Re: [REPORT] cfs-v4 vs sd-0.44

From: Gene Heskett
Date: Tue Apr 24 2007 - 03:00:35 EST

On Tuesday 24 April 2007, Ingo Molnar wrote:
>* Peter Williams <pwil3058@xxxxxxxxxxxxxx> wrote:
>> > The cases are fundamentally different in behavior, because in the
>> > first case, X hardly consumes the time it would get in any scheme,
>> > while in the second case X really is CPU bound and will happily
>> > consume any CPU time it can get.
>> Which still doesn't justify an elaborate "points" sharing scheme.
>> Whichever way you look at that that's just another way of giving X
>> more CPU bandwidth and there are simpler ways to give X more CPU if it
>> needs it. However, I think there's something seriously wrong if it
>> needs the -19 nice that I've heard mentioned.
>Gene has done some testing under CFS with X reniced to +10 and the
>desktop still worked smoothly for him.

As a data point here, and probably nothing to do with X, but I did manage to
lock it up, solid, reset button time tonight, by wanting 'smart' to get done
with an update session after amanda had started. I took both smart processes
I could see in htop all the way to -19, but when it was about done about 3
minutes later, everything came to an instant, frozen, reset button required
lockup. I should have stopped at -17 I guess. :(

>So CFS does not 'need' a reniced
>X. There are simply advantages to negative nice levels: for example
>screen refreshes are smoother on any scheduler i tried. BUT, there is a
>caveat: on non-CFS schedulers i tried X is much more prone to get into
>'overscheduling' scenarios that visibly hurt X's performance, while on
>CFS there's a max of 1000-1500 context switches a second at nice -10.
>(which, considering the cost of a context switch is well under 1%
>So, my point is, the nice level of X for desktop users should not be set
>lower than a low limit suggested by that particular scheduler's author.
>That limit is scheduler-specific. Con i think recommends a nice level of
>-1 for X when using SD [Con, can you confirm?], while my tests show that
>if you want you can go as low as -10 under CFS, without any bad
>side-effects. (-19 was a bit too much)
>> [...] You might as well just run it as a real time process.
>hm, that would be a bad idea under any scheduler (including CFS),
>because real time processes can starve other processes indefinitely.
> Ingo

Cheers, Gene
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