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Processes in UNIX

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10.3.3 Implementation of Processes in UNIX

A process in UNIX is like an iceberg: what you see is the part above the water, but there is also an important part underneath. Every process has a user part that runs the user program. However, when one of its threads makes a system call, it traps to kernel mode and begins running in kernel context, with a different memory map and full access to all machine resources. It is still the same thread, but now with more power and also its own kernel mode stack and kernel mode program counter. These are important because a system call can block part way through, for example, waiting for a disk operation to complete. The program counter and registers are then saved so the thread can be restarted in kernel mode later.

The kernel maintains two key data structures related to processes, the process table and the user structure. The process table is resident all the time and contains information needed for all processes, even those that are not currently present in memory. The user structure is swapped or paged out when its associated process is not in memory, in order not to waste memory on information that is not needed.

The information in the process table falls into the following broad categories:

  1. Scheduling parameters. Process priority, amount of CPU time consumed recently, amount of time spent sleeping recently. Together, these are used to determine which process to run next.

  2. Memory image. Pointers to the text, data, and stack segments, or, if paging is used, to their page tables. If the text segment is shared, the text pointer points to the shared text table. When the process is not in memory, information about how to find its parts on disk is here too.

  3. Signals. Masks showing which signals are being ignored, which are being caught, which are being temporarily blocked, and which are in the process of being delivered.

  4. Miscellaneous. Current process state, event being waited for, if any, time until alarm clock goes off, PID, PID of the parent process, and user and group identification.

The user structure contains information that is not needed when the process is not physically in memory and runnable. For example, although it is possible for a process to be sent a signal while it is swapped out, it is not possible for it to read a file. For this reason, information about signals must be in the process table, so they are in memory all the time, even when the process is not present in memory. On the other hand, information about file descriptors can be kept in the user structure and brought in only when the process is in memory and runnable.

The information contained in the user structure includes the following items:

  1. Machine registers. When a trap to the kernel occurs, the machine registers (including the floating-point ones, if used) are saved here.

  2. System call state. Information about the current system call, including the parameters, and results.

  3. File descriptor table. When a system call involving a file descriptor is invoked, the file descriptor is used as an index into this table to locate the in-core data structure (i-node) corresponding to this file.

  4. Accounting. Pointer to a table that keeps track of the user and system CPU time used by the process. Some systems also maintain limits here on the amount of CPU time a process may use, the maximum size of its stack, the number of page frames it may consume, and other items.

  5. Kernel stack. A fixed stack for use by the kernel part of the process.

Bearing the use of these tables in mind, it is now easy to explain how processes are created in UNIX. When a fork system call is executed, the calling process traps to the kernel and looks for a free slot in the process table for use by the child. If it finds one, it copies all the information from the parent's process table entry to the child's entry. It then allocates memory for the child's data and stack segments, and makes exact copies of the parent's data and stack segments there. The user structure (which is often kept adjacent to the stack segment), is copied along with the stack. The text segment may either be copied or shared since it is read only. At this point, the child is ready to run.

When a user types a command, say, ls on the terminal, the shell creates a new process by forking off a clone of itself. The new shell then calls exec to overlay its memory with the contents of the executable file ls. The steps involved are shown in Fig. 10-6.

Figure 10-6. The steps in executing the command ls typed to the shell.

The mechanism for creating a new process is actually fairly straightforward. A new process table slot and user area are created for the child process and filled in largely from the parent. The child is given a PID, its memory map is set up, and it is given shared access to its parent's files. Then its registers are set up and it is ready to run. semantics of fork say that no memory is shared between parent and child. However, copying memory is expensive, so all modern UNIX systems cheat. They give the child its own page tables, but have them point to the parent's pages, only marked read only. Whenever the child tries to write on a page, it gets a protection fault. The kernel sees this and then allocates a new copy of the page to the child and marks it read/write. In this way, only pages that are actually written have to be copied. This mechanism is called copy-on-write. It has the additional benefit of not requiring two copies of the program in memory, thus saving RAM.

After the child process starts running, the code running there (a copy of the shell) does an exec, system call giving the command name as a parameter. The kernel now finds and verifies the executable file, copies the arguments and environment strings to the kernel, and releases the old address space and its page tables.

Now the new address space must be created and filled in. If the system supports mapped files, as System V, BSD, and most other UNIX systems do, the new page tables are set up to indicate that no pages are in memory, except perhaps one stack page, but that the address space is backed by the executable file on disk. When the new process starts running, it will immediately get a page fault, which will cause the first page of code to be paged in from the executable file. In this way, nothing has to be loaded in advance, so programs can start quickly and fault in just those pages they need and no more. Finally, the arguments and environment strings are copied to the new stack, the signals are reset, and the registers are initialized to all zeros. At this point, the new command can start running.

Threads in UNIX

The implementation of threads depends on whether they are supported in the kernel or not. If they are not, such as in 4BSD, the implementation is entirely in a user-space library. If they are, as in System V and Solaris, the kernel has some work to do. We discussed threads in a general way in Chap. 2. Here we will just make a few remarks about kernel threads in UNIX.

The main issue in introducing threads is maintaining the correct traditional UNIX semantics. First consider fork. Suppose that a process with multiple (kernel) threads does a fork system call. Should all the other threads be created in the new process? For the moment, let us answer that question with yes. Suppose that one of the other threads was blocked reading from the keyboard. Should the corresponding thread in the new process also be blocked reading from the keyboard? If so, which one gets the next line typed? If not, what should that thread be doing in the new process? The same problem holds for many other things threads can do. In a single-threaded process, the problem does not arise because the one and only thread cannot be blocked when calling fork. Now consider the case that the other threads are not created in the child process. Suppose that one of the not-created threads holds a mutex that the one-and-only thread in the new process tries to acquire after doing the fork. The mutex will never be released and the one thread will hang forever. Numerous other problems exist too. There is no simple solution.

File I/O is another problem area. Suppose that one thread is blocked reading from a file and another thread closes the file or does an lseek to change the current file pointer. What happens next? Who knows?

Signal handling is another thorny issue. Should signals be directed at a specific thread or at the process in general? A SIGFPE (floating-point exception) should probably be caught by the thread that caused it. What if it does not catch it? Should just that thread be killed, or all threads? Now consider the SIGINT signal, generated by the user at the keyboard. Which thread should catch that? Should all threads share a common set of signal masks? All solutions to these and other problems usually cause something to break somewhere. Getting the semantics of threads right (not to mention the code) is a nontrivial business.

Threads in Linux

Linux supports kernel threads in an interesting way that is worth looking at. The implementation is based on ideas from 4.4BSD, but kernel threads were not enabled in that distribution because Berkeley ran out of money before the C library could be rewritten to solve the problems discussed above.

The heart of the Linux implementation of threads is a new system call, clone, that is not present in any other version of UNIX. It is called as follows:

pid = clone(function, stack_ptr, sharing_flags, arg); 

The call creates a new thread, either in the current process or in a new process, depending on sharing_flags. If the new thread is in the current process, it shares the address space with existing threads and every subsequent write to any byte in the address space by any thread is immediately visible to all the other threads in the process. On the other hand, if the address space is not shared, then the new thread gets an exact copy of the address space, but subsequent writes by the new thread are not visible to the old ones. These semantics are the same as fork.

In both cases, the new thread begins executing at function, which is called with arg as its only parameter. Also in both cases, the new thread gets its own private stack, with the stack pointer initialized to stack_ptr.

The sharing_flags parameter is a bitmap that allows a much finer grain of sharing than traditional UNIX systems. Five bits are defined, as listed in Fig. 10-7. Each bit controls some aspect of sharing, and each of the bits can be set independently of the other ones. The CLONE_VM bit determines whether the virtual memory (i.e., address space) is shared with the old threads or copied. If it is set, the new thread just moves in with the existing ones, so the clone call effectively creates a new thread in an existing process. If the bit is cleared, the new thread gets its own address space. Having its own address space means that the effect of its STORE instructions are not visible to the existing threads. This behavior is similar to fork, except as noted below. Creating a new address space is effectively the definition of a new process.

Figure 10-7. Bits in the sharing3flags bitmap.

The CLONE_FS bit controls sharing of the root and working directories and of the umask flag. Even if the new thread has its own address space, if this bit is set, the old and new threads share working directories. This means that a call to chdir by one thread changes the working directory of the other thread, even though the other thread may have its own address space. In UNIX, a call to chdir by a thread always changes the working directory for other threads in its process, but never for threads in another process. Thus this bit enables a kind of sharing not possible in UNIX.

The CLONE_FILES bit is analogous to the CLONE_FS bit. If set, the new thread shares its file descriptors with the old ones, so calls to lseek by one thread are visible to the other ones, again as normally holds for threads within the same process but not for threads in different processes. Similarly, CLONE_SIGHAND enables or disables the sharing of the signal handler table between the old and new threads. If the table is shared, even among threads in different address spaces, then changing a handler in one thread affects the handlers in the others. Finally, CLONE_PID controls whether the new thread gets its own PID or shares its parent's PID. This feature is needed during system booting. User processes are not permitted to enable it.

This fine-grained sharing is possible because Linux maintains separate data structures for the various items listed at the start of Sec. 10.3.3 (scheduling parameters, memory image, etc.). The process table and user structure just point to these data structures, so it is easy to make a new process table entry for each cloned thread and have it either point to the old thread's scheduling, memory, and other data structures or to copies of them. The fact that such fine-grained sharing is possible does not mean that it is useful however, especially since UNIX does not offer this functionality. A Linux program that takes advantage of it is then no longer portable to UNIX.

Let us now examine the UNIX scheduling algorithm. Because UNIX has always been a multiprogramming system, its scheduling algorithm was designed from the beginning to provide good response to interactive processes. It is a two-level algorithm. The low-level algorithm picks the process to run next from the set of processes in memory and ready to run. The high-level algorithm moves processes between memory and disk so that all processes get a chance to be in memory and run.

Each version of UNIX has a slightly different low-level scheduling algorithm, but most of them are close to the generic one we will describe now. The low-level algorithm uses multiple queues. Each queue is associated with a range of nonoverlapping priority values. Processes executing in user mode (the top of the iceberg) have positive values. Processes executing in kernel mode (doing system calls) have negative values. Negative values have the highest priority and large positive values have the lowest, as illustrated in Fig. 10-8. Only processes that are in memory and ready to run are located on the queues, since the choice must be made from this set.

Figure 10-8. The UNIX scheduler is based on a multilevel queue structure.

When the (low-level) scheduler runs, it searches the queues starting at the highest priority (i.e., most negative value) until it finds a queue that is occupied. The first process on that queue is then chosen and started. It is allowed to run for a maximum of one quantum, typically 100 msec, or until it blocks. If a process uses up its quantum, it is put back on the end of its queue, and the scheduling algorithm is run again. Thus processes within the same priority range share the CPU using a round-robin algorithm.

Once a second, each process' priority is recalculated according to a formula involving three components: priority = CPU_usage + nice + base

Based on its new priority, each process is attached to the appropriate queue of Fig. 10-8, usually by dividing the priority by a constant to get the queue number. Let us now briefly examine each of the three components of the priority formula.

CPU_usage, represents the average number of clock ticks per second that the process has had during the past few seconds. Every time the clock ticks, the CPU usage counter in the running process' process table entry is incremented by 1. This counter will ultimately be added to the process' priority giving it a higher numerical value and thus putting it on a lower-priority queue.

However, UNIX does not punish a process forever for using the CPU, so CPU_usage decays with time. Different versions of UNIX do the decay slightly differently. One way that has been used is to add the current value of CPU_usage to the number of ticks acquired in the past DT and divide the result by 2. This algorithm weights the most recent DT by ½, the one before that by ¼, and so on. This weighting algorithm is very fast because it just has one addition and one shift, but other weighting schemes have also been used.

Every process has a nice value associated with it. The default value is 0, but the allowed range is generally -20 to +20. A process can set nice to a value in the range 0 to 20 by the nice system call. A user computing p to a billion places in the background might put this call in his program to be nice to the other users. Only the system administrator may ask for better than normal service (meaning values from -20 to -1). Deducing the reason for this rule is left as an exercise for the reader.

When a process traps to the kernel to make a system call, it is entirely possible that the process has to block before completing the system call and returning to user mode. For example, it may have just done a waitpid system call and have to wait for one of its children to exit. It may also have to wait for terminal input or for disk I/O to complete, to mention only a few of the many possibilities. When it blocks, it is removed from the queue structure, since it is unable to run.

However, when the event it was waiting for occurs, it is put onto a queue with a negative value. The choice of queue is determined by the event it was waiting for. In Fig. 10-8, disk I/O is shown as having the highest priority, so a process that has just read or written a block from the disk will probably get the CPU within 100 msec. The relative priority of disk I/O, terminal I/O, etc. is hardwired into the operating system, and can only be modified by changing some constants in the source code and recompiling the system. These (negative) values are represented by base in the formula given above and are spaced far enough apart that processes being restarted for different reasons are clearly separated into different queues.

The idea behind this scheme is to get processes out of the kernel fast. If a process is trying to read a disk file, making it wait a second between read calls will slow it down enormously. It is far better to let it run immediately after each request is completed, so it can make the next one quickly. Similarly, if a process was blocked waiting for terminal input, it is clearly an interactive process, and as such should be given a high priority as soon as it is ready in order to ensure that interactive processes get good service. In this light, CPU bound processes (i.e., those on the positive queues) basically get any service that is left over when all the I/O bound and interactive processes are blocked.

Scheduling in Linux

Scheduling is one of the few areas in which Linux uses a different algorithm from UNIX. We have just examined the UNIX scheduling algorithm, so we will now look at the Linux algorithm. To start with, Linux threads are kernel threads, so scheduling is based on threads, not processes. Linux distinguishes three classes of threads for scheduling purposes:

  1. Real-time FIFO.
  2. Real-time round robin.
  3. Timesharing.

Real-time FIFO threads are the highest priority and are not preemptable except by a newly-readied real-time FIFO thread. Real-time round-robin threads are the same as real-time FIFO threads except that they are preemptable by the clock. If multiple real-time round-robin threads are ready, each one is run for its quantum, after which it goes to the end of the list of real-time round-robin threads. Neither of these classes is actually real time in any sense. Deadlines cannot be specified and guarantees are not given. These classes are simply higher priority than threads in the standard timesharing class. The reason Linux calls them real time is that Linux is conformant to the P1003.4 standard (''real-time'' extensions to UNIX) which uses those names.

Each thread has a scheduling priority. The default value is 20, but that can be altered using the nice(value) system call to a value of 20 - value. Since value must be in the range -20 to +19, priorities always fall in the range: 1 £ priority £ 40. The intention is that the quality of service is roughly proportional to the priority, with higher priority threads getting faster response time and a larger fraction of the CPU time than lower priority threads.

In addition to a priority, each thread has a quantum associated with it. The quantum is the number of clock ticks the thread may continue to run for. The clock runs at 100 Hz by default, so each tick is 10 msec, which is called a jiffy.

The scheduler uses the priority and quantum as follows. It first computes the goodness of each ready thread by applying the following rules:

if (class == real3time) goodness = 1000 + priority; 
if (class == timesharing && quantum > 0) goodness = quantum + priority; 
if (class == timesharing && quantum == 0) goodness = 0; 

Both real-time classes count for the first rule. All that marking a thread as real time does is make sure it gets a higher goodness than all timesharing threads. The algorithm has one little extra feature: if the process that ran last still has some quantum left, it gets a bonus point, so that it wins any ties. The idea here is that all things being equal, it is more efficient to run the previous process since its pages and cache blocks are likely to be loaded.

Given this background, the scheduling algorithm is very simple: when a scheduling decision is made, the thread with the highest goodness is selected. As the selected thread runs, at every clock tick, its quantum is decremented by 1. The CPU is taken away from a thread if any of these conditions occur:

  1. Its quantum hits 0.

  2. The thread blocks on I/O, a semaphore, or something else.

  3. A previously blocked thread with a higher goodness becomes ready.

Since the quanta keep counting down, sooner or later every ready thread will grind its quantum into the ground and they will all be 0. However, I/O bound threads that are currently blocked may have some quantum left. At this point the scheduler resets the quantum of all threads, ready and blocked, using the rule:

quantum = (quantum/2) + priority 

where the new quantum is in jiffies. A thread that is highly compute bound will usually exhaust its quantum quickly and have it 0 when quanta are reset, giving it a quantum equal to its priority. An I/O-bound thread may have considerable quantum left and thus get a larger quantum next time. If nice is not used, the priority will be 20, so the quantum becomes 20 jiffies or 200 msec. On the other hand, for a highly I/O bound thread, it may still have a quantum of 20 left when quanta are reset, so if its priority is 20, its new quantum becomes 20/2 + 20 = 30 jiffies. If another reset happens before it has spent 1 tick, next time it gets a quantum of 30/2 + 20 = 35 jiffies. The asymptotic value in jiffies is twice the priority. As a consequence of this algorithm, I/O-bound threads get larger quanta and thus higher goodness than compute-bound threads. This gives I/O-bound threads preference in scheduling.

Another property of this algorithm is that when compute-bound threads are competing for the CPU, ones with higher priority get a larger fraction of it. To see this, consider two compute-bound threads, A, with priority 20 and B, with priority 5. A goes first and 20 ticks later has used up its quantum. Then B gets to run for 5 quanta. At this point the quanta are reset. A gets 20 and B gets 5. This goes on forever, so A is getting 80% of the CPU and B is getting 20% of the CPU.

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