Re: Renice X for cpu schedulers

[Peter Williams]

Con Kolivas wrote:
> 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.

On a "fair" scheduler these will all get high priority (and good 
response) because their CPU bandwidth usage will be much smaller than 
their entitlement and the scheduler will be trying to help them "catch 
up".  So (as you say) they shouldn't be a problem.

> 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[1]. 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[2].

X's needs are more complex than that (from my observations) in that the 
part of X that processes input doesn't use much CPU but the part that 
does output can be quite a heavy user of CPU (e.g. do a "ls -lR /" in an 
xterm and watch X chew up the CPU).  At the same time, the part of X 
that processes input needs quick responsiveness as it's part of the 
interactive chain where this is less so for the output part.

Where X comes unstuck in the current scheduler is that when the output 
part goes on one of its CPU storms it ceases to look like an interactive 
task and gets given lower priority.  Ironically, this doesn't effect the 
output part of X but it does effect the input part and is manifest as 
crappy interactive response.  One wonders whether modifying X so that it 
has two threads: one for output and one for input; that could be 
scheduled separately might help.  I guess it would depend on whether 
there is insufficient independence between the two halves.

Part of this issue is that giving X a high static priority runs the risk 
of the CPU hog output part disrupting scheduling of other important 
tasks.  So don't give it too big a boost.

> 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 ;-)

It's worth noting that the -10 mentioned is roughly equivalent (in the 
old scheduler) to restoring interactive task status to X in those cases 
where it loses it due to a CPU storm in its output part.

> > (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 :)

I'd like to add the EBS scheduler (posted by Aurema Pty Ltd a couple of 
years back) to this list as it also recommended running X at nice -5 to -10.

Also some of the "interactive bonus" mechanisms in my SPA schedulers 
could be removed if X was reniced.  In fact, with a reniced X the 
spa_svr (server oriented scheduler which attempts to minimise the time 
tasks spend on the queue waiting for CPU access and which doesn't have 
interactive bonuses) might be usable on a work station.

> [1]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.
>
> [2] 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.

Peter
--
Peter Williams                                   pwil3058@xxxxxxxxxxxxxx

"Learning, n. The kind of ignorance distinguishing the studious."
 -- Ambrose Bierce
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