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

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Date:	Tue, 8 Aug 2006 18:49:28 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Ulrich Drepper <drepper@...il.com>, Andi Kleen <ak@...e.de>,
	Ravikiran G Thirumalai <kiran@...lex86.org>,
	"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

On Tuesday 08 August 2006 18:34, Nick Piggin wrote:
> Eric Dumazet wrote:
> > We certainly can. But if you insist of using mmap sem at all, then we
> > have a problem.
> >
> > rbtree would not reduce cacheline bouncing, so :
> >
> > We could use a hashtable (allocated on demand) of size N, N depending on
> > NR_CPUS for example. each chain protected by a private spinlock. If N is
> > well chosen, we might reduce lock cacheline bouncing. (different threads
> > fighting on different private futexes would have a good chance to get
> > different cachelines in this hashtable)
>
> See other mail. We already have a hash table ;)

Yes but still you want at FUTEX_WAIT time to tell the kernel the futex is 
private to this process.

Giving the same info at FUTEX_WAKE time could avoid the kernel to make the 
second pass (using only a private futex lookup), avoiding again the mmap_sem 
touch in case no threads are waiting anymore on this futex.

Eric
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