linux-kernel - Re: [thisops uV2 02/10] vmstat: Optimize zone counter modifications through the use of this cpu operations

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Date:	Mon, 29 Nov 2010 14:59:20 -0600 (CST)
From:	Christoph Lameter <cl@...ux.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	linux-kernel@...r.kernel.org,
	Eric Dumazet <eric.dumazet@...il.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [thisops uV2 02/10] vmstat: Optimize zone counter modifications
 through the use of this cpu operations

Some numbers for the cmpxchg implementation (cycles)

Function		Orig	Cmpxchg Fallback
--------------------------------------------------
inc_zone_page_state	170	32	196
__inc_zone_page_s	23	31	26
inc/dec pairs		347	69	379

So the fallback is always worse. cmpxchg is only better for the versions
of zone counters where we need to disable and enable interrupts.

This would suggest to only use the cmpxchg for arches that have cmpxchg
local and only in the case of the full versions.

There are 12 cases of inc_zone_page_state and 13 of dec_zone_page_state
as well as 7 cases of mod_zone_page_state.


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