linux-kernel - Re: sched_yield: delete sysctl_sched_compat

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Date:	Fri, 30 Nov 2007 11:15:27 +0800
From:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>, mingo@...e.hu,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: sched_yield: delete sysctl_sched_compat_yield

On Fri, 2007-11-30 at 13:46 +1100, Nick Piggin wrote:
> On Wednesday 28 November 2007 09:57, Arjan van de Ven wrote:
> > On Tue, 27 Nov 2007 17:33:05 +0800
> >
> > "Zhang, Yanmin" <yanmin_zhang@...ux.intel.com> wrote:
> > > If echo "1">/proc/sys/kernel/sched_compat_yield before starting
> > > volanoMark testing, the result is very good with kernel 2.6.24-rc3 on
> > > my 16-core tigerton.
> > >
> > > 1) If /proc/sys/kernel/sched_compat_yield=1, comparing with 2.6.22,
> > > 2.6.24-rc3 has more than 70% improvement;
> > > 2) If /proc/sys/kernel/sched_compat_yield=0, comparing with 2.6.22,
> > > 2.6.24-rc3 has more than 80% regression;
> > >
> > > On other machines, the volanoMark result also has much improvement if
> > > /proc/sys/kernel/sched_compat_yield=1.
> > >
> > > Would you like to change function yield_task_fair to delete codes
> > > around sysctl_sched_compat_yield, or just initiate it to 1?
> >
> > sounds like a bad idea; volanomark (well, technically the jvm behind
> > it) is abusing sched_yield() by assuming it does something it really
> > doesn't do, and as it happens some of the earlier 2.6 schedulers
> > accidentally happened to behave in a way that was nice for this
> > benchmark.
> 
> OK, why is this still happening? Haven't we been asking JVMs to use
> futexes or posix locking for years and years now? Are there any sane
> jvms that _don't_ use yield?
I think it's an issue of volanomark (a kind of java application) instead of JVM.

> 
> 
> > Todays kernel has a different behavior somewhat (and before people
> > scream "regression"; sched_yield() behavior isn't really specified and
> > doesn't make any sense at all, whatever you get is what you get....
> > it's pretty much an insane defacto behavior that is incredibly tied to
> > which decisions the scheduler makes how, and no app can depend on that
> 
> It is a performance regression. Is there any reason *not* to use the
> "compat" yield by default?
There is no, so I suggest to set sched_compat_yield=1 by default.
If sched_compat_yield=0, kernel almost does nothing but returns. When
sched_compat_yield=1, it is closer to the meaning of sched_yield man page.

> As you say, for SCHED_OTHER tasks, yield
> can do almost anything. We may as well do something that isn't a
> regression...
I just found SCHED_OTHER in man sched_setscheduler. Is it SCHED_NORMAL in
the latest kernel?

> 
> 
> > in any way. In fact, I've proposed to make sched_yield() just do an
> > msleep(1)... that'd be closer to what sched_yield is supposed to do
> > standard wise than any of the current behaviors .... ;_
> 
> What makes you say that? IIRC of all the things that sched_yeild can
> do, it is not allowed to block. So this is about the only thing that
> will break the standard...
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