Operating Systems

Remark: Discuss the diagram above (before the examples) explaining why can't tell safety without initial claims.

Remark: Lab 3 (banker) assigned. It is due in 2 weeks.

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

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

The algorithm is simple: Stay in safe states. Initially, we assume all the processes are present before execution begins and that all initial claims are given before execution begins. We will relax these assumptions very soon.

• Before execution begins, check that the system is safe. That is, check that no process claims more than the manager has). If not, then the offending process is trying to claim more of some resource than exist in the system has and hence cannot be guaranteed to complete even if run by itself. You might say that it can become deadlocked all by itself.
  
  
• When the manager receives a request, it pretends to grant it and checks if the resulting state is safe. If it is safe the request is granted, if not the process is blocked.
  
  
• When a resource is returned, the manager (politely thanks the process and then) checks to see if “the first” pending requests can be granted (i.e., if the result would now be safe). If so the request is granted. Whether or not the request was granted, the manager checks to see if the next pending request can be granted, etc..

Homework: 13.

3.5.4: The Banker's Algorithm for Multiple Resources

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

• What is a safe state?
  
  
• The same definition (if processes are run in a certain order they will all terminate).
  
  
• Checking for safety is the same idea as above. The difference is that to tell if there are enough free resources for a processes to terminate, the manager must check that for all resources, the number of free units is at least equal to the max additional need of the process.

Limitations of the banker's algorithm

• Often users don't know the maximum requests a process will make. They can estimate conservatively (i.e., use big numbers for the claim) but then the manager becomes very conservative.
  
  
• New processes arriving cause a problem (but not so bad as Tanenbaum suggests).
  • The process's claim must be less than the total number of units of the resource in the system. If not, the process is not accepted by the manager.
  • Since the state without the new process is safe, so is the state with the new process! Just use the order you had originally and put the new process at the end.
  • Insuring fairness (starvation freedom) needs a little more work, but isn't too hard either (once an hour stop taking new processes until all current processes finish).
  
  
• A resource can become unavailable (e.g., a tape drive might break). This can result in an unsafe state.

Homework: 21, 27, and 20. There is an interesting typo in 20: A has claimed 3 units of resource 5, but there are only 2 units in the entire system. Change the problem by having B both claim and be allocated 1 unit of resource 5.

3.7: Other Issues

3.7.1: Two-phase locking

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

3.7.2: 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.

3.7.3: 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.

3.8: Research on Deadlocks

Skipped.

3.9: Summary

Read.

Chapter 4: Memory Management

Also called storage management or space management.

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

• Registers, cache, central memory, disk, tape (backup)
• Move data from level to level of the hierarchy.
• How should we decide when to move data up to a higher level?
  • Fetch on demand (e.g. demand paging, which is dominant now).
  • Prefetch
    • Read-ahead for file I/O.
    • Large cache lines and pages.
    • Extreme example. Entire job present whenever running.

We will see in the next few weeks 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.

• Convert virtual addresses to physical addresses
  • Also called logical to real address translation.
  • A virtual address is the address expressed in the program.
  • A physical address is the address understood by the computer hardware.
• The translation from virtual to physical addresses is performed by the Memory Management Unit or (MMU).
• Another example of address translation is the conversion of relative addresses to absolute addresses by the linker.
• The translation might be trivial (e.g., the identity) but not in a modern general purpose OS.
• The translation might be difficult (i.e., slow).
  • Often includes addition/shifts/mask--not too bad.
  • Often includes memory references.
    • VERY serious.
    • Solution is to cache translations in a Translation Lookaside Buffer (TLB). Sometimes called a translation buffer (TB).

Homework: 6.

When is address translation performed?

1. At compile time
   • Compiler generates physical addresses.
   • Requires knowledge of where the compilation unit will be loaded.
   • No linker.
   • Loader is trivial.
   • Primitive.
   • Rarely used (MSDOS .COM files).
   
   
2. At link-edit time (the “linker lab”)
   • Compiler
     • Generates relative (a.k.a. relocatable) addresses for each compilation unit.
     • References external addresses.
   • Linkage editor
     • Converts the relocatable addr to absolute.
     • Resolves external references.
     • Misnamed ld by unix.
     • Must 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 still trivial.
   • 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.
   • No longer common.
   
   
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

1. Dynamic Loading
   • When executing a call, check if module is loaded.
   • If not loaded, call linking loader to load it and update tables.
   • Slows down calls (indirection) unless you rewrite code dynamically.
   • Not used much.
   
   
2. Dynamic Linking
   • The traditional linking described above is today often called static linking.
   • With dynamic linking, frequently used routines are not linked into the program. Instead, just a stub is linked.
   • When the routine is called, the stub checks to see if the real routine is loaded (it may have been loaded by another program).
     • If not loaded, load it.
     • If already loaded, share it. This needs some OS help so that different jobs sharing the library don't overwrite each other's private memory.
   • Advantages of dynamic linking.
     • Saves space: Routine only in memory once even when used many times.
     • Bug fix to dynamically linked library fixes all applications that use that library, without having to relink the application.
   • Disadvantages of dynamic linking.
     • New bugs in dynamically linked library infect all applications.
     • Applications “change” even when they haven't changed.

4.1: Basic Memory Management (Without Swapping or Paging)

Entire process remains in memory from start to finish and does not move.

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

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

The “good old days” when everything was easy.

• No address translation done by the OS (i.e., address translation is not performed dynamically during execution).
• Either reload the OS for each job (or don't have an OS, which is almost the same), or protect the OS from the job.
  • One way to protect (part of) the OS is to have it in ROM.
  • Of course, must have the OS (read-write) data in ram.
  • Can have a separate OS address space only accessible in supervisor mode.
  • Might just put some drivers in ROM (BIOS).
• The user employs overlays if the memory needed by a job exceeds the size of physical memory.
  • Programmer breaks program into pieces.
  • A “root” piece is always memory resident.
  • The root contains calls to load and unload various pieces.
  • Programmer's responsibility to ensure that a piece is already loaded when it is called.
  • No longer used, but we couldn't have gotten to the moon in the 60s without it (I think).
  • Overlays have been replaced by dynamic address translation and other features (e.g., demand paging) that have the system support logical address sizes greater than physical address sizes.
  • Fred Brooks (leader of IBM's OS/360 project and author of “The mythical man month”) remarked that the OS/360 linkage editor was terrific, especially in its support for overlays, but by the time it came out, overlays were no longer used.