|From:||Con Kolivas <kernel-AT-kolivas.org>|
|To:||Ingo Molnar <mingo-AT-elte.hu>|
|Subject:||Renice X for cpu schedulers|
|Date:||Thu, 19 Apr 2007 21:59:35 +1000|
|Cc:||Nick Piggin <npiggin-AT-suse.de>, Bill Huey <billh-AT-gnuppy.monkey.org>, Mike Galbraith <efault-AT-gmx.de>, Peter Williams <pwil3058-AT-bigpond.net.au>, William Lee Irwin III <wli-AT-holomorphy.com>, linux-kernel-AT-vger.kernel.org, ck list <ck-AT-vds.kolivas.org>, Matt Mackall <mpm-AT-selenic.com>, Andrew Morton <akpm-AT-linux-foundation.org>, Linus Torvalds <torvalds-AT-linux-foundation.org>, Thomas Gleixner <tglx-AT-linutronix.de>, Arjan van de Ven <arjan-AT-infradead.org>|
Ok, there are 3 known schedulers currently being "promoted" as solid replacements for the mainline scheduler which address most of the issues with mainline (and about 10 other ones not currently being promoted). The main way they do this is through attempting to maintain solid fairness. There is enough evidence mounting now from the numerous test cases fixed by much fairer designs that this is the way forward for a general purpose cpu scheduler which is what linux needs. Interactivity of just about everything that needs low latency (ie audio and video players) are easily managed by maintaining low latency between wakeups and scheduling of all these low cpu users. The one fly in the ointment for linux remains X. I am still, to this moment, completely and utterly stunned at why everyone is trying to find increasingly complex unique ways to manage X when all it needs is more cpu. Now most of these are actually very good ideas about _extra_ features that would be desirable in the long run for linux, but given the ludicrous simplicity of renicing X I cannot fathom why people keep promoting these alternatives. At the time of 2.6.0 coming out we were desparately trying to get half decent interactivity within a reasonable time frame to release 2.6.0 without rewiring the whole scheduler. So I tweaked the crap out of the tunables that were already there. So let's hear from the 3 people who generated the schedulers under the spotlight. These are recent snippets and by no means the only time these comments have been said. Without sounding too bold, we do know a thing or two about scheduling. CFS: On Thursday 19 April 2007 16:38, Ingo Molnar wrote: > hmmmm. How about the following then: default to nice -10 for all > (SCHED_NORMAL) kernel threads and all root-owned tasks. Root _is_ > special: root already has disk space reserved to it, root has special > memory allocation allowances, etc. I dont see a reason why we couldnt by > default make all root tasks have nice -10. This would be instantly loved > by sysadmins i suspect ;-) > > (distros that go the extra mile of making Xorg run under non-root could > also go another extra one foot to renice that X server to -10.) Nicksched: On Wednesday 18 April 2007 15:00, Nick Piggin wrote: > What's wrong with allowing X to get more than it's fair share of CPU > time by "fiddling with nice levels"? That's what they're there for. and Staircase-Deadline: On Thursday 19 April 2007 09:59, Con Kolivas wrote: > Remember to renice X to -10 for nicest desktop behaviour :) The one caveat I can think of is that when you share X sessions across multiple users -with a fair cpu scheduler-, having them all nice 0 also makes the distribution of cpu across the multiple users very even and smooth, without the expense of burning away the other person's cpu time they'd like for compute intensive non gui things. If you make a scheduler that always favours X this becomes impossible. I've had enough users offlist ask me to help them set up multiuser environments just like this with my schedulers because they were unable to do it with mainline, even with SCHED_BATCH, short of nicing everything +19. This makes the argument for not favouring X within the scheduler with tweaks even stronger.  Nick was promoting renicing X with his Nicksched alternative at the time of 2.6.0 and while we were not violently opposed to renicing X, Nicksched was still very new on the scene and didn't have the luxury of extended testing and reiteration in time for 2.6 and he put the project on hold for some time after that. The correctness of his views on renicing certainly have become more obvious over time. So yes go ahead and think up great ideas for other ways of metering out cpu bandwidth for different purposes, but for X, given the absurd simplicity of renicing, why keep fighting it? Again I reiterate that most users of SD have not found the need to renice X anyway except if they stick to old habits of make -j4 on uniprocessor and the like, and I expect that those on CFS and Nicksched would also have similar experiences. -- -ck
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