CODEWITHSUNDEEP
linuxlinux-kernelscheduling
keye
keye

Reputation: 125

Does sched_setscheduler() /sched_getscheduler() only affect scheduling policy of the specified process and its child processes?

I am confused about how sched_setscheduler() works in Linux.

My understanding is Linux uses CFS (SCHED_NORMAL) internally for kernel processes scheduling. When user space starts a program (process), a clone() call would be trigged to create a corresponding scheduling entity in kernel space.

So assume there are user process A and process B. Process A calls sched_setscheduler(pid_A, SCHED_RR) and spawns child processes A1, A2. Process B calls sched_setscheduler(pid_B, SCHED_NORMAL) and spawns child processes B1, B2.

How would Linux schedule process A and B in this case as they are using different scheduling policy? Does Linux still uses its internal default scheduling policy to choose between A and B, but leave A, A2, A1 compete each other using SCHED_RR and B, B1, B2 with SCHED_NORMAL ?

Upvotes: 0

Views: 507

Answers (1)

merlin2011
merlin2011

Reputation: 75555

The Linux kernel has a series of "scheduling classes" which are modules that decide what thread should run when a core goes idle.

Since SCHED_RR is a real-time scheduling class, it will get to decide what to schedule onto an idle core before SCHED_NORMAL does. That means that in some sense, A and its children have strict priority over B and its children.

However, I believe Linux reserves something like 5% CPU for non-realtime scheduling classes, so B and its children should not be starved of CPU.

Upvotes: 1

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