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

[Dan Magenheimer]

> From: Seth Jennings [mailto:sjenning@xxxxxxxxxxxxxxxxxx]
> Sent: Thursday, September 29, 2011 8:32 AM
> To: gregkh@xxxxxxx
> Cc: cascardo@xxxxxxxxxxxxxx; Dan Magenheimer; rdunlap@xxxxxxxxxxxx; devel@xxxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx; 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@xxxxxxxxxxxxxxxxxx>

Good catch Seth!

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