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

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Date:	Mon, 14 May 2007 14:07:37 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Eric Dumazet <dada1@...mosbay.com>, Ingo Molnar <mingo@...e.hu>,
	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>
Subject: Re: [PATCH 0/2] convert mmap_sem to a scalable rw_mutex

On Fri, May 11, 2007 at 07:18:33PM +0200, Peter Zijlstra wrote:
> On Fri, 2007-05-11 at 18:52 +0200, Eric Dumazet wrote:
> > 
> > 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)
> 
> Yeah, that is true; there are two occurences, the one in
> rw_mutex_read_unlock() is not strictly needed for correctness.
> 
> Write locks are indeed quite expensive. But given the ratio of
> reader:writer locks on mmap_sem (I'm not all that familiar with other
> rwsem users) this trade-off seems workable.

I guess the problem with that logic is assuming the mmap_sem read side
always needs to be scalable. Given the ratio of threaded:unthreaded
apps, maybe the trade-off swings away from favour?

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