Work, And Its Abstractions

We introduced the idea of work on the first day of class. Today we filled in some more details about process management in distributed systems, and dug much more deeply into threads and thread scheduling. Review the relevant portion of the lecture notes from the first day before diving into these. We did a brief review in class, but it is not repeated here.

Creating New Tasks

One of the functions of the operating system is to provide a mechanism for existing tasks to create new tasks. When this happens, we call the original task the parent. The new task is called the child. It is possible for one task to have many children. In fact, even the children can have children.

In UNIX, child tasks can either share resources with the parent or obtain new resources. But existing resources are not partitioned.

In UNIX when a new task is created, the child is a clone of the parent. The new task can either continue to execute with a copy of the parent image, or load another image. Well talk more about this soon, when we talk about the fork() and exec-family() of calls.

After a new task is created, the parent may either wait()/waitpid() for the child to finish or continue and execute concurrently (real or imaginary) with the child.

Fork -- A traditional implementation

fork() is the system call that is used to create a new task on UNIX systems. In a traditional implementation, it creates a new task by making a nearly exact copy of the parent. Why nearly exact? Some things don't make sense to be duplicated exactly, the ID number, for example.

The fork() call returns the ID of the child process in the parent and 0 in the child. Other than this type of subtle differences, the two tasks are very much alike. Execution picks up at the same point in both.

If execution picks up at the same point in both, how can fork() return something different in each? The answer is very straightforward. The stack is duplicated and a different value is placed on top of each. (If you don't remeber what the stack is, don't worry, we'll talk about it soon -- just realize that the return value is different).

The difference in the return value of the fork() is very significant. Most programmers check the result of the fork in order to determine whether they are currently the child or parent. Very often the child and parent to very different things.

The Exec-family() of calls

Since the child will often serve a very different purpose that its parent, it is often useful to replace the child's memory space, that was cloned form the parent, with that of another program. By replace, I am referring to the following process:

1. Deallocate the process' memory space (memory pages, stack, etc).
2. Allocate new resources
3. Fill these resources with the state of a new process.
4. (Some of the parent's state is preserved, the group id, interrupt mask, and a few other items.)

Copying all of the pages of memory associated with a process is a very expensive thing to do. It is even more expensive considering that very often the first act of the child is to deallocate this recently created space.

One alternative to a traditional fork implementation is called copy-on-write. the details of this mechanism won't be completely clear until we study memory management, but we can get the flavor now.

The basic idea is that we mark all of the parent's memory pages as read-only, instead of duplicating them. If either the parent or any child try to write to one of these read-only pages, a page-fault occurs. At this point, a new copy of the page is created for the writing process. This adds some overhead to page accesses, but saves us the cost of unnecessarly copying pages.

vfork()

Another alternative is also available -- vfork(). vfork is even faster, but can also be dangerous in the worng hands. With vfork(), we do not duplicate or mark the parent's pages, we simply loan them, and the stack frame to the child process. During this time, the parent remains blocked (it can't use the pages). The dangerous part is this: any changes the child makes will be seen by the aprent process.

vfork() is most useful when it is immediately followed by an exec_(). This is because an exec() will create a completely new process-space, anyway. There is no reason to create a new task space for the child, just to have it throw it away as part of an exec(). Instead, we can loan it the parent's space long enough for it to get started (exec'd).

Although there are several (4) different functions in the exec-family, the only difference is the way they are parameterizes; under-the-hood, they all work identically (and are often one).

After a new task is created, the parent will often want to wait for it (and any siblings) to finish. We discussed the defunct and zombie states last class. The wait-family of calls is used for this purpose.

The Conceptual Thread

In our discussion of tasks we said that a task is an operating system abstraction that represents the state of a program in execution. We learned that this state included such things as the registers, the stack, the memory, and the program counter, as well as software state such as "running," "blocked", &c. We also said that the processes on a system compete for the systems resources, especially the CPU(s).

Today we discussed another operating system abstraction called the thread. A thread, like a task, represents a discrete piece of work-in-progress. But unlike tasks, threads cooperate in their use of resources and in fact share many of them.

We can think of a thread as a task within a task. Among other things threads introduce concurrency into our programs -- many threads of control may exist. Older operating systems didn't support threads. Instead of tasks, they represented work with an abstraction known as a process. The name process, e.g. first do ___, then do ____, if x then do ____, finally do ____, suggests only one thread of control. The name task, suggests a more general abstraction. For historical reasons, colloquially we often say process when we really mean task. From this point forward I'll often say process when I mean task -- I'll draw our attention to the difference, if it is important.

A Process's Memory

Before we talk about how a tasks' memory is laid out, let's first look at the simpler case of a process -- a task with only one thread of control.

Please note that the heap grows upward through dynamic allocation (like malloc) and the stack grows downward as stack frames are added throguh function calls. Such things as return addresses, return values, parameters, local variables, and other state are stored in the runtime stack.

• The text area holds the program code.
• The data area holds global and static variables that must be stored in the executible, since they are initialized.
• The bss area holds variables that are unitialized in the sense that their need not be persistently stored on disk -- they can be plugged in later.

The Implementation of Threads

All of the threads within a process exist within the context of that process. They share the code section, data section, and operating system resources such as open files.

But they do not share all resources. Since each thread executes independently, each thread has its own understanding of the stack and of the registers.

The good part about sharing so many resources is that switching execution among threads is less expensive than it is among processes.

The bad part is that unlike the protection that exists among processes, the operating system can not prevent threads from interfering with each other -- they share the same process space.

Kernel Threads

The most primitive implementations of threads were invisible to the user. They existed only within the kernel. Many of the kernel daemons, such at the page daemon, were implemented as threads. Implementing different operating system functions as threads made sense for many reasons:

• There was no need for protection, since the kernel developers trust themselves. Please note that we lose memory protection once we go from processes to threads -- they all play in the same address space.
• The different OS functions shared many of the kernel's resources
• They can be created and destroyed very cheaply, so they can be easily used for things like I/O requests and other intermitent activities.
• It is very cheap to switch among them to handle various tasks
• It is very easy to thing of kernel activities in terms of separate threads instead of functions within one monolithic kernel.

User Threads

But the UNIX developers couldn't keep such a great thing as their own private secret for long. Users began to use threads via thread libraries. Thread libraries gave users the illusion of threads, without any involvement form the kernel. When a process containing threads is executed, the thread scheduler, within the process, is run. This scheduler selects which thread should run and for how long. If a thread should block, the scheduler can select to run another thread within the same process.

This implementation of threads is actually much more than an illusion. It gives users the ability to write very efficient programs. These programs can switch among threads and share resurces with very little overhead. To switch threads, the registers must be saved and restored and the stack must be switched. No expensive context switch is required.

Another advantage is that user level threads are implemented entirely by a thread library -- from the interface to the scheduling. The kernel doesn't see them or know about them.

Kernel-Supported User-Level Threads

Kernel threads are great for kernel writers and user threads answer many of the needs of users, but they are not perfect. Consider these examples:

• On a multiprocessor system, only one thread within a process can execute at a time
• A process that consists of many threads, each of which may be able to execute at any time, will not get any more CPU time than a process containing only one thread
• If any thread within a process makes a system call, all threads within that process will be blocked because of the context switch.
• If any user thread blocks waiting for I/O or a resource, the entire process blocks. (Thread libraries usually replace blocking calls with non-blocking calls whenever possible to mitigate this.)

To address these needs, we need to have a kernel supported user thread. That is to say, we need a facility for threads to share resources within a process, but we also need the ability of the kernel to preempt, schedule, and dispatch threads. This type of thread is called a kernel supported user thread or a light-weight process (LWP). A light-weight process is in contrast with a heavy-weight process otherwise known as a process or task.

Our model of the universe has gone from looking like this:

To looking like this:

A More Complex Model

In some models, such as that used by Solaris, it is also possible to assign several kernel-supported threads to a single process without assigning them to specific user-level threads. In this case, the process will have more opportunities to be seen by the OS's CPU scheduler. On multiprocessor systems, the maximum level of concurrency is determined by the number of LWPs assigned to the process (of course this is further limited by the number of threads that are runnable within the process and the number of available CPUs).

In the context of Solaris, an LWP is a user-visible kernel thread. In some ways, it might be better to view a Solaris LWP as a virtual light weight processor (this is Kesden nomenclature!). This is because pools of LWPs can be assigned to the same task. Threads within that task are then scheduled to run on available LWPs, much like processes are scheduled to run on available processors.

In truth, LWPs are anything but light weight. They are lighter weight than (heavy weight) processes -- but they require far more overhead than user-level threads without kernel support. Context-switching among user-level threads within a process is much, much cheaper than context switching among LWPs. But, switching among LWPs can lead to greater concurrency for a task when user-level threads block within the kernel (as opposed to within the process such that the thread scheduler can run another).

The diagram below shows LWP's associated with tasks and kernel threads, as well as kernel threads without an associated LWP and several different associations between user-level threads and the LWP(s) assigned to the process.

LWP's offer a convenient and flexible compromise between user-threads and separate processes. But it is important to realize that they are bulky structures:

• Communications, even within a process, among LWPs requires kernel involvement (read as: 2 context switches)
• LWPs are scheduled by the kernel, so blocking an LWP requires kernel involvement.
• LWPs are very flexible, and very general -- this means that they are very big. LWPs consume a great deal of resources
• LWPs are expensive to create and destroy, because this involves the kernel
• LWPs are unpoliced, so users can create many of them, consuming system resources, and starving other users processes by getting more CPU time than similar processes with fewer LWPs.

Another Model

You might observe that threads within the same task share many of the same resources -- the most significant difference is that they have different stacks. This is exactly how Linux implements threads -- by leveraging the machinery it uses to create new processes.

In Linux, the system call beneath a fork() is known as clone(). It does everything that fork() does -- but wiht a lot more flexibility. It allows a newly created child to be created from the template of its parent by either copying, sharing, or recreating various resources from the parent. So, you can imagine that if all of the important resources are copies -- clone() is, in effect, a fork().

But, imagine two different processes that, in essence, share everything but the stack. These processes are sharing global memory, so they are really in the smae memory context -- but they have differnt stacks, so can be doing different things. They are, in effect, threads.

So, as Linux evolved, they created kernel-supported threads by leveraging clone() to create new processes that shared resources. They then created a new task abstraction that aggregated related processes. So, in the Linux model, within the kernel, a task is genuinely a collection of processes. From the user's perspective, these thread-like processes are presented as threads by the user-level thread library.

Tasks in Distributed Systems

In distributed systems, we find that the various resources needed to perform a task are scattered across a network. This blurs the distinction between a process and a task and, for that matter, a task and a thread. In the context of distributed systems, a process and a thread are interchangable terms -- they represent something that the user wants done.

But, task has an interesting and slightly nuianced meaning. A task is the collection of resources configured to solve a particular problem. A task contains not only the open files and communication channels -- but also the threads (a.k.a. processes). Distributed Systems people see a task as the enviornment in which work is done -- and the thread (a.k.a. process) as the instance of that work, in progress.

I like to explain that a task is a factory -- all of the means of production scattered across many assembly lines. The task contains the machinery and the supplies -- as well the processes that are ongoing and making use of them.

Although this is traditionally a Distributed Systems view of the world, it it is pretty applicable to modern operating systems that implement kernel-supported threads, whether by a Linux-style close() or a Solaris-style first-class task abstraction.

Scheduling Basics

Scheduling the access of processes to non-sharable resources is a fundamental part of an operating system's job. The same is true of the thread scheduler within a user-level thread library. The CPU is the most important among these resource, because it typically has the highest contention. The high cost of additional CPUs, both in terms of the price of the processor and the cost of the technology in massively parallel systems, ensures that most systems don't have a sufficient surplus of CPUs to allow for their wasteful use.

The primary objective of CPU scheduling is to ensure that as many jobs are running at a time as is possible. On a single-CPU system, the goal is to keep one job running at all times.

Multiprogramming allows us to keep many jobs ready to run at all times. Although we can not concurrently run more jobs than we have available processors, we can allow each processor to be running one job, while other jobs are waiting for I/O or other events.

Observation: The CPU-I/O Burst Cycle

During our discussion of scheduling, I may make reference to the CPU-I/O burst cycle. This is a reference to the observation that programs usually have a burst of I/O (when the collect data) followed by a burst of CPU (when they process it and request more). These bursts form a cycle of execution.

Some processes have long bursts of CPU usage, followed by short bursts of I/O. We say that thee jobs are CPU Bound.

Some processes have long bursts of I/O, followed by short bursts of CPU. We say that thee jobs are I/O Bound.

The CPU Scheduler

Some schedulers are only invoked after a job finishes executing or voluntarily yield the CPU. This type of scheduler is called a non-premptive scheduler.

1. A process blocks itself waiting for a resource or event
2. A process terminates

But, in order to support interaction, most modern schedulers are premeptive. They make use of a hardware timer to interrupt running jobs. When a scheduler hardware interrupt occurs, the scheduler's ISR (Interrupt Service Routine) is invoked and it runs. When this happens, it can continue the previous task or run another. Before starting a new job, the scheduler sets the hardware timer to generate an interrupt after a particular amount of time. This time is known as the time quantum. It is the amount of time that a job can run without interruption.

A preemptive scheduler may be invoked under the following four circumstances:

1. A process blocks itself waiting for a resource or event
2. A process terminates
3. A process moves from running to ready (interrupt)
4. A process moves from waiting to ready (blocking condition satisfied)

The Dispatcher

Once the CPU scheduler selects a process for execution, it is the job of the dispatcher to start the selected process. Starting this process involves three steps:

1. Switching context
2. Switching to user mode
3. Jumping to the proper location in the program to start or resume execution

The latency introduced by the dispatch is called the dispatch latency. Obviously, this should be as small as possible -- but is most critical in real-time system, those systems that must meet deadlines associated with real world events. These systems are often associted with manufacturing systems, monitoring systems, &c. Actually, admission is also much more important in these systems -- a job isn't automically admitted, it is only admitted if the system can verify that enough resources are available to meet the deadlines associated with the job. But real-time systems are a different story -- back to today's tale.

So, given a collection of tasks, how might the OS (or the user-level thread scheduler) decide which to place on an available CPU?

First Come, First Serve (FCFS)
First In, First Out (FIFO)

FCFS is the simplest algorithm. It should make sense to anyone who has waited in line at the deli, bank, or check-out line, or to anyone who has ever called a customer service telephone number, "Your call will be answered in the order in which it was received."

The approach is very simple. When a job is submitted, it enters the ready queue. The oldest job (has been in the ready queue for the longest time) in the ready queue is always selected to be dispatched. The algorithm is non-premptive, so the job will run until it voluntarily gives up the CPU by blocking or terminating. After a blocked process is satisfied and returns to the ready queue, it enters at the end of the line.

This algorithm is very easy to implement, and it is also very fair and consequently starvation-free. No characteristic of a job bias its placement in the queue. But it does have several disadvantages.

• Because short jobs can be scheduled after very long jobs, the average wait time can be increased. Unlike the grocery store, the algorithm has no "Express Lane."
• Since the algorithm is non-premptive, there can be problems on interactive systems. A long job may prevent responsiveness to the user.
• A student actually pointed out this more subtle property during class in the form of a question (good job!) It is possible for one CPU bound process to hog the CPU for a long time. In the meantime several I/O bound processes, that could execute concurrently with the CPU-bound process, can't start.

  I didn't give this phenomenon a name in class, but I probably should have -- it is called the convoy effect, because cycle after cycle, the I/O bound processes will "follow" the CPU bound processes, without overlapping.

Bonus Material

Everything in this section goes above and beyond lecture. It is just here for those who happen to be curious for more real-world detail.

Shortest-Job-First (SJF)

Another apporach is to consider the expected length of each processes's next CPU burst and to run the process with the shortest burst next. This algorithm optimizes the average waiting time of the processes. This is because moving a shorter job ahead of a longer job helps the shorter job more than it hurts the longer job. Recall my lunchroom story -- those who ate early had no lines, although there wasn't anyone to vouch for this.

Unfortunately, we have no good way of knowing for sure the length of any jobs next CPU burst. In practice this can be estimated using an exponential average of the jobs recent CPU usage. In the past, programmers estimated it -- but if their jobs went over their estimate, they were killed. programmers got very good at "The Price Is Right."

Sometimes this algorithm is premeptive. A job can be prempted if another job arrives that has a shorter execution time. This is flavor is often called shortest-remaining-time-first (SRTF).

Shortest-Time-To-Completion-First (STCF)

Shortest CPU time to Completion First (STCF). The process that will complete first runs whenever possible. The other processes only run when the first process is busy with an I/O event.

But, much like SJF, the CPU time is not known in advance.

Priority Scheduling (PRI)

Priority scheduling is designed to strictly enforce the goals of a system. Important jobs always run before less important jobs. If this scheduling discipline is implemented preemeptively, more important jobs will preempt less important jobs, even if they are currently running.

The bigest problem with priority-based scheduling is starvation. it is possible that low priority jobs will never execute, if more important jobs continually arrive.

Round Robin Scheduling (RR)

Round Robin scheduling can be thought of as a preemptive version of FCFS. Jobs are processed in a FCFS order from the run queue. As with FCFS, if they block, the next process can be scheduled. And when a blocked process returns to the ready queue, it is placed at the end of the list.

The difference is that each process is given a time quantum or time slice. A hardware timer interrupt preemept the running process, if it is still running after this fixed amount of time. The scheduler can then dispatch the next process in the queue.

With an appropriate time quantum, this process offeres a better average case performance than FCFS without the guesswork of SJF.

If the time quantum is very, very small, an interesting effect is produced. If there are N processes, each process executes as if it were running on its own private CPU running at 1/N th the speed. This effect is called processor sharing.

If the time quantum is very, very large -- large enough that the processes generally complete before it expires, this approach approximates FCFS.

The time quantum can be selected to balance the two effects.

Multilevel Queue Scheduling (MQS)

MQS is similar to PRI, except that the jobs arrive sorted by their priority. For example, all system jobs may have a higher priority than interactive jobs, which enjoy a higher priority than batch jobs. Jobs of different priorities are placed into different queues.

In some implementations, jobs in all higher priority queues must be executed before jobs in any lower priority queue. This absolute approach can lead to starvation in the same way as its simplier cousin, PRI. In some preemptive implementations, a lower-priority process will be returned to its ready queue, if a higher-priority process arrives.

Another approach is to time-slice among the queues. Higher priority queus can be given longer or more frequent time slices. This approach prevents absolute starvation.

Multilevel Feedback Queue (MFQ)

In the multilevel queuing system we described above, there is no social mobility. When aprocess arrives, it is placed in a queue based on its initial classification. It remains in this queue throughout its lifetime.

In a MFQ system, a process's priority can change as the result of its CPU utilization. Processes that have longer CPU bursts are lowered in importance. Processes that are I/O bound and frequently release the CPU prematurely by blocking are increased in importance.

This system prevents starvation and gives I/O bound processes the change to be dispatched and overlap with CPU bound processes. It fights the convoy effect.

Scheduling among queues can be done by varying the lenght or frequence of the time slice. Scheduling within each queue can also be varied.

Although this system sounds very pretty, it is very complex. In general this type of system is defined by the following parameters:

• the number of queues
• the method of scheduling for each queue
• the method of scheudling among the queues
• the method of promoting a process
• the method of demoting a process

Traditional UNIX Scheduling - Introduction

I thought it would be interesting to spend some time considering scheduling in a real-world operating system. Today we'll talk about what I call "traditional" UNIX scheduling. This scheduling system was used, with little variation through AT&T SVR3 and 4.3BSD. Newer UNIX's use more sophisticated approaches, but this is a good place to start.

Priorities

The scheduling in these systems was priority based. The priority of a process ranged from 0 - 127. Counterintuitively, lower priorities represent more important processes.

The range of priorities is partitioned. Priorities in the range of 0 - 49 are reserved for system processes. Both user and system processes may have priorities over the full range from 0 - 127. Ths prevents user processes from interfereing with more important system tasks.

Parameters

A processes ultimate scheduling priority varies with several factors. The accounting of these factors is kept in the proc structure, which contains the following fields:

• p_pri - current scheduling priority
• p_usrpri - the process's priority in user mode
• p_cpu - a measure of the prcoess's recent CPU usage
• p_nice - a user supplied measure of the process's importance.

Elevated Priority in System Mode

p_pri is a number in the range of 0 - 127 that represents the priority of the process. This is the value of that the scheduler considers when selecting a process to be dispatched. This value is normally the same as p_usrpri. It however, may be lowered (making the process more important) while the process is making a system call.

Traditional UNIX systems did not have preemptive kernels. This meant that only one process could be in the kernel at a time. If a process blocked while in a system call, other user processes could run, but not other system calls or functions. For this reason a process which had blocked while in a system call often would have its p_pri value lowered so that it would expeditiously complete its work in the kernel and return to user mode. This allowed other processes that blocked waiting to enter the kernel to make progress. Once the system call is complete, the process's p_pri is reset to its p_usrpri.

User Mode Priority

The priority that a process within the kernel receives after returning from the blocked queue is called its sleep priority. There is a specific sleep priority associated with every blocking condition. For example, the sleep priority associated with terminal I/O was 28 and disk I/0 was 20.

The user mode scheduling priority depends on three factors:

• the default user mode priority, PUSER, typically 50
• the process's recent CPU usuage, p_cpu
• the nice value assigned by the user, p_nice

The p_usr value is a system-wide default. In most implementations it was 50, indicating the most important level of scheduling for a user process.

Let's be Nice

The p_nice value defaults to 0, but can be increased by users who want to be nice. Remember that the likelihood of a process to be dispatched is inversly proportional to the priority. By increasing the process's nice value, the process is deacreasing its likelihood of being scheduled. Processes are usually "niced" if they are long-running, non-interactive backgorund processes.

Tracking CPU usage

p_cpu is a measure of the process's recent CPU usage. It ranges from 0 - 127 and is initially 0. Ever 1/10th of a second, the ISR that handles clock ticks increments the p_cpu for the current process.

Every 1 second another ISR decreases the p_cpu of all processes, running or not. This reduction is called the decay. SVR3 used a fixed decay of 1/2. The problem with a fixed decay is that it elevates the priority of nearly all processes if the load is very high, since very few processes are getting CPU. This makes the p_cpu field nearly meaningless.

The designers of 4.3BSD remedied this side-effect by using a variable decay that is a fuction of the systems load average, the average number of processes in the run queue over the last second. This formula follows:

decay_factor = (2*load_average)/ (2*load_average + 1)

User Mode Priority - Final Formula

The scheduler computes the process's user priority form these factors as follows:

p_usrpri = PUSER + (p_cpu/4) + (2*p_nice)

Implementation

Artifacts of the old DEC VAX systems made it much more efficient to collapse the 127 priorities into 32 queues. So these systems in effect had 32 queues each holding processes in a range of 4 priority levels (0-3, 4-7, 8-11, 12-15, 16-19, etc).

The system maintained a 32-bit mask. Each bit represented a single queue. If the bit was set, there were jobs in the queue. If the bit was 0, the queue was empty. The system charged from low-bit to high-bit in this mask until it found a non-empty queue. It would then select a job Round Robin (RR) from this queue to be dispatched.

The round-robin scheduling with a time quantum of 100mS only applied to processes in the same queue. If a process arrived in a lower priority (more important) queue, that process would be scheduled at the end of the currently executing process's quantum.

High priority (less important) processes would not execute until all lower priority (more important) queues were empty.

The queues would be check by means of the bit mask every time a process blocked or a time quantum expired.

Analysis

This method of scheduling proved viable for general purpose systems, but it does have several limitations:

• it falls apart with a massive number of processes - the overhead of recomputing priorities for every process every second becomes too high
• nice values are a very weak and underutilized way for applications to affect their priorities -- how often doyou nice your jobs?
• A non-preemptive kernel means that important processes may have to wait for lower priority processes in the kernel. The temporary sleep priority is a weak solution.

Sharing or Fighting?

Cooperating processes (or threads) often share resources. Some of these resources can be concurrently used by any number of processes. Others can only be used by one process at a time.

The air in this room can be shared by everyone without coordination -- we don't have to coordinate our breathing. But the printer wouldn't be much use to any of us if all of us were to use it at the same time. I'm not sure exactly how that would work -- perhaps it would print all of our images superimposed, or perhaps a piece here-or-there from each of our jobs. But in any case, we would want to do something to coordinate our use of the resource. Another example might be an interesection -- unless we have some way of controlling our use of an intersection (stop signs? traffic lights?) -- smack!

The policy that defines how a resource should be shared is known as a sharing discipline. The most common sharing discipline is mutual exclusion, but many others are possible. When the mutual exclusion policy is used, the use of a resource by one process excludes its use by any other process.

How do we manipulate a resource from within a program? With code, of course. The portion of a program that manipulates a resource in a way that requires mutual exclusion, or some other type of protection, is known as a critical section. We also noted that in practice, even a single line of HLL code can create a critical section, because one line of HLL code may be translated into several lines of interruptable machine-language code.

Characteristics of a Solution

A solution to the mutual exclusion problem must satisfy three conditions:

1. Mutual Exclusion: Only one process can be in the critical section at a time -- otherwise what critical section?.
2. Progress: No process is forced to wait for an available resource -- otherwise very wasteful.
3. Bounded Waiting: No process can wait forever for a resource -- otherwise an easy solution: no one gets in.

Disabling Interrupts

It is actually possible to synchronize an arbitrary number of processes within an operating system without hardware support. But, this is a tangled, ticklish, and generally unfortunate businesss. In practice, in order to implement a general-purpose solution in the real world, we're going to need a small amount of hardware support to implement very basic concurrency control primitives, from which we can build up richer abstractions or simply correct, easily readable solutions to straight-forward problems.

One rudimentary form of synchronization supported by hardware is frequently used within the kernel: disabling interrupts. The kernel often guarantees that its critical sections are mutually exclusive simply by maintaining control over the execution. Most kernels disable interrupts around many critical regions ensuring that they can not be interrupted. Interrupts are reenabled immediately after the critical section.

Unfortunately, this approach only works in kernel mode and can result in a delayed service of interrupts -- or lost interrupts (if two of the same type occur before interrupts are reenabled). And it only works on uniprocesors, becuase diabling interrupts can be very time-consuming and messy if several processors are involved.

    Disable_interrupts();

    <<< critical section >>>

    Enable_interrupts();
    

Special Instructions

Another approach is to build into the hardware very simple instructions that can give us a limited guarantee of mutual exclusion in hardware. From these small guarantees, we can build more complex constructs in software. This is the approach that is, virtually universally, in use in modern hardware.

Test-and-Set

One such instruction that is commonly implemented in hardware is test-and-set. This instruction allows the atomic testing and setting of a value.

The semantics of the instruction are below. Please remember that it is atomic. It is not interruptable.

    TS (<mem loc>)
    {
       if (<memloc> == 0)
       {
           <mem loc> = 1;
           return 0;
       }
       else
       {
          return 1;
       }
    

Given the test-and-set instruction, we can build a simple synchronization primiative called the mutex or spin-lock. A process busy-waits until the test-and-set succeeds, at which time it can move into the critical section. When done, it can mark the critical section as available by setting the mutex's value back to 0 -- it is assumed that this operation is atomic.

    Acquire_mutex(<mutex>) /* Before entering critical section */
    {
        while(TS(<mutex>))
    }

    Release_mutex(<mutex>) /* After exiting critical section */
    {
        <mutex> = 0;
    }
    

Compare-and-Swap

Another common hardware-supplied instruction that is useful for building synchronization primatives is compare-and-swap. Much like test-and-set, it is atomic in hardware -- the pseudo-code below just illustrates the semantics, it is not meant to in any way suggest that the instruction is interruptable.

    CS (<mem loc>, <expected value>, <new value>)
    {
        if (<memloc> == <expected value>))
        {
           <mem loc> = <new value>;
           return 0;
        }
        else
        {
           return 1;
        }
    

You may also want to note that test-and-set is just a special case of compare-and-swap:

TS(x) == CS (x, 0, 1)

The pseudo-code below illustration the creation of a mutex (spin lock) using compare-and-swap:

    Acquire_mutex(<mutex>) /* Before entering critical section */
    {
       while (CS (<mutex>, 1, 0))
       ;
    }

    Release_mutex(<mutex>) /* After exiting critical section */
    {
       <mutex> =  1;
    }