linux-kernel - Re: [GIT PULL] sched/core for v2.6.32

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Date:	Sun, 13 Sep 2009 00:07:11 +0200 (CEST)
From:	Jesper Juhl <jj@...osbits.net>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Tony Luck <tony.luck@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>
Subject: Re: [GIT PULL] sched/core for v2.6.32

On Sat, 12 Sep 2009, Ingo Molnar wrote:

> 
> * Tony Luck <tony.luck@...el.com> wrote:
> 
> > On Fri, Sep 11, 2009 at 4:34 PM, Jesper Juhl <jj@...osbits.net> wrote:
> > > That may be so; but most people I've ever talked to about multiple
> > > processes, fork, vfork and the like, have mostly assumed child-runs-first.
> > > That is just my personal experience.
> > > So I get worried when that assumption is made false.
> > 
> > With multi-core cpus becoming (being?) the norm, almost all 
> > systems are SMP now.  So child and parent can surely end up 
> > running in parallel very often.  So applications that make 
> > assumptions about child running first are going to be frequently 
> > surprised.  Aren't they?
> 
> We had parent-runs-first briefly, in v2.6.23 - this got changed by 
> v2.6.24 - but yes, it did trigger at least one app bug that i 
> remember (dont remember which one it was though).
> 
> We are almost two years later now - maybe it works fine now.
> 
> In any case, as a precaution i made the sched_child_runs_first 
> sysctl knob unconditional (previously it was under 
> CONFIG_SCHED_DEBUG).
> 
> So if an old distro is upgraded with a new kernel (and user-space is 
> not updated), it can be worked around by putting this into 
> /etc/sysctl.conf:
> 
>     kernel.sched_child_runs_first = 1
> 

Always goo to have a workaround if something unexpectedly breaks. Makes 
perfect sense for that knob to now be unconditional - and with it, 
changing the behaviour is a much less risky operation. Glad to have it.

> You are right to suggest that due to SMP and due to the general 
> non-determinism of preemption we _never_ made any 'promise' to run 
> the child first.
> 
> It was a statistical property based on performance considerations - 
> and now we flipped it around based on latency and for kbuild 
> performance/throughput reasons: Serge Belyshev reported a 7% 
> increase on a quad due to this change and i measured a 1.5% 
> peak-kbuild performance increase.
> 
Impressive. I wouldn't have expected that much gain by running the parent 
first. Actually I personally would have expected child-first to perform 
better since (in my experience) it's usually the child that's just forked 
that matters the most. A server process that spawns a child to service a 
request wants the child to run asap and process that forks, then execs 
cares primarily about the child.

> So it's worth it for multiple reasons and even in the worst-case 
> problems can be worked around easily and without rebooting the 
> system.
> 
Perfect :-)


-- 
Jesper Juhl <jj@...osbits.net>             http://www.chaosbits.net/
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