linux-kernel - Re: Questions about linux scheduler

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Message-ID: <20091029140542.21970ff2@bike.lwn.net>
Date:	Thu, 29 Oct 2009 14:05:42 -0600
From:	Jonathan Corbet <corbet@....net>
To:	Daniel Rodrick <daniel.rodrick@...il.com>
Cc:	Linux Newbie <linux-newbie@...r.kernel.org>,
	Kernel Newbies <kernelnewbies@...linux.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Questions about linux scheduler

Hi, Daniel,

I'm not a scheduler expert by any stretch, but I can try to play one on
the net...

> I'm following the Robert Love's book and am trying to understand the
> Linux O(1) scheduler. 

Note that the O(1) scheduler is no more; it was replaced by the
completely fair scheduler in 2.6.23.  For the purposes of your
questions things haven't changed a lot, but it's completely different
internally.

> So here is my understanding. The kernel allows
> the applications to specify two types of priorities
> 
> * Realtime Priorities: Range from 0 to 99
> * Non-realtime priorities: Also called "nice" values range from -20 to +19.

You're really talking about different scheduling classes.  There are
two realtime classes (FIFO and RR), both of which trump the interactive
class (SCHED_OTHER).

> * A total of 140 priorities (100 RT + 40 non-RT) - these priorities
> are static - do not change over time.

Sort of, but realtime priorities are really an entirely different scale.

> * A lower priority process will run only if there are no runnable
> processes in priority above it - this automatically means that all RT
> processes get to run before non-RT processes.

That is true for the realtime classes.  SCHED_OTHER will give
lower-priority processes a bit of time even in the presence of runnable
high-priority processes.

>  * tasks on the same priority level are scheduled round robin

SCHED_RR does that, SCHED_FIFO does not.  SCHED_OTHER is
fairness-based, which has RR-like characteristics but is not quite the
same.

Hope that helps,

jon

Jonathan Corbet / LWN.net / corbet@....net
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