From: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Cc: Arjan van de Ven <arjan@infradead.org>,
Andrew Morton <akpm@linux-foundation.org>,
mingo@elte.hu, LKML <linux-kernel@vger.kernel.org>
Subject: Re: sched_yield: delete sysctl_sched_compat_yield
Date: Fri, 30 Nov 2007 12:32:09 +0800 [thread overview]
Message-ID: <1196397129.25646.78.camel@ymzhang> (raw)
In-Reply-To: <200711301429.15664.nickpiggin@yahoo.com.au>
On Fri, 2007-11-30 at 14:29 +1100, Nick Piggin wrote:
> On Friday 30 November 2007 14:15, Zhang, Yanmin wrote:
> > On Fri, 2007-11-30 at 13:46 +1100, Nick Piggin wrote:
> > > On Wednesday 28 November 2007 09:57, Arjan van de Ven wrote:
>
> > > > 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.
>
> volanomark itself and not the jvm is calling sched_yield()? Do we have
> any non-toy threaded java apps? (what's JAVA in the kernel-perf tests?)
I run lots of well-known benchmarks and volanoMark is the one who gets the largest
impact from sched_yield.
As for real-applications which use sched_yield, mostly, they are not open sources.
Yesterday, I got to know someone was using sched_yield in his network C programs,
but he didn't want to share the sources with me.
>
>
> > > > 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.
>
> sched_yield() is really only defined for posix realtime scheduling
> AFAIK, which talks about priority lists.
>
> SCHED_OTHER is defined to be a single priority, below the rest of the
> realtime priorities. So at first you *might* say that the process
> should then be made to run only after all other SCHED_OTHER processes,
> however there is no such ordering requirement for SCHED_OTHER
> scheduling. The SCHED_OTHER scheduler can run any task at any time.
>
> That said, I think people would *expect* that call be much closer to
> the compat behaviour than the current default. And that's definitely
> what Linux has done in the past. So there really does need to be a
> good reason to change it like this IMO.
That's indeed what I am thinking.
I am running many testing(SPECjbb/SPECjbb2005/cpu2000/iozone/dbench/tbench...) to
see if there is any regression if sched_compat_yield=1. I think there is no
regression and the testing is just to double-check.
>
>
> > > 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?
>
> Yes, SCHED_NORMAL is SCHED_OTHER. Don't know why it got renamed...
Thanks.
next prev parent reply other threads:[~2007-11-30 4:34 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-11-27 9:33 sched_yield: delete sysctl_sched_compat_yield Zhang, Yanmin
2007-11-27 11:17 ` Ingo Molnar
2007-11-27 22:57 ` Arjan van de Ven
2007-11-30 2:46 ` Nick Piggin
2007-11-30 2:51 ` Arjan van de Ven
2007-11-30 3:02 ` Nick Piggin
2007-11-30 3:15 ` Zhang, Yanmin
2007-11-30 3:29 ` Nick Piggin
2007-11-30 4:32 ` Zhang, Yanmin [this message]
2007-11-30 10:08 ` Ingo Molnar
2007-12-03 4:27 ` Nick Piggin
2007-12-03 8:45 ` Ingo Molnar
2007-12-03 9:17 ` Nick Piggin
2007-12-03 9:35 ` Zhang, Yanmin
2007-12-03 9:57 ` Ingo Molnar
2007-12-03 10:15 ` Nick Piggin
2007-12-03 10:33 ` Ingo Molnar
2007-12-03 11:02 ` Nick Piggin
2007-12-03 11:37 ` Ingo Molnar
2007-12-03 17:04 ` David Schwartz
2007-12-03 17:37 ` Chris Friesen
2007-12-03 19:12 ` David Schwartz
2007-12-03 19:56 ` Chris Friesen
2007-12-03 21:39 ` Mark Lord
2007-12-03 21:48 ` Ingo Molnar
2007-12-03 21:57 ` Mark Lord
2007-12-03 22:05 ` Ingo Molnar
2007-12-03 22:18 ` Mark Lord
2007-12-03 22:33 ` Ingo Molnar
2007-12-04 0:18 ` Nick Piggin
2007-12-04 0:30 ` David Schwartz
2007-12-04 2:09 ` Nick Piggin
2007-12-04 1:02 ` Nick Piggin
2007-12-03 9:41 ` Zhang, Yanmin
2007-12-03 10:17 ` Ingo Molnar
2007-12-03 9:29 ` Zhang, Yanmin
2007-12-03 10:05 ` Ingo Molnar
2007-12-04 6:40 ` Zhang, Yanmin
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