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 nonempty 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(2) and sched_get_priority_max(2) 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 calling 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, 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(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 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: 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, 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 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).

Privileges and resource limits

In Linux kernels before 2.6.12, only privileged (CAP_SYS_NICE) processes can set a nonzero 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 nonzero 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 kernels, they can make arbitrary changes to scheduling policy and priority.

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 algorithm and parameters across a fork(2). 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(2) or mlockall(2).

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.

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 and others. Until the patches have been completely merged into the mainline kernel (this is expected to be around kernel version 2.6.24 or 2.6.25), the realtime-preempt patches must be installed to achieve the best realtime performance. These patches are named:

and can be downloaded from http://people.redhat.com/mingo/realtime-preempt/.

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.