Scheduling Policies¶

The scheduler is the kernel component that decides which runnable process will be executed by the CPU next. Each process has an associated scheduling policy and a static scheduling priority, sched_priority; these are the settings that are modified by sched_setscheduler(). The scheduler makes it decisions based on knowledge of the scheduling policy and static priority of all processes on the system.

For processes scheduled under one of the normal scheduling policies (SCHED_OTHER, SCHED_IDLE, SCHED_BATCH), sched_priority is not used in scheduling decisions (it must be specified as 0).

Processes scheduled under one of the real-time policies (SCHED_FIFO, SCHED_RR) have a sched_priority value in the range 1 (low) to 99 (high). (As the numbers imply, real-time processes always have higher priority than normal processes.) Note well: POSIX.1-2001 only requires an implementation to support a minimum 32 distinct priority levels for the real-time policies, and some systems supply just this minimum. Portable programs should use sched_get_priority_min(2) and sched_get_priority_max(2) to find the range of priorities supported for a particular policy.

Conceptually, the scheduler maintains a list of runnable processes for each possible sched_priority value. In order to determine which process runs next, the scheduler looks for the non-empty list with the highest static priority and selects the process at the head of this list.

A process's scheduling policy determines where it will be inserted into the list of processes with equal static priority and how it will move inside this list.

All scheduling is preemptive: if a process with a higher static priority becomes ready to run, the currently running process will be preempted and returned to the wait list for its static priority level. 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, which means that when a SCHED_FIFO processes becomes runnable, it will always immediately preempt any currently running SCHED_OTHER, SCHED_BATCH, or SCHED_IDLE process. SCHED_FIFO is a simple scheduling algorithm without time slicing. For processes scheduled under the SCHED_FIFO policy, the following rules apply:

• 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(2) 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(2) 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(2).

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 using sched_rr_get_interval(2).

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 the special 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 value (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: Scheduling batch processes¶

(Since Linux 2.6.16.) SCHED_BATCH can only be used at static priority 0. This policy is similar to SCHED_OTHER in that it schedules the process according to its dynamic priority (based on the nice value). The difference is 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 with respect to wakeup behaviour, so that this process is mildly disfavored 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).

SCHED_IDLE: Scheduling very low priority jobs¶

(Since Linux 2.6.23.) SCHED_IDLE can only be used at static priority 0; the process nice value has no influence for this policy.

This policy is intended for running jobs at extremely low priority (lower even than a +19 nice value with the SCHED_OTHER or SCHED_BATCH policies).

Privileges and resource limits¶

In Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE) processes can set a non-zero static priority (i.e., set a real-time scheduling policy). 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 static priority for the SCHED_RR and SCHED_FIFO policies. The rules for changing scheduling policy and priority are as follows:

• 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 maximum of its current priority and its RLIMIT_RTPRIO soft limit.
• If the RLIMIT_RTPRIO soft limit is 0, then the only permitted changes are to lower the priority, or to switch to a non-real-time policy.
• 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.
• Special rules apply for the SCHED_IDLE: an unprivileged process operating under this policy cannot change its policy, regardless of the value of its RLIMIT_RTPRIO resource limit.

Privileged (CAP_SYS_NICE) processes ignore the RLIMIT_RTPRIO limit; as with older kernels, they can make arbitrary changes to scheduling policy and priority. See getrlimit(2) for further information on RLIMIT_RTPRIO.

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 "slow interrupt" interrupt handler.

Miscellaneous¶

Child processes inherit the scheduling policy and parameters across a fork(2). The scheduling policy 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(2) or mlockall(2).

Since a non-blocking infinite 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. See also the description of the RLIMIT_RTTIME resource limit in getrlimit(2).

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

Real-time features in the mainline Linux kernel¶

From kernel version 2.6.18 onwards, however, Linux is gradually becoming equipped with real-time capabilities, most of which are derived from the former realtime-preempt patches developed by Ingo Molnar, Thomas Gleixner, Steven Rostedt, and others. Until the patches have been completely merged into the mainline kernel (this is expected to be around kernel version 2.6.30), they must be installed to achieve the best real-time performance. These patches are named:

patch-kernelversion-rtpatchversion

and can be downloaded from http://www.kernel.org/pub/linux/kernel/projects/rt/.

Without the patches and prior to their full inclusion into the mainline kernel, the kernel configuration offers only the three preemption classes CONFIG_PREEMPT_NONE, CONFIG_PREEMPT_VOLUNTARY, and CONFIG_PREEMPT_DESKTOP which respectively provide no, some, and considerable reduction of the worst-case scheduling latency.

With the patches applied or after their full inclusion into the mainline kernel, the additional configuration item CONFIG_PREEMPT_RT becomes available. If this is selected, Linux is transformed into a regular real-time operating system. The FIFO and RR scheduling policies that can be selected using sched_setscheduler() are then used to run a process with true real-time priority and a minimum worst-case scheduling latency.