much larger. Why?

  1. The total size of the active processes is no longer limited to the size of physical memory. Since the total size of the processes is greater, the total size of the page tables is greater and hence concerns over the size of the page table are more acute.
  2. With demand paging an important question is the choice of a victim page to page out. Data in the page table can be useful in this choice.

We must be able access to the page table very quickly since it is needed for every memory access.

Unfortunate laws of hardware.

So we can't just say, put the page table in fast processor registers, and let it be huge, and sell the system for $1500.




For now, put the (one-level) page table in main memory.


Contents of a PTE

Each page has a corresponding page table entry (PTE). The information in a PTE is for use by the hardware. Information set by and used by the OS is normally kept in other OS tables. The page table format is determined by the hardware so access routines are not portable. The following fields are often present.

  1. The valid bit. This tells if the page is currently loaded (i.e., is in a frame). If set, the frame pointer is valid. It is also called the presence or presence/absence bit. If a page is accessed with the valid bit zero, a page fault is generated by the hardware.
  2. The frame number. This is the main reason for the table. It is needed for virtual to physical address translation.
  3. TheModified bit. Indicates that some part of the page has been written since it was loaded. This is needed when the page is evicted so the OS can know that the page must be written back to disk.
  4. The referenced bit. Indicates that some word in the page has been referenced. Used to select a victim: unreferenced pages make good victims by the locality property.
  5. Protection bits. For example one can mark text pages as execute only. This requires that boundaries between regions with different protection are on page boundaries. Normally many consecutive (in logical address) pages have the same protection so many page protection bits are redundant. Protection is more naturally done with segmentation.

Multilevel page tables

Recall the previous diagram. Most of the virtual memory is the unused space between the data and stack regions. However, with demand paging this space does not waste real memory. But the single large page table does waste real memory.

The idea of multi-level page tables (a similar idea is used in Unix inode-based file systems) is to add a level of indirection and have a page table containing pointers to page tables.

Do an example on the board

The VAX used a 2-level page table structure, but with some wrinkles (see Tanenbaum for details).

Naturally, there is no need to stop at 2 levels. In fact the SPARC has 3 levels and the Motorola 68030 has 4 (and the number of bits of Virtual Address used for P#1, P#2, P#3, and P#4 can be varied).

3.3.4: Associative memory (TLBs)

Note: Tanenbaum suggests that ``associative memory'' and ``translation lookaside buffer'' are synonyms. This is wrong. Associative memory is a general structure and translation lookaside buffer is a special case.

An associative memory is a content addressable memory. That is you access the memory by giving the value of some field and the hardware searches all the records and returns the record whose field contains the requested value.

For example

Name  | Animal | Mood     | Color
======+========+==========+======
Moris | Cat    | Finicky  | Grey
Fido  | Dog    | Friendly | Black
Izzy  | Iguana | Quiet    | Brown
Bud   | Frog   | Smashed  | Green
If the index field is Animal and Iguana is given, the associative memory returns
Izzy  | Iguana | Quiet    | Brown

A Translation Lookaside Buffer or TLB is an associate memory where the index field is the page number. The other fields include the frame number, dirty bit, valid bit, and others.

Homework: 15.

3.3.5: Inverted page tables

Keep a table indexed by frame number with the entry f containing the number of the page currently loaded in frame f.

3.4: Page Replacement Algorithms

These are solutions to the replacement question.

Good solutions take advantage of locality.

Pages belonging to processes that have terminated are of course perfect choices for victims.

Pages belonging to processes that have been blocked for a long time are good choices as well.

Random

A lower bound on performance. Any decent scheme should do better.

3.4.1: The optimal page replacement algorithm (opt PRA) (aka Belady's min PRA)

Replace the page whose next reference will be furthest in the future.

3.4.2: The not recently used (NRU) PRA

Divide the frames into four classes and make a random selection from the lowest nonempty class.

  1. Not referenced, not modified
  2. Not referenced, modified
  3. Referenced, not modified
  4. Referenced, modified

Assumes that in each PTE there are two extra flags R (sometimes called U, for used) and M (often called D, for dirty).

Also assumes that a page in a lower priority class is cheaper to evict.

We again have the prisoner problem, we do a good job of making little ones out of big ones, but not the reverse. Need more resets.

Every k clock ticks, reset all R bits

What if the hardware doesn't set these bits?

3..4.3: FIFO PRA

Simple but poor since usage of the page is ignored.

Belady's Anomaly: Can have more frames yet generate more faults. Example given later.


================ Start Lecture #7 ================

3.4.4: Second chance PRA

Similar to the FIFO PRA but when time choosing a victim, if the page at the head of the queue has been referenced (R bit set), don't evict it. Instead reset R and move the page to the rear of the queue (so it looks new). The page is being a second chance.

What if all frames have been referenced?
Becomes the same as fifo (but takes longer).

Might want to turn off the R bit more often (say every k clock ticks).

3.4.5: Clock PRA

Same algorithm as 2nd chance, but a better (and I would say obvious) implementation: Use a circular list.

Do an example.

LIFO PRA

This is terrible! Why?
Ans: All but the last frame are frozen once loaded so you can replace only one frame. This is especially bad after a phase shift in the program when it is using all new pages.

3.4.6:Least Recently Used (LRU) PRA

When a page fault occurs, choose as victim that page that has been unused for the longest time, i.e. that has been least recently used.

LRU is definitely

Homework: 19, 20

A hardware cutsie in Tanenbaum

3.4.7: Approximating LRU in Software

The Not Frequently Used (NFU) PRA

The Aging PRA

NFU doesn't distinguish between old references and recent one. The following modification does distinguish.

R counter
110000000
001000000
110100000
111010000
001101000
000110100
110011010
111001101
001100110

Homework: 21, 25

3.5: Modeling Paging Algorithms

3.5.1: Belady's anomaly

Consider a system that has no pages loaded and that uses the FIFO PRU.
Consider the following ``reference string'' (sequences of pages referenced).

 0 1 2 3 0 1 4 0 1 2 3 4

If we have 3 frames this generates 9 page faults (do it).

If we have 4 frames this generates 10 page faults (do it).

Theory has been developed and certain PRA (so called ``stack algorithms'') cannot suffer this anomaly for any reference string. FIFO is clearly not a stack algorithm. LRU is.

Repeat the above calculations for LRU.

3.6: Design issues for (demand) Paging

3.6.1 & 3.6.2: The Working Set Model and Local vs Global Policies

I will do these in the reverse order (which makes more sense). Also Tanenbaum doesn't actually define the working set model, but I shall.

A local PRA is one is which a victim page is chosen among the pages of the same process that requires a new page. That is the number of pages for each process is fixed. So LRU means the page least recently used by this process.

If we apply global LRU indiscriminately with some sort of RR processor scheduling policy, and memory is somewhat over-committed, then by the time we get around to a process, all the others have run and have probably paged out this process.

If this happens each process will need to page fault at a high rate; this is called thrashing. It is therefore important to get a good idea of how many pages a process needs, so that we can balance the local and global desires.

The working set policy (Peter Denning)

The goal is to specify which pages a given process needs to have memory resident in order for the give process to run without too many page faults.

The idea of the working set policy is to ensure that each process keeps its working set in memory.

Interesting questions include:

... various approximations to the working set, have been devised.

  1. Wsclock
    • Use the aging algorithm above to maintain a counter for each PTE and declare a page whose counter is above a certain threshold to be part of the working set.
    • Apply the clock algorithm globally (i.e. to all pages) but refuse to page out any page in a working set, the resulting algorithm is called wsclock.
    • What if we find there are no pages we can page out?
      Answer: Reduce the MPL.
  2. Page Fault Frequency (PFF)
    • For each process keep track of the page fault frequency, which is the number of faults divided by the number of references.
    • Actually, must use a window or a weighted calculation since you are really interested in the recent page fault frequency.
    • If the PFF is too high, allocate more frames to this process. Either
      1. Raise its number of frames and use local policy; or
      2. Bar its frames from eviction (for a while) and use a global policy.
    • What if there are not enough frames?
      Answer: Reduce the MPL.

3.6.3: Page size

3.6.4: Implementation Issues

Don't worry about instruction backup. Very machine dependent and modern implementations tend to get it right.

Locking (pinning) pages

We discussed pinning jobs already. The same (mostly I/O) considerations apply to pages.

Shared pages

Really should share segments.


================ Start Lecture #8 ================

Note: Lab 2 handed out, due in three weeks, 20 November 2000. It is available on the web.

Backing Store

The issue is where on disk do we put pages.

Paging Daemons

Done earlier

Page Fault Handling

What happens when a process, say process A, gets a page fault?
  1. The hardware detects the fault and traps to the kernel (switches to supervisor mode and saves state).

  2. Some assembly language code save more state, establishes the C-language (or another programming language) environment, and ``calls'' the OS.

  3. The OS determines that a page fault occurred and which page was referenced.

  4. If the virtual address is invalid, process A is killed. If the virtual address is valid, the OS must find a free frame. If there is no free frames, the OS selects a victim frame. Call the process owning the victim frame, process B. (If the page replacement algorithm is local process B is process A.)

  5. If the victim frame is dirty, the OS schedules an I/O write to copy the frame to disk. Thus, if the victim frame is dirty, process B is blocked (it might already be blocked for some other reason). Process A is also blocked since it needs to wait for this frame to be free. The process scheduler is invoked to perform a context switch.

    • Tanenbaum ``forgot'' some here.
    • The process selected by the scheduler (say process C) runs.
    • Perhaps C is preempted for D or perhaps C blocks and D runs and then perhaps D is blocked and E runs, etc.
    • When the I/O to write the victim frame completes, a Disk interrupt occurs. Assume processes C is running at the time.
    • Hardware trap / assembly code / OS determines I/O done.
    • Processes A is moved from blocked to ready as is process B (unless B is also blocked for some other reason).
    • The scheduler picks a process to run, maybe A, maybe B, maybe C, maybe another processes.
    • At some point the scheduler does pick process A to run. Recall that at this point A is still executing OS code.

  6. Now the O/S has a clean frame (this may be much later in wall clock time if a victim frame had to be written). The O/S schedules an I/O to read the desired page into this clean frame. Process A is blocked (perhaps for the second time) and hence the process scheduler is invoked to perform a context switch.

  7. A Disk interrupt occurs when the I/O completes (trap / asm / OS determines I/O done). The PTE is updated.

  8. The O/S may need to fix up process A (e.g. reset the program counter to re-execute the instruction that caused the page fault).

  9. Process A is placed on the ready list and eventually is chosen by the scheduler to run. Recall that process A is executing O/S code.

  10. The OS returns to the first assembly language routine.

  11. The assembly language routine restores registers, etc. and ``returns'' to user mode.

Process A is unaware that all this happened.

3.7: Segmentation

Up to now, the virtual address space has been contiguous.

Homework: 29.

** Two Segments

Late PDP-10s and TOPS-10

** Three Segments

Traditional Unix shown at right.

  1. Shared text marked execute only.
  2. Data segment (global and static variables).
  3. Stack segment (automatic variables).

** Four Segments

Just kidding.

** General (not necessarily demand) Segmentation

** Demand Segmentation

Same idea as demand paging applied to segments.

The following table mostly from Tanenbaum compares demand paging with demand segmentation.

Consideration Demand
Paging
Demand
Segmentation
Programmer aware NoYes
How many addr spaces 1Many
VA size > PA size YesYes
Protect individual
procedures separately
NoYes
Accommodate elements
with changing sizes
NoYes
Ease user sharing NoYes
Why invented let the VA size
exceed the PA size
Sharing, Protection,
independent addr spaces

Internal fragmentation YesNo, in principle
External fragmentation NoYes
Placement question NoYes
Replacement question YesYes

** 3.7.2: Segmentation with paging

Combines both segmentation and paging to get advantages of both at a cost in complexity. This is very common now.

Homework: 30.

Some last words on memory management.

Chapter 4: File Systems

Requirements

  1. Size: Store very large amounts of data.
  2. Persistence: Data survives the creating process.
  3. Access: Multiple processes can access the data concurrently.

Solution: Store data in files that together form a file system.

4.1: Files

4.1.1: File Naming

Very important. A major function of the file system.

4.1.2: File structure

A file is a

  1. Byte stream
    • Unix, dos, windows (I think).
    • Maximum flexibility.
    • Minimum structure.

  2. (fixed size) Record stream: Out of date
    • 80-character records for card images.
    • 133-character records for line printer files. Column 1 was for control (e.g., new page) Remaining 132 characters were printed.

  3. Varied and complicated beast.
    • Indexed sequential.
    • B-trees.
    • Supports rapidly finding a record with a specific key.
    • Supports retrieving (varying size) records in key order.
    • Treated in depth in database courses.


================ Start Lecture #9 ================

4.1.3: File types

Examples

  1. (Regular) files.

  2. Directories: studied below.

  3. Special files (for devices). Uses the naming power of files to unify many actions.
        dir             # prints on screen
        dir > file      # result put in a file
        dir > /dev/tape # results written to tape
        
  4. ``Symbolic'' Links (similar to ``shortcuts''): Also studied below.

``Magic number'': Identifies an executable file.

Strongly typed files:

4.1.4: File access

There are basically two possibilities, sequential access and random access (a.k.a. direct access). Previously, files were declared to be sequential or random. Modern systems do not do this. Instead all files are random and optimizations are applied when the system dynamically determines that a file is (probably) being accessed sequentially.

  1. With Sequential access the bytes (or records) are accessed in order (i.e., n-1, n, n+1, ...). Sequential access is the most common and gives the highest performance. For some devices (e.g. tapes) access ``must'' be sequential.
  2. With random access, the bytes are accessed in any order. Thus each access must specify which bytes are desired.

4.1.5: File attributes

A laundry list of properties that can be specified for a file For example:

Driver as a process (Tanenbaum) (less detailed than above)

5.2.4: Device-Independent I/O Software

The device-independent code does most of the functionality, but not necessarily most of the code since there can be many drivers all doing essentially the same thing in slightly different ways due to slightly different controllers.

5.2.5: User-Space Software

A good deal of I/O code is actually executed in user space. Some is in library routines linked into user programs and some is in daemon processes.

Homework: 6, 7, 8.

5.3: Disks

The ideal storage device is

  1. Fast
  2. Big (in capacity)
  3. Cheap
  4. Impossible

Disks are big and cheap, but slow.

5.3.1: Disk Hardware

Show a real disk opened up and illustrate the components

Overlapping I/O operations is important. Many controllers can do overlapped seeks, i.e. issue a seek to one disk while another is already seeking.

Despite what Tanenbaum says, modern disks cheat and do not have the same number of sectors on outer cylinders as on inner one. Often the controller ``cover for them'' and protect the lie.

Again contrary to Tanenbaum, it is not true that when one head is reading from cylinder C, all the heads can read from cylinder C with no penalty.

Choice of block size

5.3.2: Disk Arm Scheduling Algorithms

These algorithms are relevant only if there are several I/O requests pending. For many PCs this is not the case. For most commercial applications, I/O is crucial.

  1. FCFS (First Come First Served): Simple but has long delays.

  2. Pick: Same as FCFS but pick up requests for cylinders that are passed on the way to the next FCFS request.

  3. SSTF (Shortest Seek Time First): Greedy algorithm. Can starve requests for outer cylinders and almost always favors middle requests.

  4. Scan (Look, Elevator): The method used by an old fashioned jukebox (remember ``Happy Days'') and by elevators. The disk arm proceeds in one direction picking up all requests until there are no more requests in this direction at which point it goes back the other direction. This favors requests in the middle, but can't starve any requests.

  5. C-Scan (C-look, Circular Scan/Look): Similar to Scan but only service requests when moving in one direction. When going in the other direction, go directly to the furthest away request. This doesn't favor any spot on the disk. Indeed, it treats the cylinders as though they were a clock, i.e. after the highest numbered cylinder comes cylinder 0.

  6. N-step Scan: This is what the natural implementation of Scan gives.
    • While the disk is servicing a Scan direction, the controller gathers up new requests and sorts them.
    • At the end of the current sweep, the new list becomes the next sweep.

Minimizing Rotational Latency

Use Scan, which is the same as C-Scan. Why?
Because the disk only rotates in one direction.

Homework: 9, 10.


================ Start Lecture #13 ================

RAID (Redundant Array of Inexpensive Disks)

5.3.3: Error Handling

Disks error rates have dropped in recent years. Moreover, bad block forwarding is done by the controller (or disk electronic) so this topic is no longer as important for OS.

5.3.4: Track Caching

Often the disk/controller caches a track, since the seek penalty has already been paid. In fact modern disks have megabyte caches that hold recently read blocks. Since modern disks cheat and don't have the same number of blocks on each track, it is better for the disk electronics (and not the OS or controller) to do the caching since it is the only part of the system to know the true geometry.

5.3.5: Ram Disks

5.4: Clocks

Also called timers.

5.4.1: Clock Hardware


5.4.2: Clock Software

  1. TOD: Bump a counter each tick (clock interupt). If counter is only 32 bits must worry about overflow so keep two counters: low order and high order.

  2. Time quantum for RR: Decrement a counter at each tick. The quantum expires when counter is zero. Load this counter when the scheduler runs a process.

  3. Accounting: At each tick, bump a counter in the process table entry for the currently running process.

  4. Alarm system call and system alarms:
    • Users can request an alarm at some future time and the system also needs to do things specific future times (e.g. turn off floppy motor).
    • The conceptually simplest solution is to have one timer for each event. Instead, we simulate many timers with just one.
    • The data structure on the right works well.
    • The time in each list entry is the time after the preceding entry that this entry's alarm is to ring. For example, if the time is zero, this event occurs at the same time as the previous event. The other entry is a pointer to the action to perform.
    • At each tick, decrement next-signal.
    • When next-signal goes to zero, process the first entry on the list and any others following immediately after with a time of zero (which means they are to be simultaneous with this alarm). Then set next-signal to the value in the next alarm.

  5. Profiling
    • Want a histogram giving how much time was spent in each 1KB (say) block of code.
    • At each tick check the PC and bump the appropriate counter.
    • At the end of the run can assign the 1K blocks to software modules.
    • If use fine granularity (say 10B instead of 1KB) get higher accuracy but more memory overhead.

Homework: 12

5.5: Terminals

5.5.1: Terminal Hardware

Quite dated. It is true that modern systems can communicate to a hardwired ascii terminal, but most don't. Serial ports are used, but they are normally connected to modems and then some protocol (SLIP, PPP) is used not just a stream of ascii characters.

5.5.2: Memory-Mapped Terminals

Keyboards

Tanenbaum description of keyboards is correct.

5.5.3: Input Software

5.5.4: Output Software

Again too dated and the truth is too complicated to deal with in a few minutes.

Homework: 16.

Chapter 6: Deadlocks

A deadlock occurs when a every member of a set of processes is waiting for an event that can only be caused by a member of the set.

Often the event waited for is the release of a resource.

In the automotive world deadlocks are called gridlocks.

Reward: One point extra credit on the final exam for anyone who brings a real (e.g., newspaper) picture of an automotive deadlock. You must bring the clipping to the final and it must be in good condition. Hand it in with your exam paper.

For a computer science example consider two processes A and B that each want to print a file currently on tape.

  1. A has obtained ownership of the printer and will release it after printing one file.
  2. B has obtained ownership of the tape drive and will release it after reading one file.
  3. A tries to get ownership of the tape drive, but is told to wait for B to release it.
  4. B tries to get ownership of the printer, but is told to wait for A to release the printer.

Bingo: deadlock!

6.1: Resources:

The resource is the object granted to a process.

6.2: Deadlocks

To repeat: A deadlock occurs when a every member of a set of processes is waiting for an event that can only be caused by a member of the set.

Often the event waited for is the release of a resource.

6.2.1: (Necessary) Conditions for Deadlock

The following four conditions (Coffman; Havender) are necessary but not sufficient for deadlock. Repeat: They are not sufficient.

  1. Mutual exclusion: A resource can be assigned to at most one process at a time (no sharing).
  2. Hold and wait: A processing holding a resource is permitted to request another.
  3. No preemption: A process must release its resources; they cannot be taken away.
  4. Circular wait: There must be a chain of processes such that each member of the chain is waiting for a resource held by the next member of the chain.

6.2.2: Deadlock Modeling

On the right is the Resource Allocation Graph, also called the Reusable Resource Graph.

Homework: 1.

Consider two concurrent processes P1 and P2 whose programs are.

P1: request R1       P2: request R2
    request R2           request R1
    release R2           release R1
    release R1           release R2

On the board draw the resource allocation graph for various possible executions of the processes, indicating when deadlock occurs and when deadlock is no longer avoidable.

There are four strategies used for dealing with deadlocks.

  1. Ignore the problem
  2. Detect deadlocks and recover from them
  3. Avoid deadlocks by carefully deciding when to allocate resources.
  4. Prevent deadlocks by violating one of the 4 necessary conditions.

6.3: Ignoring the problem--The Ostrich Algorithm

The ``put your head in the sand approach''.

6.4: Detecting Deadlocks and Recovering from them

6.4.1: Detecting Deadlocks with single unit resources

Consider the case in which there is only one instance of each resource.

To find a directed cycle in a directed graph is not hard. The algorithm is in the book. The idea is simple.

  1. For each node in the graph do a depth first traversal (hoping the graph is a DAG (directed acyclic graph), building a list as you go down the DAG.
  2. If you ever find the same node twice on your list, you have found a directed cycle and the graph is not a DAG and deadlock exists among the processes in your current list.
  3. If you never find the same node twice, the graph is a DAG and no deadlock occurs.
  4. The searches are finite since the list size is bounded by the number of nodes.

6.4.2: Detecting Deadlocks with multiple unit resources

This is more difficult.

6.4.3: Recovery from deadlock

Preemption

Perhaps you can temporarily preempt a resource from a process. Not likely.

Rollback

Database (and other) systems take periodic checkpoints. If the system does take checkpoints, one can roll back to a checkpoint whenever a deadlock is detected. Somehow must guarantee forward progress.

Kill processes

Can always be done but might be painful. For example some processes have had effects that can't be simply undone. Print, launch a missile, etc.


================ Start Lecture #14 ================

Remark: We are doing 6.6 before 6.5 since 6.6 is easier.

6.6: Deadlock Prevention

Attack one of the coffman/havender conditions

6.6.1: Attacking Mutual Exclusion

Idea is to use spooling instead of mutual exclusion. Not possible for many kinds of resources

6.6.2: Attacking Hold and Wait

Require each processes to request all resources at the beginning of the run. This is often called One Shot.

6.6.3: Attacking No Preempt

Normally not possible.

6.6.4: Attacking Circular Wait

Establish a fixed ordering of the resources and require that they be requested in this order. So if a process holds resources #34 and #54, it can request only resources #55 and higher.

It is easy to see that a cycle is no longer possible.

6.5: Deadlock Avoidance

Let's see if we can tiptoe through the tulips and avoid deadlock states even though our system does permit all four of the necessary conditions for deadlock.

An optimistic resource manager is one that grants every request as soon as it can. To avoid deadlocks with all four conditions present, the manager must be smart not optimistic.

6.5.1 Resource Trajectories

6.5.2: Safe States

Avoiding deadlocks given some extra knowledge.

Definition: A state is safe if there one can find an ordering of the processes such that if the processes are run in this order, they will all terminate (assuming none exceeds its claim).

Give an example of all four possibilities. A state that is

  1. Safe and deadlocked
  2. Safe and not deadlocked
  3. Not safe and deadlocked
  4. Not safe and not deadlocked

A manager can determine if a state is safe.

The manager then follows the following procedure, which is part of Banker's Algorithms discovered by Dijkstra, to determine if the state is safe.

  1. If there are no processes remaining, the state is safe.

  2. Seek a process P whose max additional requests is less than what remains (for each resource).
    • If no such process can be found, then the state is not safe.
    • The banker (manager) knows that if it refuses all requests excepts those from P, then it will be able to satisfy all of P's requests. Why?
      Look at how P was chosen.

  3. The banker now pretends that P has terminated (since the banker knows that it can guarantee this will happen). Hence the banker pretends that all of P's currently held resources are returned. This makes the banker richer and hence perhaps a process that was not eligible to be chosen as P previously, can now be chosen.

  4. Repeat these steps.

Example 1

processclaimcurrent
X31
Y115
Z1910
total16

Example 2

processclaimcurrent
X31
Y115
Z1912
total18

Assume that Z now requests 2 units and we grant them.

Remark: An unsafe state is not necessarily a deadlocked state. Indeed, if one gets lucky all processes may terminate successfully. A safe state means that the manager can guarantee that no deadlock will occur.

6.5.3: The Banker's Algorithm (Dijkstra) for a Single Resource

The algorithm is simple: Stay in safe states.

6.5.4: The Banker's Algorithm for Multiple Resources

At a high level the algorithm is identical: Stay in safe states.

Limitations of the banker's algorithm

Homework: 11, 14 (do not hand in).

6.7: Other Issues

6.7.1: Two-phase locking

This is covered (MUCH better) in a database text. We will skip it.

6.7.2: Non-resource deadlocks

You can get deadlock from semaphores as well as resources. This is trivial. Semaphores can be considered resources. P(S) is request S and V(S) is release S. The manager is the module implementing P and V. When the manager returns from P(S), it has granted the resource S.

6.7.3: Starvation

As usual FCFS is a good cure. Often this is done by priority aging and picking the highest priority process to get the resource. Also can periodically stop accepting new processes until all old ones get their resources. The End: Good luck on the final