linux-kernel - Re: [RFC] NUMA futex hashing

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Date:	08 Aug 2006 11:57:59 +0200
From:	Andi Kleen <ak@...e.de>
To:	Ravikiran G Thirumalai <kiran@...lex86.org>
Cc:	"Shai Fultheim (Shai@...lex86.org)" <shai@...lex86.org>,
	pravin b shelar <pravin.shelar@...softinc.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] NUMA futex hashing

Ravikiran G Thirumalai <kiran@...lex86.org> writes:

> Current futex hash scheme is not the best for NUMA.   The futex hash table is 
> an array of struct futex_hash_bucket, which is just a spinlock and a 
> list_head -- this means multiple spinlocks on the same cacheline and on NUMA 
> machines, on the same internode cacheline.  If futexes of two unrelated 
> threads running on two different nodes happen to hash onto adjacent hash 
> buckets, or buckets on the same internode cacheline, then we have the 
> internode cacheline bouncing between nodes.

When I did some testing with a (arguably far too lock intensive) benchmark
on a bigger box I got most bouncing cycles not in the futex locks itself,
but in the down_read on the mm semaphore.

I guess that is not addressed?

-Andi
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