This lecture looks at process management from the O/S point of view. It covers process state, process creation and process scheduling.

There is also an introduction to inter-process communication (IPC).

Reference: Tanenbaum Chapter 2

Process states

When a process is using the CPU, it is actually running and doing some work. However, it will not always stay in that state.

If a process does I/O and the device is not ready or just slow, the process will become blocked and be unable to do any more work until the I/O is complete.

In a multitasking environment, a process is not allowed to use the CPU all the time. At intervals (maybe signalled by a timer interrupt) it will be stopped by the O/S so that another process can run. In this state it is not running, but is runnable.

The process states and transitions are

Process table

The O/S maintains information about each process in a process table. Entries in this table are often called process control blocks and must contain information about for each process.

Process state

The process state must contain all the information needed so that the process can be loaded into memory and run. This includes Information about the state that the process is in (runnning, runnable, blocked) and why will need to be stored.

In addition it may contain extra fields such as:

Memory state

Pointers to the various memory areas used by the program need to be kept, so that they can be relocated back as needed.

Resource state

The process will generally have files open, be in particular directory, have a certain user ID, etc. The information about these will need to be stored also.

For example, in Unix each process has a file table. The first entry in this (file descriptor zero) is for the processes' standard input, the second entry is for standard output, the third for standard error. Additional entries are made when the process opens more files.

Linux process state

In Linux, the information stored per process is defined by the structure struct task_struct { /* these are hardcoded - don't touch */ volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */ long counter; long priority; unsigned long signal; unsigned long blocked; /* bitmap of masked signals */ unsigned long flags; /* per process flags, defined below */ int errno; int debugreg[8]; /* Hardware debugging registers */ struct exec_domain *exec_domain; /* various fields */ struct linux_binfmt *binfmt; struct task_struct *next_task, *prev_task; struct sigaction sigaction[32]; unsigned long saved_kernel_stack; unsigned long kernel_stack_page; int exit_code, exit_signal; unsigned long personality; int dumpable:1; int did_exec:1; int pid,pgrp,session,leader; int groups[NGROUPS]; /* * pointers to (original) parent process, youngest child, younger siblin g, * older sibling, respectively. (p->father can be replaced with * p->p_pptr->pid) */ struct task_struct *p_opptr, *p_pptr, *p_cptr, *p_ysptr, *p_osptr; struct wait_queue *wait_chldexit; /* for wait4() */ unsigned short uid,euid,suid,fsuid; unsigned short gid,egid,sgid,fsgid; unsigned long timeout; unsigned long it_real_value, it_prof_value, it_virt_value; unsigned long it_real_incr, it_prof_incr, it_virt_incr; long utime, stime, cutime, cstime, start_time; struct rlimit rlim[RLIM_NLIMITS]; unsigned short used_math; char comm[16]; /* virtual 86 mode stuff */ struct vm86_struct * vm86_info; unsigned long screen_bitmap; unsigned long v86flags, v86mask, v86mode; /* file system info */ int link_count; struct tty_struct *tty; /* NULL if no tty */ /* shm stuff */ struct shm_desc *shm; struct sem_undo *semun; /* ldt for this task - used by Wine. If NULL, default_ldt is used */ struct desc_struct *ldt; /* tss for this task */ struct tss_struct tss; /* filesystem information */ struct fs_struct fs[1]; /* open file information */ struct files_struct files[1]; /* memory management info */ struct mm_struct mm[1]; };

Process creation

When a process starts, the O/S has to build an entry for it in the process table. The process state will be marked as ``runnable''.

Here is the Linux fork() routine:

/* * Ok, this is the main fork-routine. It copies the system process * information (task[nr]) and sets up the necessary registers. It * also copies the data segment in its entirety. */ int do_fork(unsigned long clone_flags, unsigned long usp, struct pt_regs *regs) { int nr; unsigned long new_stack; struct task_struct *p; if(!(p = (struct task_struct*)__get_free_page(GFP_KERNEL))) goto bad_fork; new_stack = get_free_page(GFP_KERNEL); if (!new_stack) goto bad_fork_free; nr = find_empty_process(); if (nr < 0) goto bad_fork_free; *p = *current; if (p->exec_domain && p->exec_domain->use_count) (*p->exec_domain->use_count)++; if (p->binfmt && p->binfmt->use_count) (*p->binfmt->use_count)++; p->did_exec = 0; p->kernel_stack_page = new_stack; *(unsigned long *) p->kernel_stack_page = STACK_MAGIC; p->state = TASK_UNINTERRUPTIBLE; p->flags &= ~(PF_PTRACED|PF_TRACESYS); p->pid = last_pid; p->p_pptr = p->p_opptr = current; p->p_cptr = NULL; p->signal = 0; p->it_real_value = p->it_virt_value = p->it_prof_value = 0; p->it_real_incr = p->it_virt_incr = p->it_prof_incr = 0; p->leader = 0; /* process leadership doesn't inherit */ p->tty_old_pgrp = 0; p->utime = p->stime = 0; p->cutime = p->cstime = 0; p->start_time = jiffies; p->mm->swappable = 0; /* don't try to swap it out before it's set up * / task[nr] = p; SET_LINKS(p); nr_tasks++; /* copy all the process information */ copy_thread(nr, clone_flags, usp, p, regs); if (copy_mm(clone_flags, p)) goto bad_fork_cleanup; p->semundo = NULL; copy_files(clone_flags, p); copy_fs(clone_flags, p); /* ok, now we should be set up.. */ p->mm->swappable = 1; p->exit_signal = clone_flags & CSIGNAL; p->counter = current->counter >> 1; p->state = TASK_RUNNING; /* do this last, just in case */ return p->pid; bad_fork_cleanup: task[nr] = NULL; REMOVE_LINKS(p); nr_tasks--; bad_fork_free: free_page(new_stack); free_page((long) p); bad_fork: return -EAGAIN; }

Process scheduling

The process scheduler is responsible for changing the state of processes.

A scheduler must compromise between certain desirable things:

Stopping processes

There are two ways in which processes can stop: voluntarily stop, or be forced to stop. If there is a mechanism to stop processes, then the scheduling is preemptive, otherwise it is non-preemptive or co-operative.

OS2 and Windows use non-preemptive schedulers. Windows NT and Unix use preemptive schedulers.

To keep the CPU busy as much as possible, whenever a process blocks, it should be replaced.

To only allow a process to run for a maximum amount of time, timer interrupts can be set to stop the process.

When a process stops or is stopped, the registers have to be copied into the process control block. Any values in the PCB that have changed will need to be set. (Because of the register copy, the O/S code to do that would have to be in Assembler.)

Round robin scheduling

This is one of the simplest scheduling algorithms. The O/S maintains a queue of runnable processes. When a running process is stopped, or a blocked process becomes runnable, it is placed on the end of the queue.

When the CPU becomes free, the process at the head of the queue is loaded and made the running process.

Each process is allowed to run for a fixed maximum time before it is stopped. This fixed time must be carefully tuned. If it is too long, then response time for processes currently not running is too slow. If the time is too short, then the system will spend proportionally more time swapping processes and will just appear to be running slowly.

Priority scheduling

In round robin, all processes have the same priority. In priority scheduling they may have different ones. The O/S will maintain lists of processes, one for each priority. A runnable process from the highest priority list will be chosen.

Priorities may be static (set at creation time) or dynamic. In Unix, the longer a process runs, the lower its priority becomes.

Within a queue, a process may be chosen say by round robin.

Shortest job first

This is often used on batch systems. In these it is quite common to know in advance about how long a job will take. Then the list of jobs can be ordered on time required. The one needing the shortest time can be scheduled first. This gives the best average time.

This cannot easily be used in interactive systems because you don't know how long a command will take.

Two level scheduling

The above schemes assume either that all processes are in memory, or that only one process (the currently running one) is in memory. Then the cost of context switching is about equal.

The cost of context switching to a process that is in memory is that the registers have to be loaded, and a jump made to the program counter.

The extra cost of context switching to a process that is not in memory is that it first has to be loaded from disk. This will be significantly slower.

A two-level scheduler will maintain two lists, one for processes in main memory, the other for those on disk. One scheduler will work on those in memory, switching between them.

Periodically another scheduler will move processes between the lists so that those on disk can have a chance to run.

Linux Scheduling

Here is part of the Linux scheduling function (the wakeup stuff belonging to alalrm calls is omitted) asmlinkage void schedule(void) { int c; struct task_struct * p; struct task_struct * next; unsigned long ticks; /* check alarm, wake up any interruptible tasks that have got a signal */ /* code omitted here - Jan */ /* this is the scheduler proper: */ c = -1000; next = p = &init_task; for (;;) { if ((p = p->next_task) == &init_task) goto confuse_gcc2; if (p->state == TASK_RUNNING) { nr_running++; if (p->counter > c) c = p->counter, next = p; } } confuse_gcc2: if (!c) { for_each_task(p) p->counter = (p->counter >> 1) + p->priority; } if (current == next) return; kstat.context_swtch++; switch_to(next); } The full scheduling code is available as sched.c and sched.h where switch_to is implemented as a macro for speed: /* * switch_to(n) should switch tasks to task nr n, first * checking that n isn't the current task, in which case it does nothing. * This also clears the TS-flag if the task we switched to has used * tha math co-processor latest. */ #define switch_to(tsk) \ __asm__("cli\n\t" \ "xchgl %%ecx,_current\n\t" \ "ljmp %0\n\t" \ "sti\n\t" \ "cmpl %%ecx,_last_task_used_math\n\t" \ "jne 1f\n\t" \ "clts\n" \ "1:" \ : /* no output */ \ :"m" (*(((char *)&tsk->, \ "c" (tsk) \ :"cx")

Windows 95 Scheduling

When Windows95 runs an MSDOS application it does so using a ``virtual 8086''. This is a process that has its own 1M address space. The process does not have access to any other resources. So an MSDOS application cannot crash the system any more.

Since MSDOS is not multitasking, the only way it can create new processes is as sequential ones. Scheduling is not an issue here, since each process has to run to completion before a parent can resume. Such multiple processes created within MSDOS are not visible to Windows95.

All Windows 3.1 applications run within a shared address space. It is not possible to change the scheduling algorithm without breaking many Windows 3.1 applications, so the cooperative scheduling mechanism must remain for these.

It is possible for one Windows 3.1 application to lock out others, just as it happens under Windows 3.1. There is no change in this. However, whena Windows 3.1 application crashes, it can only bring down the other Windows 3.1 applications that are running.

Windows 32-bit applications each run within their own protected memory space. No Windows 32-bit application can crash another by corrupting its memory because it does not have access (unless deliberately shared).

Each 32-bit application may use threads internally, so that a 32-bit application may be one process with multiple threads.

An MSDOS application is one process and one thread.

Each Windows 3.1 application is one process and one thread.

The Windows 95 scheduler manages threads rather than processes. The scheduler is pre-emptive so that one process cannot lock out others (see caveats later). The scheduler uses priority scheduling where a thread can have a priority between 0 and 31. If multiple processes have the same priority, round robin is used.

The time slice defaults to 20ms, but the scheduler has the ability to change this. It can also temporarily bump up priorities if needed (e.g. in response to a mouse click). I don't know the full details of the algorithm.

The caveat is as follows: Windows 3.1 16-bit system code is not ``re-entrant''. That is, it is not possible to interrupt a Windows 3.1 system call and restart it, later continuing with the interrupted call. This is why Windows 3.1 scheduling must be cooperative rather than preemptive because the system calls cannot be interrupted.

To ensure that two processes do not attempt to run Windows 3.1 system calls simultaneously, Windows 95 uses a ``semaphore'' (see next lecture) to control access.

Some Windows 95 32-bit code calls the old Windows 3.1 16-bit code. Not much, but some. So if a Windows 3.1 application won't terminate and release the semaphore, then the 32-bit code can't call the 16-bit code. So a 32-bit application will block at that point.

If all 32-bit applications hit 16-bit code, then they will all block and the system will. Killing the offending 16-bit application will allow the rest to proceed.

Unix process startup

When a system boots Unix, it creates a single process called ``init''. This init acts as the root of the process tree.

init forks a set of copies of itself, one for each terminal line that is connected to it. Each one of these type the login message and then blocks waiting for terminal input.

When the user name and password are typed, init checks that the user is valid, and if so changes to the user's home directory, resets the user ID of the process to the user, and ``exec''s a shell.

At this point the init for that terminal line has been replaced by the login shell for that user. The user does whatever they want to do, and eventually logs out.

Logging out terminates the login shell. In the meantime, the init at the top of the process tree has done a wait, waiting on any of its children. The login shell is in fact a direct child of this toplevel init because it came from exec-ing a child. So when the shell terminates, the toplevel init wakes up.

The toplevel init then forks a new init for that terminal line and starts a wait again, for another child to terminate.

Intro to InterProcess Communication

Processes do not run in isolation from each other. Generally they need to communicate with each other.
Example: Any two processes in a pipeline are communicating. One sends a stream of bytes to the other.
Example: Access to the lineprinter is controlled by a single process called ``lpd'' (the lineprinter daemon). Each time a user runs ``lpr'' this has to communicate with ``lpd'' and send it the file to print.
Example: Your home directories are stored on the machine ``willow''. Each time you access a file the O/S has to make a connection to willow, request the file from a suitable process on willow and accept responses from it.

There are many models of IPC, with different advantages and problems.

Shared memory

If two processes share the same piece of memory then they can use this to communicate. For example, one may write information in this shared area and the other may read it.

This can be a very fast method of information transfer because RAM can be used. Synchronisation is a major problem - if the first process keeps writing data, how can it ensure that the second one reads it?


A pipe acts like a channel betwen two processes. When one process writes into the pipe the other process can read from it. A pipe can usually buffer information so that the writer can place a lot of information in the pipe before the child has to read it. When the pipe becomes full the writer has to suspend.

Pipes can be un-named. This is the norm in Unix where a process creates a pipe and then forks, so that the two processes share the pipe between them.

If the processes do not come from a common ancestor then they can only share a pipe if they can both name it (otherwise they could not find it). Named pipes usually appear as though they were files in the file system.


Pipes carry unstructured data - you put bytes in one end and get the same bytes out the other. Streams are designed to carry record information - you put records in at one end and get the same records out the other. Each record must contain a field saying how large it is.


Sockets are more like ports that you can send data to. A process will be ``listening'' at a port and will accept data sent to it.


Signals are a fairly crude method if IPC. A process may send a signal to another such as ``wake up'' or ``die''. The other process can respond to these signals in various ways.
This page is, copyright Jan Newmarch.
It is maintained by Jan Newmarch.
Last modified: 29 August, 1995