Linux – Which process scheduling algorithm is used by Linux

linuxprocessscheduling

The Linux operating system switches between running processes to enable multi-tasking.

What algorithm is used to determine when to suspend the execution of the currently running process and which process should run next?

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In newer kernels, the Completely Fair Scheduler is used (it replaces the O(1) scheduler of older kernels).

The CFS stores the planned task in a red-black tree and uses the spent CPU time amount for the process has run as a key. This allows the scheduler to pick the process with the least amount of run-time (which is stored in the left-mode node of the tree) efficiently.

Once the task is about to be run, it is removed from the tree, and then when it runs, it is re-added with the new CPU time used.

Processes that are "sleeping" for a long time will automatically get a priority boost, since they don't have a large spent CPU time.

Therefore, it is "fair", since processes that are sleeping get as much CPU time as processes that are constantly running

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Linux – Which Scheduling algorithm is used in Linux

The current Linux task scheduler is called Completely Fair Scheduler (CFS). You should have a look at http://people.redhat.com/mingo/cfs-scheduler/sched-design-CFS.txt for more details. The design is quite complex and in my view not suitable for RTOS.

A common technique in realtime systems is rate-monotonic scheduling, because it has strong guarantees if certain assumptions hold (e.g. static task priorities and fixed execution time and rate). There are a whole lot other algorithms and there has been a lot of research. So it's basically all about the properties you need and what you know about your task and what is fixed.

Kernel Process Scheduling – Real Time Processes Scheduling in Linux

Question 1

It is possible for an user to use real time priority for a process as well. This configuration could be set from /etc/security/limits.conf file. I see the below contents in that file.

# /etc/security/limits.conf
#
#Each line describes a limit for a user in the form:
#
#<domain>        <type>  <item>  <value>

If we check the item section, we see the below entry which enables to set a real time priority for the users.

#        - rtprio - max realtime priority

Question 2 and Question 3

To set scheduling policy to SCHED_FIFO, enter:

chrt -f -p [1..99] {pid}

To set scheduling policy to SCHED_RR, enter:

chrt -r -p [1..99] {pid}

So to answer question 3, we should verify the scheduling algorithms available and the priorities using the chrt -m command and then use any scheduling algorithm that suits our need. To set different priorities, we could use the commands as above.

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Related Solutions

Linux – Which Scheduling algorithm is used in Linux

Kernel Process Scheduling – Real Time Processes Scheduling in Linux

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