From: Eric Dumazet <dada1@cosmosbay.com>
To: Nick Piggin <nickpiggin@yahoo.com.au>
Subject: Re: [RFC] NUMA futex hashing
Date: Tue, 8 Aug 2006 18:59:04 +0200
User-Agent: KMail/1.9.1
Cc: Ulrich Drepper <drepper@gmail.com>, Andi Kleen <ak@suse.de>,
       Ravikiran G Thirumalai <kiran@scalex86.org>,
       "Shai Fultheim (Shai@scalex86.org)" <shai@scalex86.org>,
       pravin b shelar <pravin.shelar@calsoftinc.com>,
       linux-kernel@vger.kernel.org
References: <20060808070708.GA3931@localhost.localdomain> <44D8BD29.4010102@yahoo.com.au> <200608081849.28458.dada1@cosmosbay.com>
In-Reply-To: <200608081849.28458.dada1@cosmosbay.com>
MIME-Version: 1.0
Content-Type: text/plain;
  charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
Message-Id: <200608081859.04727.dada1@cosmosbay.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1489
Lines: 33

On Tuesday 08 August 2006 18:49, Eric Dumazet wrote:
> 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.

After looking at kernel/futex.c, I realize we also can avoid the atomic ops 
(and another cacheline bouncing) done in get_key_refs()/drop_key_refs(), 
touching the inode i_count or mm_count refcounter)

Eric
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/