linux-kernel - Re: sched_yield: delete sysctl_sched_compat

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Date:	Mon, 3 Dec 2007 20:17:19 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: sched_yield: delete sysctl_sched_compat_yield

On Monday 03 December 2007 19:45, Ingo Molnar wrote:
> * Nick Piggin <nickpiggin@...oo.com.au> wrote:
> > On Friday 30 November 2007 21:08, Ingo Molnar wrote:
> > > * Nick Piggin <nickpiggin@...oo.com.au> wrote:
> > > > Haven't we been asking JVMs to use futexes or posix locking for years
> > > > and years now? [...]
> > >
> > > i'm curious, with what JVM was it tested and where's the source so i
> > > can fix their locking for them? Can the problem be reproduced with:
> >
> > Sure, but why shouldn't the compat behaviour be the default, and the
> > sysctl go away?
> >
> > It makes older JVMs work better, it is slightly closer to the old
> > behaviour, and it is arguably a less surprising result.
>
> as far as desktop apps such as firefox goes, the exact opposite is true.
> We had two choices basically: either a "more agressive" yield than
> before or a "less agressive" yield. Desktop apps were reported to hurt
> from a "more agressive" yield (firefox for example gets some pretty bad
> delays), so we defaulted to the less agressive method. (and we defaulted
> to that in v2.6.23 already)

Yeah, I doubt the 2.6.23 scheduler will be usable for distros though...


> Really, in this sense volanomark is another 
> test like dbench - we care about it but not unconditionally and in this
> case it's a really silly API use that is at the center of the problem.

Sure, but do you whether _real_ java server applications are OK? Is it
possible to reduce the aggressiveness of yield to a mid-way? Are the
firefox tests also like dbench (ie. were they done with make -j huge or
some other insane scheduler loads)


> Talking about the default alone will not bring us forward, but we can
> certainly add helpers to identify SCHED_OTHER::yield tasks - a once per
> bootup warning perhaps?

I don't care about keeping the behaviour for future apps. But for older
code out there, it is very important to still work well.

I was just talking about the default because I didn't know the reason
for the way it was set -- now that I do, we should talk about trying to
improve the actual code so we don't need 2 defaults.
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