linux-kernel - Re: [PATCH 0/2] convert mmap_sem to a scalable rw

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Message-ID: <46449F61.2060004@cosmosbay.com>
Date:	Fri, 11 May 2007 18:52:49 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Oleg Nesterov <oleg@...sign.ru>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 0/2] convert mmap_sem to a scalable rw_mutex

Ingo Molnar a écrit :
> * Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> 
>> I was toying with a scalable rw_mutex and found that it gives ~10% 
>> reduction in system time on ebizzy runs (without the MADV_FREE patch).
>>
>> 2-way x86_64 pentium D box:
>>
>> 2.6.21
>>
>> /usr/bin/time ./ebizzy -m -P
>> 59.49user 137.74system 1:49.22elapsed 180%CPU (0avgtext+0avgdata 0maxresident)k
>> 0inputs+0outputs (0major+33555877minor)pagefaults 0swaps
>>
>> 2.6.21-rw_mutex
>>
>> /usr/bin/time ./ebizzy -m -P
>> 57.85user 124.30system 1:42.99elapsed 176%CPU (0avgtext+0avgdata 0maxresident)k
>> 0inputs+0outputs (0major+33555877minor)pagefaults 0swaps
> 
> nice! This 6% runtime reduction on a 2-way box will i suspect get 
> exponentially better on systems with more CPUs/cores.

As long you only have readers, yes.

But I personally find this new rw_mutex not scalable at all if you have some 
writers around.

percpu_counter_sum is just a L1 cache eater, and O(NR_CPUS)



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