linux-kernel - Re: tbench regression with 2.6.33-rc1

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Date:	Mon, 11 Jan 2010 13:08:00 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Lin Ming <ming.m.lin@...el.com>
Cc:	mingo@...e.hu, tglx@...utronix.de,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Mike Galbraith <efault@....de>
Subject: Re: tbench regression with 2.6.33-rc1

On Fri, 2009-12-25 at 19:11 +0800, Lin Ming wrote:
> Hi,
> 
> Test machine: 16 cpus (4P/2Core/HT), 8G mem
> tbench test command:
> tbench_srv &
> tbench 32
> 
> Compared with 2.6.32, tbench has ~4% regression in 2.6.33-rc1.
> 
> >From vmstat data, the context switch number also drop ~4%.
> perf top data does not show much differences.
> 
> But lockstat data shows huge difference in rq->lock, as below.
> See the attachment for the full lockstat data.
> 
> Any clue of this regression?

Nope, I thought to see the same on a dual-socket machine, but when
bisecting I ended up on a user-space perf commit, which is pretty much
impossible.

I did notice some variance in the numbers between boots, maybe it was
large enough to fool me.. (~2800 MB/s was the good one, ~2200 MB/s was
the bad one).

perf itself also didn't really provide clue, perf record -ag on the
workload didn't really show anything scheduler related. vmstat 1 did
show a proportional drop in context switch rate between the kernels
though.. most odd.

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