linux-kernel - RE: [PATCH] staging: zcache: reduce tmem bucket lock contention

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Date:	Mon, 3 Oct 2011 11:51:53 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>, gregkh@...e.de
Cc:	cascardo@...oscopio.com, rdunlap@...otime.net,
	devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org
Subject: RE: [PATCH] staging: zcache: reduce tmem bucket lock contention

> From: Seth Jennings [mailto:sjenning@...ux.vnet.ibm.com]
> Sent: Thursday, September 29, 2011 8:32 AM
> To: gregkh@...e.de
> Cc: cascardo@...oscopio.com; Dan Magenheimer; rdunlap@...otime.net; devel@...verdev.osuosl.org; linux-
> kernel@...r.kernel.org; Seth Jennings
> Subject: [PATCH] staging: zcache: reduce tmem bucket lock contention
> 
> tmem uses hash buckets each with their own rbtree and lock to
> quickly lookup tmem objects.  tmem has TMEM_HASH_BUCKETS (256)
> buckets per pool.  However, because of the way the tmem_oid is
> generated for frontswap pages, only 16 unique tmem_oids are being
> generated, resulting in only 16 of the 256 buckets being used.
> This cause high lock contention for the per bucket locks.
> 
> This patch changes SWIZ_BITS to include more bits of the offset.
> The result is that all 256 hash buckets are potentially used resulting in a
> 95% drop in hash bucket lock contention.
> 
> Based on v3.1-rc7
> 
> Signed-off-by: Seth Jennings <sjenning@...ux.vnet.ibm.com>

Good catch Seth!

Acked-by: Dan Magenheimer <dan.magenheimer@...cle.com>
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