linux-kernel - Re: SMP performance degradation with sysbench

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Date:	Tue, 13 Mar 2007 22:56:35 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	Andrea Arcangeli <andrea@...e.de>,
	Anton Blanchard <anton@...ba.org>,
	Rik van Riel <riel@...hat.com>,
	Lorenzo Allegrucci <l_allegrucci@...oo.it>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Jens Axboe <jens.axboe@...cle.com>
Subject: Re: SMP performance degradation with sysbench

Eric Dumazet wrote:
> On Tuesday 13 March 2007 12:12, Nick Piggin wrote:
> 
>>I guess googlemalloc (tcmalloc?) isn't suitable for a general purpose
>>glibc allocator. But I wonder if there are other improvements that glibc
>>can do here?
> 
> 
> I cooked a patch some time ago to speedup threaded apps and got no feedback.

Well that doesn't help in this case. I tested and the mmap_sem contention
is not an issue.

> http://lkml.org/lkml/2006/8/9/26
> 
> Maybe we have to wait for 32 core cpu before thinking of cache line 
> bouncings...

The idea is a good one, and I was half way through implementing similar
myself at one point (some java apps hit this badly).

It is just horribly sad that futexes are supposed to implement a
_scalable_ thread synchronisation mechanism, whilst fundamentally
relying on an mm-wide lock to operate.

I don't like your interface, but then again, the futex interface isn't
exactly pretty anyway.

You should resubmit the patch, and get the glibc guys to use it.

-- 
SUSE Labs, Novell Inc.
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