linux-kernel - Re: [rfc] [patch 1/2 ] Process private hash tables for private futexes

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Message-ID: <20090324031925.GF7278@localdomain>
Date:	Mon, 23 Mar 2009 20:19:25 -0700
From:	Ravikiran G Thirumalai <kiran@...lex86.org>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	shai@...lex86.org, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [rfc] [patch 1/2 ] Process private hash tables for private
	futexes

On Mon, Mar 23, 2009 at 10:57:55PM +0100, Eric Dumazet wrote:
>Ravikiran G Thirumalai a écrit :
>> 
[...]
>> Hmm!  How about
>> a) Reduce hash table size for private futex hash and increase hash table
>>    size for the global hash?
>> 
>> OR, better,
>> 
>> b) Since it is multiple spinlocks on the same cacheline which is a PITA
>>    here, how about keeping the global table, but just add a dereference
>>    to each hash slot, and interleave the adjacent hash buckets between
>>    nodes/cpus? So even without needing to  lose out memory from padding,
>>    we avoid false sharing on cachelines due to unrelated futexes hashing
>>    onto adjacent hash buckets?
>> 
>
>Because of jhash(), futex slots are almost random. No need to try to interleave
>them. Since you have a "cache line" of 4096 bytes, you need almost 4 pages
>per cpu to avoid in a statistical way false sharing.

How did you come up with that number?  So there is no way adjacent
cachelines will never ever be used in the global hash??
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