Re: interactive task starvation

[Willy Tarreau <willy@w.ods.org>]

On Wed, Mar 22, 2006 at 12:13:15AM +1100, Con Kolivas wrote:
> On Wednesday 22 March 2006 00:10, Mike Galbraith wrote:
> > On Tue, 2006-03-21 at 22:53 +1100, Con Kolivas wrote:
> > > On Tuesday 21 March 2006 22:18, Ingo Molnar wrote:
> > > > great work by Mike! One detail: i'd like there to be just one default
> > > > throttling value, i.e. no grace_g tunables [so that we have just one
> > > > default scheduler behavior]. Is the default grace_g[12] setting good
> > > > enough for your workload?
> > >
> > > I agree. If anything is required, a simple on/off tunable makes much more
> > > sense. Much like I suggested ages ago with an "interactive" switch which
> > > was rather unpopular when I first suggested it.
> >
> > Let me try to explain why on/off is not sufficient.
> >
> > You notice how Willy said that his notebook is more responsive with
> > tunables set to 0,0?  That's important, because it's absolutely true...
> > depending what you're doing.  Setting tunables to 0,0 cuts off the idle
> > sleep logic, and the sleep_avg divisor - both of which were put there
> > specifically for interactivity - and returns the scheduler to more or
> > less original O(1) scheduler.  You and I both know that these are most
> > definitely needed in a Desktop environment.  For instance, if Willy
> > starts editing code in X, and scrolls while something is running in the
> > background, he'll suddenly say hey, maybe this _ain't_ more responsive,
> > because all of a sudden the starvation added with the interactivity
> > logic will be sorely missed as my throttle wrings X's neck.
> >
> > How long should Willy be able to scroll without feeling the background,
> > and how long should Apache be able to starve his shell.  They are one
> > and the same, and I can't say, because I'm not Willy.  I don't know how
> > to get there from here without tunables.  Picking defaults is one thing,
> > but I don't know how to make it one-size-fits-all.  For the general
> > case, the values delivered will work fine.  For the apache case, they
> > absolutely 100% guaranteed will not.
> 
> So how do you propose we tune such a beast then? Apache users will use off, 
> everyone else will have no idea but to use the defaults.

What you describe is exactly a case for a tunable. Different people with
different workloads want different values. Seems fair enough. After all,
we already have /proc/sys/vm/swappiness, and things like that for the same
reason : the default value should suit most users, and the ones with
knowledge and different needs can tune their system. Maybe grace_{g1,g2}
should be renamed to be more explicit, may be we can automatically tune
one from the other and let only one tunable. But if both have a useful
effect, I don't see a reason for hiding them.

> Cheers,
> Con

Cheers,
Willy

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