linux-kernel - Re: niced tasks on SMT system

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Date:   Thu, 28 Dec 2017 21:58:32 +0300
From:   Ozgur <ozgur@...sey.org>
To:     Pavel Machek <pavel@....cz>, Mike Galbraith <efault@....de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        kernel list <linux-kernel@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "hpa@...or.com" <hpa@...or.com>, "x86@...nel.org" <x86@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>
Subject: Re: niced tasks on SMT system



28.12.2017, 21:54, "Pavel Machek" <pavel@....cz>:
> Hi!
>
>>  > Ok, so I'm compiling, and I'd like to run a flight simulator.
>>  >
>>  > Flightgear normally does 20fps on my system... kinda low but playable.
>>  >
>>  > I have reniced make -j 5 fo kernel running. Scheduler gives 100% of
>>  > one of CPUs to Flightgear (good), but the smt sibling is used by the
>>  > compilation, and I'm down to 9 fps. Not good.
>>  >
>>  > Even with single-threaded make, I have 10-13fps.
>>  >
>>  > Is there way to learn which CPUs are SMT siblings?
>>
>>  cat /sys/devices/system/cpu/cpu{N}/topology/thread_siblings
>
> Thanks for a hint.
>
> Well, something is definitely wrong there:
>
> pavel@duo:/data/l/linux-n900$ cat
> /sys/devices/system/cpu/cpu0/topology/thread_siblings
> 03

Ops,

do you fly on N900 via flightgear? :)
I have two N900 and one run gentoo.

> I believe that means that cpu0 & cpu1 are sharing physical cpu. Yet,
> when I run flightgear and "nice while1". flightgear goes to CPU#0 and
> while1 to CPU#1. Would not it be nice if while1 went preferably to
> CPUs #2 and #3? Ok. after a while while1 moved to cpu#2, good.
>
>>  > Is there way to disable SMT during runtime?
>>
>>  You could offline them, but wouldn't it be better to tell each which
>>  CPUs they can use, or perhaps partition your box with cpusets?
>
> Let me try offlining first... and yes, SMT seems to be problem here.
>
> | CPU0 | CPU1 | CPU2 | CPU3 | |
> | flightgear | off | off | off | 21 fps |
> | flightgear | while1 | off | off | 17 fps |
> | flightgear+while1 | off | off | off | 10 fps |
> | flightgear | off | while1 | off | 21 fps |
>
>>  > Can I do something to improve Flightgear performance and still do
>>  > compilation?
>>
>>  Sure, run everything associated with your game as RT, and everything
>>  not game gets the leftover cycles.  If there are none, box will
>>   throttle RT to save itself from it's psycho :) driver and you'll know
>>  that you need a bigger box. (it's likely your phone)
>
>>  Oh yeah, echo NO_RT_RUNTIME_SHARE > /sys/kernel/debug/sched_features
>>  before you try that, otherwise the throttle won't help.
>
> Sure, but that is not a problem I have.
>
> Scheduler already (correctly) decides that the game is important, and
> gives game all the time it wants (100% of one CPU). That is not a problem.
>
> Problem seems to be that it schedules other tasks to SMT sibling, and
> that slows down the game quite significantly.
>
> Best regards,
>
>                                                                         Pavel
> --
> (english) http://www.livejournal.com/~pavelmachek
> (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html