Operating Systems

Chapter 2: Process and Thread Management

Homework solutions posted, give password.

TAs assigned

Tanenbaum's chapter title is “Processes and Threads”. I prefer to add the word management. The subject matter is processes, threads, scheduling, interrupt handling, and IPC (InterProcess Communication--and Coordination).

2.1: Processes

Definition: A process is a program in execution.

• We are assuming a multiprogramming OS that can switch from one process to another.
• Sometimes this is called pseudoparallelism since one has the illusion of a parallel processor.
• The other possibility is real parallelism in which two or more processes are actually running at once because the computer system is a parallel processor, i.e., has more than one processor.
• We do not study real parallelism (parallel processing, distributed systems, multiprocessors, etc) in this course.

2.1.1: 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: The time used by just this processes. Virtual time progresses at a rate independent of other processes. Actually, this is false, the virtual time is typically incremented a little during systems calls used for process switching; so if there are more other processors more “overhead” virtual time occurs.
  
  
• Virtual memory: The memory as viewed by the process. Each process typically believes it has a contiguous chunk of memory starting at location zero. Of course this can't be true of all processes (or they would be using the same memory) and in modern systems it is actually true of no processes (the memory assigned is not contiguous and does not include location zero).
  
  Think of the individual modules that are input to the linker. Each numbers its addresses from zero; the linker eventually translates these relative addresses into absolute addresses. That is the linker provides to the assembler a virtual memory in which addresses start at zero.

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.

2.1.2: Process Creation

From the users or external viewpoint there are several mechanisms for creating a process.

1. System initialization, including daemon (see below) processes.
2. Execution of a process creation system call by a running process.
3. A user request to create a new process.
4. Initiation of a batch job.

But looked at internally, from the system's viewpoint, the second method dominates. Indeed in unix only one process is created at system initialization (the process is called init); all the others are children of this first process.

Why have init? That is why not have all processes created via method 2?
Ans: Because without init there would be no running process to create any others.

Definition of daemon

Many systems have daemon process lurking around to perform tasks when they are needed. I was pretty sure the terminology was related to mythology, but didn't have a reference until a student found “The {Searchable} Jargon Lexicon” at http://developer.syndetic.org/query_jargon.pl?term=demon

daemon: /day'mn/ or /dee'mn/ n. [from the mythological meaning, later rationalized as the acronym `Disk And Execution MONitor'] A program that is not invoked explicitly, but lies dormant waiting for some condition(s) to occur. The idea is that the perpetrator of the condition need not be aware that a daemon is lurking (though often a program will commit an action only because it knows that it will implicitly invoke a daemon). For example, under {ITS}, writing a file on the LPT spooler's directory would invoke the spooling daemon, which would then print the file. The advantage is that programs wanting (in this example) files printed need neither compete for access to nor understand any idiosyncrasies of the LPT. They simply enter their implicit requests and let the daemon decide what to do with them. Daemons are usually spawned automatically by the system, and may either live forever or be regenerated at intervals. Daemon and demon are often used interchangeably, but seem to have distinct connotations. The term `daemon' was introduced to computing by CTSS people (who pronounced it /dee'mon/) and used it to refer to what ITS called a dragon; the prototype was a program called DAEMON that automatically made tape backups of the file system. Although the meaning and the pronunciation have drifted, we think this glossary reflects current (2000) usage.

As is often the case, wikipedia.org proved useful. Here is the first paragraph of a more thorough entry. The wikipedia also has entries for other uses of daemon.

In Unix and other computer multitasking operating systems, a daemon is a computer program that runs in the background, rather than under the direct control of a user; they are usually instantiated as processes. Typically daemons have names that end with the letter "d"; for example, syslogd is the daemon which handles the system log.

2.1.3: Process Termination

Again from the outside there appear to be several termination mechanism.

1. Normal exit (voluntary).
2. Error exit (voluntary).
3. Fatal error (involuntary).
4. Killed by another process (involuntary).

And again, internally the situation is simpler. In Unix terminology, there are two system calls kill and exit that are used. Kill (poorly named in my view) sends a signal to another process. If this signal is not caught (via the signal system call) the process is terminated. There is also an “uncatchable” signal. Exit is used for self termination and can indicate success or failure.

2.1.4: Process Hierarchies

Modern general purpose operating systems permit a user to create and destroy processes.

• In unix this is done by the fork system call, which creates a child process, and the exit system call, which terminates the current process.
• After a fork both parent and child keep running (indeed they have the same program text) and each can fork off other processes.
• A process tree results. The root of the tree is a special process created by the OS during startup.
• A process can choose to wait for children to terminate. For example, if C issued a wait() system call it would block until G finished.

Old or primitive operating system like MS-DOS are not fully multiprogrammed, so when one process starts another, the first process is automatically blocked and waits until the second is finished.

2.1.5: Process States and Transitions

The diagram on the right contains much information.

• Consider a running process P that issues an I/O request
  • The process blocks
  • At some later point, a disk interrupt occurs and the driver detects that P's request is satisfied.
  • P is unblocked, i.e. is moved from blocked to ready
  • At some later time the operating system scheduler looks for a ready job to run and picks P.
  
  
• A preemptive scheduler has the dotted line preempt;
  A non-preemptive scheduler doesn't.
  
  
• The number of processes changes only for two arcs: create and terminate.
  
  
• Suspend and resume are medium term scheduling
  • Done on a longer time scale.
  • Involves memory management as well. As a result we study it later.
  • Sometimes called two level scheduling.

Homework: 1.

One can organize an OS around the scheduler.

• Write a minimal “kernel” (a micro-kernel) consisting of the scheduler, interrupt handlers, and IPC (interprocess communication).
  
  
• The rest of the OS consists of kernel processes (e.g. memory, filesystem) that act as servers for the user processes (which of course act as clients).
  
  
• The system processes also act as clients (of other system processes).
  
  
• The above is called the client-server model and is one Tanenbaum likes. His “Minix” operating system works this way.
  
  
• Indeed, there was reason to believe that the client-server model would dominate OS design. But that hasn't happened.
  
  
• Such an OS is sometimes called server based.
  
  
• Systems like traditional unix or linux would then be called self-service since the user process serves itself.
  • That is, the user process switches to kernel mode and performs the system call.
  • To repeat: the same process changes back and forth from/to user<-->system mode and services itself.

2.1.6: 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 process control block (PCB).

I normally refer to a process table entry as a PTE, but this is bad. I recently realized that I use PTE for two different things, Process Table Entry and Page Table Entry. Since the latter is very common, I must stop using the former. Please correct me if I slip up.

• One entry per process.
  
  
• The central data structure for process management.
  
  
• A process state transition (e.g., moving from blocked to ready) is reflected by a change in the value of one or more fields in the PCB.
  
  
• We have converted an active entity (process) into a data structure (PCB). Finkel calls this the level principle “an active entity becomes a data structure when looked at from a lower level”.
  
  
• The PCB contains a great deal of information about the process. For example,
  • Saved value of registers when process not running
  • Program counter (i.e., the address of the next instruction)
  • Stack pointer
  • CPU time used
  • Process id (PID)
  • Process id of parent (PPID)
  • User id (uid and euid)
  • Group id (gid and egid)
  • Pointer to text segment (memory for the program text)
  • Pointer to data segment
  • Pointer to stack segment
  • UMASK (default permissions for new files)
  • Current working directory
  • Many others

2.1.6A: An addendum on Interrupts

This should be compared with the addendum on transfer of control.

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.

• Tanenbaum calls the interrupt handler the interrupt service routine.
• Actually one can have different priorities of interrupts and the interrupt vector contains one pointer for each level. This is why it is called a vector.

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 disk interrupts are unlikely to be for the currently running process (because the process that initiated the disk access is likely blocked).

Actions by P prior to the interrupt:

1. Who knows??
   This is the difficulty of debugging code depending on interrupts, the interrupt can occur (almost) anywhere. Thus, we do not know what happened just before the interrupt.

Executing the interrupt itself:

2. The hardware saves the program counter and some other registers (or switches to using another set of registers, the exact mechanism is machine dependent).
   
   
3. Hardware loads new program counter from the interrupt vector.
   • Loading the program counter causes a jump.
   • Steps 2 and 3 are similar to a procedure call. But the interrupt is asynchronous.
   
   
4. As with a trap, the hardware automatically switches the system into privileged mode. (It might have been in supervisor mode already, that is an interrupt can occur in supervisor mode).

Actions by the interrupt handler (et al) upon being activated

5. An assembly language routine saves registers.
   
   
6. The assembly routine sets up new stack. (These last two steps are often called setting up the C environment.)
   
   
7. The assembly routine calls a procedure in a high level language, often the C language (Tanenbaum forgot this step).
   
   
8. The C procedure does the real work.
   • Determines what caused the interrupt (in this case a disk completed an I/O)
     • How does it figure out the cause?
       • It might know the priority of the interrupt being activated.
       • The controller might write information in memory before the interrupt
       • The OS can read registers in the controller
   • Mark process Q as ready to run.
     • That is move Q to the ready list (note that again we are viewing Q as a data structure).
     • The state of Q is now ready (it was blocked before).
     • The code that Q needs to run initially is likely to be OS code. For example, Q probably needs to copy the data just read from a kernel buffer into user space.
   • Now we have at least two processes ready to run, namely P and Q. There may be arbitrarily many others.
   
   
9. The scheduler decides which process to run (P or Q or something else). This loosely corresponds to g calling other procedures in the simple f calls g case we discussed previously. Eventually the scheduler decides to run P.

Actions by P when control returns

10. The C procedure (that did the real work in the interrupt processing) continues and returns to the assembly code.
    
    
11. Assembly language restores P's state (e.g., registers) and starts P at the point it was when the interrupt occurred.

Properties of interrupts

• Phew.
• Unpredictable (to an extent). We cannot tell what was executed just before the interrupt occurred. That is, the control transfer is asynchronous; it is difficult to ensure that everything is always prepared for the transfer.
• The user code is unaware of the difficulty and cannot (easily) detect that it occurred. This is another example of the OS presenting the user with a virtual machine environment that is more pleasant than reality (in this case synchronous rather asynchronous behavior).
• Interrupts can also occur when the OS itself is executing. This can cause difficulties since both the main line code and the interrupt handling code are from the same “program”, namely the OS, and hence might well be using the same variables. We will soon see how this can cause great problems even in what appear to be trivial cases.
• The interprocess control transfer is neither stack-like nor queue-like. That is if first P was running, then Q was running, then R was running, then S was running, the next process to be run might be any of P, Q, or R (or some other process).
• The system might have been in user-mode or supervisor mode when the interrupt occurred. The interrupt processing starts in supervisor mode.

2.2: Threads

The idea is to have separate threads of control (hence the name) running in the same address space. An address space is a memory management concept. For now think of an address space as the memory in which a process runs and the mapping from the virtual addresses (addresses in the program) to the physical addresses (addresses in the machine). Each thread is somewhat like a process (e.g., it is scheduled to run) but contains less state (e.g., the address space belongs to the process in which the thread runs.

2.2.1: The Thread Model

A process contains a number of resources such as address space, open files, accounting information, etc. In addition to these resources, a process has a thread of control, e.g., program counter, register contents, stack. The idea of threads is to permit multiple threads of control to execute within one process. This is often called multithreading and threads are often called lightweight processes. Because threads in the same process share so much state, switching between them is much less expensive than switching between separate processes.

Individual threads within the same process are not completely independent. For example there is no memory protection between them. This is typically not a security problem as the threads are cooperating and all are from the same user (indeed the same process). However, the shared resources do make debugging harder. For example one thread can easily overwrite data needed by another and if one thread closes a file other threads can't read from it.

2.2.2: Thread Usage

Often, when a process A is blocked (say for I/O) there is still computation that can be done. Another process B can't do this computation since it doesn't have access to the A's memory. But two threads in the same process do share memory so that problem doesn't occur.

An important modern example is a multithreaded web server. Each thread is responding to a single WWW connection. While one thread is blocked on I/O, another thread can be processing another WWW connection.
Question: Why not use separate processes, i.e., what is the shared memory?
Ans: The cache of frequently referenced pages.

A common organization is to have a dispatcher thread that fields requests and then passes this request on to an idle thread.

Another example is a producer-consumer problem (c.f. below) in which we have 3 threads in a pipeline. One thread reads data from an I/O device into a buffer, the second thread performs computation on the input buffer and places results in an output buffer, and the third thread outputs the data found in the output buffer. Again, while one thread is blocked the others can execute.

Question: Why does each thread block?

Answer:

1. The first thread blocks while waiting for the device to supply the data. It also blocks if the input buffer is full.
   
   
2. The second thread blocks when either the input buffer is empty or the output buffer is full.
   
   
3. The third thread blocks when the output device is busy (it might also block waiting for the output request to complete, but this is not necessary). It also blocks if the output buffer is empty.

Homework: 9.

A final (related) example is that an application that wishes to perform automatic backups can have a thread to do just this. In this way the thread that interfaces with the user is not blocked during the backup. However some coordination between threads may be needed so that the backup is of a consistent state.

2.2.3: Implementing threads in user space

Write a (threads) library that acts as a mini-scheduler and implements thread_create, thread_exit, thread_wait, thread_yield, etc. The central data structure maintained and used by this library is the thread table, the analogue of the process table in the operating system itself.

Advantages

• Requires no OS modification.
• Requires no OS modification.
• Requires no OS modification.
• Very fast since no context switching.
• Can customize the scheduler for each application.

Disadvantages

• Blocking system calls can't be executed directly since that would block the entire process. For example the producer consumer example, if implemented in the natural manner would not work well as whenever an I/O was issued that caused the process to block, all the threads would be unable to run (but see just below).
• Similarly a page fault would block the entire process (i.e., all the threads).
• A thread with an infinite loop prevents all other threads in this process from running.
• Re-doing the effort of writing a scheduler.

Possible methods of dealing with blocking system calls

• Perhaps the OS supplies a non-blocking version of the system call, e.g. non-blocking read.
• Perhaps the OS supplies another system call that tell if the blocking system call will in fact block. For example, a unix select() can be used to tell if a read would block. It might not block if
  • The requested disk block is in the buffer cache (see I/O chapter).
  • The request was for a keyboard or mouse or network event that has already happened.

2.2.4: Implementing Threads in the Kernel

Move the thread operations into the operating system itself. This naturally requires that the operating system itself be (significantly) modified and is thus not a trivial undertaking.

• Thread-create and friends are now system calls and hence much slower than with user-mode threads. They are, however, still much faster than creating/switching/etc processes since there is so much shared state that does not need to be recreated.
• A thread that blocks causes no particular problem. The kernel can run another thread from this process or can run another process.
• Similarly a page fault, or infinite loop in one thread does not automatically block the other threads in the process.

2.2.5: Hybrid Implementations

One can write a (user-level) thread library even if the kernel also has threads. This is sometimes called the M:N model since M user mode threads run on each of N kernel threads. Then each kernel thread can switch between user level threads. Thus switching between user-level threads within one kernel thread is very fast (no context switch) and we maintain the advantage that a blocking system call or page fault does not block the entire multi-threaded application since threads in other processes of this application are still runnable.

2.2.6: Scheduler Activations

Skipped

2.2.7: Popup Threads

The idea is to automatically issue a thread-create system call upon message arrival. (The alternative is to have a thread or process blocked on a receive system call.) If implemented well, the latency between message arrival and thread execution can be very small since the new thread does not have state to restore.

Making Single-threaded Code Multithreaded

Definitely NOT for the faint of heart.

• There often is state that should not be shared. A well-cited example is the unix errno variable that contains the error number (zero means no error) of the error encountered by the last system call. Errno is hardly elegant (even in normal, single-threaded, applications), but its use is widespread. If multiple threads issue faulty system calls the errno value of the second overwrites the first and thus the first errno value may be lost.
• Much existing code, including many libraries, are not re-entrant.
• Managing the shared memory inherent in multi-threaded applications opens up the possibility of race conditions that we will be studying next.
• What should be done with a signal sent to a process. Does it go to all or one thread?
• How should stack growth be managed. Normally the kernel grows the (single) stack automatically when needed. What if there are multiple stacks?

2.3: Interprocess Communication (IPC) and Coordination/Synchronization

2.3.1: Race Conditions

A race condition occurs when two (or more) processes are about to perform some action. Depending on the exact timing, one or other goes first. If one of the processes goes first, everything works, but if another one goes first, an error, possibly fatal, occurs.

Imagine two processes both accessing x, which is initially 10.

• One process is to execute x <-- x+1
• The other is to execute x <-- x-1
• When both are finished x should be 10
• But we might get 9 and might get 11!
• Show how this can happen (x <-- x+1 is not atomic)
• Tanenbaum shows how this can lead to disaster for a printer spooler