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SubjectRe: 2.6.0-test2-mm3 osdl-aim-7 regression
On Thu, 7 Aug 2003 15:11, Nick Piggin wrote:
> What is the need for this round robining? Don't processes get a calculated
> timeslice anyway?

Nice to see you taking an unhealthy interest in the scheduler tweaks Nick.
This issue has been discussed before but it never hurts to review things.
I've uncc'ed the rest of the people in case we get carried away again. First
let me show you Ingo's comment in the relevant code section:

* Prevent a too long timeslice allowing a task to monopolize
* the CPU. We do this by splitting up the timeslice into
* smaller pieces.
* Note: this does not mean the task's timeslices expire or
* get lost in any way, they just might be preempted by
* another task of equal priority. (one with higher
* priority would have preempted this task already.) We
* requeue this task to the end of the list on this priority
* level, which is in essence a round-robin of tasks with
* equal priority.

I was gonna say second blah blah but I think the first paragraph explains the

Must we do this? No.

Should we? Probably.

How frequently should we do it? Once again I'll quote Ingo who said it's a
difficult question to answer.

The more frequently you round robin the lower the scheduler latency between
SCHED_OTHER tasks of the same priority. However, the longer the timeslice the
more benefit you get from cpu cache. Where is the sweet spot? Depends on the
hardware and your usage requirements of course, but Ingo has empirically
chosen 25ms after 50ms seemed too long. Basically cache trashing becomes a
real problem with timeslices below ~7ms on modern hardware in my limited
testing. A minor quirk in Ingo's original code means _occasionally_ a task
will be requeued with <3ms to go. It will be interesting to see if fixing
this (which O12.2+ does) makes a big difference or whether we need to
reconsider how frequently (if at all) we round robin tasks.


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