Scheduling Policies

These policies either require superuser privilege (ie. run as root) or realtime capabilities for unprivileged users in the form of a PAM module. They include SCHED_FIFO and SCHED_RR.

REALTIME POLICIES

SCHED_FIFO

These processes schedule according to their realtime priority which is unrelated to the nice value. The highest priority process runs indefinitely, never releasing the cpu except to an even higher priority realtime task or voluntarily. Only proper realtime code should ever use this policy as the potential for hardlocking a machine is high if the process runs away. Audio applications for professional performance such as jack use this policy.

SCHED_RR

These run similar to SCHED_FIFO except that if more than one process has the same realtime priority, they will run for short periods each and share the cpu.

NON REALTIME POLICIES

These policies do not require special privileges to use and include SCHED_NORMAL and SCHED_BATCH in mainline and -ck. -ck also includes two extra unprivileged policies, SCHED_ISO and SCHED_IDLEPRIO.

SCHED_NORMAL

This is how most normal applications are run. The amount of cpu each process consumes and the latency it will get is mostly determined by the 'nice' value. They run for short periods and share cpu amongst all other processes running with the same policy, across all nice values. Known as SCHED_OTHER in most of the rest of the world, including glibc headers as per POSIX.1.

SCHED_BATCH

Similar to SCHED_NORMAL in every way except that specifying a task as batch means you are telling the kernel that this process should not ever be considered an interactive task. This means that you want these tasks to get the same share of cpu time as the same nice level SCHED_NORMAL tasks would have, but you do not care what latency they have.

SCHED_ISO

Unique to -ck this is a scheduling policy designed for pseudo-realttime scheduling without requiring superuser privileges (unlike SCHED_RR and SCHED_FIFO). When scheduled SCHED_ISO, a task can receive very low latency scheduling, and can take the full cpu like SCHED_RR, but unlike the realtime tasks they cannot starve the machine as an upper limit to their cpu usage is specified in a tunable (see below). It is designed for realtime like behaviour without risk to hanging for programs not really coded safely enough to be run realtime such as ordinary audio and video playback software. SCHED_ISO does not take a realtime priority, but nice levels like other normal tasks, although the nice value is largely ignored except when the task uses more than its cpu limit.

SCHED_IDLEPRIO

Also unique to -ck this is a scheduling policy designed for tasks to purely use idle time. This means that if anything else is running, these tasks will not even progress at all. This policy is useful for performing prolonged computing tasks such as distributed computing clients (setiathome, foldingathome, etc) and prevents these tasks from stealing away any cpu time from other applications. Also it can be useful for conducting large compilations of software without affecting other tasks. These tasks are also flagged in -ck for less aggressive memory usage and disk read bandwidth, but these affects are not potent, and if the task uses a lot of memory and disk it will be noticeable. SCHED_IDLEPRIO takes a nice value. However this nice value only determines the cpu distribution between all idleprio tasks, allowing many idleprio tasks to be running with different nice values. This might be desirable, for example, when using a distributed computing client at nice 19 and compiling software at nice 0 when both are SCHED_IDLEPRIO.