linux-kernel - Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to 3.6-rc5 on AMD chipsets

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Message-ID: <20120915151827.GA17439@liondog.tnic>
Date:	Sat, 15 Sep 2012 17:18:27 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Mike Galbraith <efault@....de>
Cc:	Nikolay Ulyanitsky <lystor@...il.com>,
	linux-kernel@...r.kernel.org,
	Andreas Herrmann <andreas.herrmann3@....com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: 20% performance drop on PostgreSQL 9.2 from kernel 3.5.3 to
 3.6-rc5 on AMD chipsets - bisected

On Sat, Sep 15, 2012 at 04:47:39PM +0200, Mike Galbraith wrote:
> Increasing /proc/sys/kernel/sched_min_granularity_ns to roughly half
> of sched_latency_ns should also help. That will allow LAST_BUDDY to do
> it's job, try to hand the CPU back to a preempted task if possible.

Just for my n00b scheduler understanding: this way you're practically
extending the timeslice of the task so that it gets done without being
preempted and the lock-holding period of the preempted task gets smaller
and thus you get more completed transactions in postgres during the
benchmark run?

> The change that increased sched_nr_latency to 8 should have injured
> postgress as well. ATM, it's disabled unless you're massively loaded.
>
> I _think_ it's about preemption, but it doesn't matter, patch is
> toast.

In any case, if you wanna retry the buddy thing for 3.7, ping me and I
can run it on the assortment of machines I have here.

Thanks.

-- 
Regards/Gruss,
    Boris.
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