Re: sched_yield: delete sysctl_sched_compat_yield

From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Arjan van de Ven <arjan@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
	mingo@elte.hu, LKML <linux-kernel@vger.kernel.org>
Subject: Re: sched_yield: delete sysctl_sched_compat_yield
Date: Fri, 30 Nov 2007 13:46:22 +1100	[thread overview]
Message-ID: <200711301346.22573.nickpiggin@yahoo.com.au> (raw)
In-Reply-To: <20071127145747.5f4d0aca@laptopd505.fenrus.org>

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@linux.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?

> 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? As you say, for SCHED_OTHER tasks, yield
can do almost anything. We may as well do something that isn't a
regression...

> 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...