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SETSCHEDULER !!!SETSCHEDULER NAME SYNOPSIS DESCRIPTION RETURN VALUE ERRORS CONFORMING TO BUGS NOTE SEE ALSO ---- !!NAME sched_setscheduler, sched_getscheduler - set and get scheduling algorithm/parameters !!SYNOPSIS __#include __ __int sched_setscheduler(pid_t__ ''pid''__, int__ ''policy''__, const struct sched_param *__''p''__);__ __int sched_getscheduler(pid_t__ ''pid''__);__ struct sched_param { ... int ''sched_priority''__; ... }; __ !!DESCRIPTION __sched_setscheduler__ sets both the scheduling policy and the associated parameters for the process identified by ''pid''. If ''pid'' equals zero, the scheduler of the calling process will be set. The interpretation of the parameter ''p'' depends on the selected policy. Currently, the following three scheduling policies are supported under Linux: ''SCHED_FIFO'', ''SCHED_RR'', and ''SCHED_OTHER''; their respective semantics is described below. __sched_getscheduler__ queries the scheduling policy currently applied to the process identified by ''pid''. If ''pid'' equals zero, the policy of the calling process will be retrieved. __Scheduling Policies__ The scheduler is the kernel part that decides which runnable process will be executed by the CPU next. The Linux scheduler offers three different scheduling policies, one for normal processes and two for real-time applications. A static priority value ''sched_priority'' is assigned to each process and this value can be changed only via system calls. Conceptually, the scheduler maintains a list of runnable processes for each possible ''sched_priority'' value, and ''sched_priority'' can have a value in the range 0 to 99. In order to determine the process that runs next, the Linux scheduler looks for the non-empty list with the highest static priority and takes the process at the head of this list. The scheduling policy determines for each process, where it will be inserted into the list of processes with equal static priority and how it will move inside this list. ''SCHED_OTHER'' is the default universal time-sharing scheduler policy used by most processes, ''SCHED_FIFO'' and ''SCHED_RR'' are intended for special time-critical applications that need precise control over the way in which runnable processes are selected for execution. Processes scheduled with ''SCHED_OTHER'' must be assigned the static priority 0, processes scheduled under ''SCHED_FIFO'' or ''SCHED_RR'' can have a static priority in the range 1 to 99. Only processes with superuser privileges can get a static priority higher than 0 and can therefore be scheduled under ''SCHED_FIFO'' or ''SCHED_RR''. The system calls __sched_get_priority_min__ and __sched_get_priority_max__ can be used to to find out the valid priority range for a scheduling policy in a portable way on all POSIX.1b conforming systems. All scheduling is preemptive: If a process with a higher static priority gets ready to run, the current process will be preempted and returned into its wait list. The scheduling policy only determines the ordering within the list of runnable processes with equal static priority. __SCHED_FIFO: First In-First out scheduling__ ''SCHED_FIFO'' can only be used with static priorities higher than 0, that means that when a ''SCHED_FIFO'' processes becomes runnable, it will always preempt immediately any currently running normal ''SCHED_OTHER'' process. ''SCHED_FIFO'' is a simple scheduling algorithm without time slicing. For processes scheduled under the ''SCHED_FIFO'' policy, the following rules are applied: A ''SCHED_FIFO'' process that has been preempted by another process of higher priority will stay at the head of the list for its priority and will resume execution as soon as all processes of higher priority are blocked again. When a ''SCHED_FIFO'' process becomes runnable, it will be inserted at the end of the list for its priority. A call to __sched_setscheduler__ or __sched_setparam__ will put the ''SCHED_FIFO'' process identified by ''pid'' at the end of the list if it was runnable. A process calling __sched_yield__ will be put at the end of the list. No other events will move a process scheduled under the ''SCHED_FIFO'' policy in the wait list of runnable processes with equal static priority. A ''SCHED_FIFO'' process runs until either it is blocked by an I/O request, it is preempted by a higher priority process, or it calls __sched_yield__. __SCHED_RR: Round Robin scheduling__ ''SCHED_RR'' is a simple enhancement of ''SCHED_FIFO''. Everything described above for ''SCHED_FIFO'' also applies to ''SCHED_RR'', except that each process is only allowed to run for a maximum time quantum. If a ''SCHED_RR'' process has been running for a time period equal to or longer than the time quantum, it will be put at the end of the list for its priority. A ''SCHED_RR'' process that has been preempted by a higher priority process and subsequently resumes execution as a running process will complete the unexpired portion of its round robin time quantum. The length of the time quantum can be retrieved by __sched_rr_get_interval__. __SCHED_OTHER: Default Linux time-sharing scheduling__ ''SCHED_OTHER'' can only be used at static priority 0. ''SCHED_OTHER'' is the standard Linux time-sharing scheduler that is intended for all processes that do not require special static priority real-time mechanisms. The process to run is chosen from the static priority 0 list based on a dynamic priority that is determined only inside this list. The dynamic priority is based on the nice level (set by the __nice__ or __setpriority__ system call) and increased for each time quantum the process is ready to run, but denied to run by the scheduler. This ensures fair progress among all ''SCHED_OTHER'' processes. __Response time__ A blocked high priority process waiting for the I/O has a certain response time before it is scheduled again. The device driver writer can greatly reduce this response time by using a request_irq__(9). __Miscellaneous__ Child processes inherit the scheduling algorithm and parameters across a __fork__. Memory locking is usually needed for real-time processes to avoid paging delays, this can be done with __mlock__ or __mlockall__. As a non-blocking end-less loop in a process scheduled under ''SCHED_FIFO'' or ''SCHED_RR'' will block all processes with lower priority forever, a software developer should always keep available on the console a shell scheduled under a higher static priority than the tested application. This will allow an emergency kill of tested real-time applications that do not block or terminate as expected. As ''SCHED_FIFO'' and ''SCHED_RR'' processes can preempt other processes forever, only root processes are allowed to activate these policies under Linux. POSIX systems on which __sched_setscheduler__ and __sched_getscheduler__ are available define ''_POSIX_PRIORITY_SCHEDULING'' in '' !!RETURN VALUE On success, __sched_setscheduler__ returns zero. On success, __sched_getscheduler__ returns the policy for the process (a non-negative integer). On error, -1 is returned, ''errno'' is set appropriately. !!ERRORS __ESRCH__ The process whose ID is ''pid'' could not be found. __EPERM__ The calling process does not have appropriate privileges. Only root processes are allowed to activate the ''SCHED_FIFO'' and ''SCHED_RR'' policies. The process calling __sched_setscheduler__ needs an effective uid equal to the euid or uid of the process identified by ''pid'', or it must be a superuser process. __EINVAL__ The scheduling ''policy'' is not one of the recognized policies, or the parameter ''p'' does not make sense for the ''policy''. !!CONFORMING TO POSIX.1b (formerly POSIX.4) !!BUGS As of linux-1.3.81, ''SCHED_RR'' has not yet been tested carefully and might not behave exactly as described or required by POSIX.1b. !!NOTE Standard Linux is a general-purpose operating system and can handle background processes, interactive applications, and soft real-time applications (applications that need to usually meet timing deadlines). This man page is directed at these kinds of applications. Standard Linux is ''not'' designed to support hard real-time applications, that is, applications in which deadlines (often much shorter than a second) must be guaranteed or the system will fail catastrophically. Like all general-purpose operating systems, Linux is designed to maximize average case performance instead of worst case performance. Linux's worst case performance for interrupt handling is much poorer than its average case, its various kernel locks (such as for SMP) produce long maximum wait times, and many of its performance improvement techniques decrease average time by increasing worst-case time. For most situations, that's what you want, but if you truly are developing a hard real-time application, consider using hard real-time extensions to Linux such as RTLinux (http://www.rtlinux.org) or use a different operating system designed specifically for hard real-time applications. !!SEE ALSO sched_setparam(2), __sched_getparam__(2), sched_yield(2), __sched_get_priority_max__(2), sched_get_priority_min(2), nice(2), setpriority(2), getpriority(2), mlockall(2), munlockall(2), mlock(2), munlock(2) ''Programming for the real world - POSIX.4'' by Bill O. Gallmeister, O'Reilly '' IEEE Std 1003.1b-1993'' (POSIX.1b standard)'' ISO/IEC 9945-1:1996'' - This is the new 1996 revision of POSIX.1 which contains in one single standard POSIX.1(1990), POSIX.1b(1993), POSIX.1c(1995), and POSIX.1i(1995). ----
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sched_getscheduler(2)
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syscalls(2)
nice(2)
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