sched_setscheduler, sched_getscheduler - 設置和獲取調度算法/參數

內容簡介

#include <sched.h>

int sched_setscheduler(pid_t pid**, int** policy**,** const struct sched_param **param*);

int sched_getscheduler(pid_t pid**);**

struct sched_param { ... int sched_priority**; ... };**

描述

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  param  depends on the selected policy. Currently, the following three scheduling policies are supported under Linux: SCHED_FIFO ,  SCHED_RR ,  SCHED_OTHER , and  SCHED_BATCH ; their respective semantics are 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.

調度策略

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_BATCH is intended for "batch" style execution of 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 or SCHED_BATCH 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. The system calls sched_get_priority_min() andsched_get_priority_max() can be used to find out the valid priority range for a scheduling policy in a portable way on all POSIX.1-2001 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：先入先出調度

SCHED_FIFO  can only be used with static priorities higher than 0, which means that when a  SCHED_FIFO  processes becomes runnable, it will always immediately preempt any currently running  SCHED_OTHER  or  SCHED_BATCH  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  (or  SCHED_RR ) process identified by  pid  at the start of the list if it was runnable. As a consequence, it may preempt the currently running process if it has the same priority. (POSIX.1-2001 specifies that the process should go to the end of the list.) 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：輪循調度

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 using  sched_rr_get_interval (2).

SCHED_OTHER：默認的Linux分時調度

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  nice (2) or  setpriority (2)) 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.

SCHED_BATCH：調度批處理

(Since Linux 2.6.16.)  SCHED_BATCH  can only be used at static priority 0. This policy is similar to  SCHED_OTHER , except that this policy will cause the scheduler to always assume that the process is CPU-intensive. Consequently, the scheduler will apply a small scheduling penalty so that this process is mildly disfavoured in scheduling decisions. This policy is useful for workloads that are non-interactive, but do not want to lower their nice value, and for workloads that want a deterministic scheduling policy without interactivity causing extra preemptions (between the workload’s tasks).

權限和資源限制

In Linux kernels before 2.6.12, only privileged ( CAP_SYS_NICE ) processes can set a non-zero static priority. The only change that an unprivileged process can make is to set the  SCHED_OTHER  policy, and this can only be done if the effective user ID of the caller of  sched_setscheduler () matches the real or effective user ID of the target process (i.e., the process specified by  pid ) whose policy is being changed.

Since Linux 2.6.12, the RLIMIT_RTPRIO resource limit defines a ceiling on an unprivileged process’s priority for the SCHED_RR and SCHED_FIFO policies. If an unprivileged process has a non-zero RLIMIT_RTPRIO soft limit, then it can change its scheduling policy and priority, subject to the restriction that the priority cannot be set to a value higher than the RLIMIT_RTPRIO soft limit. If the RLIMIT_RTPRIO soft limit is 0, then the only permitted change is to lower the priority. Subject to the same rules, another unprivileged process can also make these changes, as long as the effective user ID of the process making the change matches the real or effective user ID of the target process. See getrlimit(2) for further information on RLIMIT_RTPRIO. Privileged (CAP_SYS_NICE) processes ignore this limit; as with older older kernels, they can make arbitrary changes to scheduling policy and priority.

響應時間

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 "slow interrupt" interrupt handler.

雜項

Child processes inherit the scheduling algorithm and parameters across a  fork (). The scheduling algorithm and parameters are preserved across  execve (2).

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_RRwill 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.

POSIX systems on which sched_setscheduler() and sched_getscheduler() are available define _POSIX_PRIORITY_SCHEDULING in <unistd.h>.

返回值

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, and errno  is set appropriately.

錯誤

標籤

描述

EINVAL

The scheduling policy is not one of the recognized policies, or the parameter param does not make sense for the policy.

EPERM

The calling process does not have appropriate privileges.

ESRCH

The process whose ID is pid could not be found.

遵循於

POSIX.1-2001. The  SCHED_BATCH  policy is Linux specific.

注意

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 RTAI (http://www.rtai.org) or use a different operating system designed specifically for hard real-time applications.

另請參閱

• getpriority (2)

• mlock (2)

• mlockall (2)

• munlock (2)

• munlockall (2)

• nice (2)

• sched_get_priority_max (2)

• sched_get_priority_min (2)

• sched_getaffinity (2)

• sched_getparam (2)

• sched_rr_get_interval (2)

• sched_setaffinity (2)

• sched_setparam (2)

• sched_yield (2)

• setpriority (2)

Programming for the real world - POSIX.4 by Bill O. Gallmeister, O’Reilly & Associates, Inc., ISBN 1-56592-074-0