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From: Jakub Jelinek <jakub@redhat.com>
To: Eric Dumazet <dada1@cosmosbay.com>
Cc: Andrea Arcangeli <andrea@suse.de>,
	Nick Piggin <nickpiggin@yahoo.com.au>,
	Anton Blanchard <anton@samba.org>, Rik van Riel <riel@redhat.com>,
	Lorenzo Allegrucci <l_allegrucci@yahoo.it>,
	linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
	Suparna Bhattacharya <suparna@in.ibm.com>,
	Jens Axboe <jens.axboe@oracle.com>
Subject: Re: SMP performance degradation with sysbench
Date: Tue, 13 Mar 2007 07:27:17 -0500	[thread overview]
Message-ID: <20070313122716.GV355@devserv.devel.redhat.com> (raw)
In-Reply-To: <200703131302.45216.dada1@cosmosbay.com>

On Tue, Mar 13, 2007 at 01:02:44PM +0100, Eric Dumazet wrote:
> On Tuesday 13 March 2007 12:42, Andrea Arcangeli wrote:
> 
> > My wild guess is that they're allocating memory after taking
> > futexes. If they do, something like this will happen:
> >
> >      taskA		taskB		taskC
> >      user lock
> > 			mmap_sem lock
> >      mmap sem -> schedule
> > 					user lock -> schedule
> >
> > If taskB wouldn't be there triggering more random trashing over the
> > mmap_sem, the lock holder wouldn't wait and task C wouldn't wait too.
> >
> > I suspect the real fix is not to allocate memory or to run other
> > expensive syscalls that can block inside the futex critical sections...
> 
> glibc malloc uses arenas, and trylock() only. It should not block because if 
> an arena is already locked, thread automatically chose another arena, and 
> might create a new one if necessary.

Well, only when allocating it uses trylock, free uses normal lock.
glibc malloc will by default use the same arena for all threads, only when
it sees contention during allocation it gives different threads different
arenas.  So, e.g. if mysql did all allocations while holding some global
heap lock (thus glibc wouldn't see any contention on allocation), but
freeing would be done outside of application's critical section, you would
see contention on main arena's lock in the free path.
Calling malloc_stats (); from e.g. atexit handler could give interesting
details, especially if you recompile glibc malloc with -DTHREAD_STATS=1.

	Jakub

  reply	other threads:[~2007-03-13 12:29 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-02-25 17:44 Lorenzo Allegrucci
2007-02-25 23:46 ` Rik van Riel
2007-02-26 13:36   ` Nick Piggin
2007-02-26 13:41     ` Nick Piggin
2007-02-26 22:04     ` Pete Harlan
2007-02-26 22:36       ` Dave Jones
2007-02-27  0:32         ` Hiro Yoshioka
2007-02-27  0:43           ` Rik van Riel
2007-02-27  4:03             ` Hiro Yoshioka
2007-02-27  4:31               ` Rik van Riel
2007-02-27  8:14                 ` J.A. Magallón
2007-02-27 14:02                   ` Rik van Riel
2007-02-27 14:56                     ` Paulo Marques
2007-02-27 20:40                       ` Nish Aravamudan
2007-02-28  2:21                       ` Bill Davidsen
2007-02-28  2:52                         ` Nish Aravamudan
2007-03-01  0:20                           ` Nish Aravamudan
2007-02-27 19:05                     ` Lorenzo Allegrucci
2007-03-01 16:57                       ` Lorenzo Allegrucci
2007-02-28  1:27     ` Nish Aravamudan
2007-02-28  2:22       ` Nick Piggin
2007-02-28  2:51         ` Nish Aravamudan
2007-03-12 22:00     ` Anton Blanchard
2007-03-13  5:11       ` Nick Piggin
2007-03-13  9:45         ` Andrea Arcangeli
2007-03-13 10:06           ` Nick Piggin
2007-03-13 10:31             ` Andrea Arcangeli
2007-03-13 10:37               ` Nick Piggin
2007-03-13 10:57                 ` Andrea Arcangeli
2007-03-13 11:12                   ` Nick Piggin
2007-03-13 11:40                     ` Eric Dumazet
2007-03-13 11:56                       ` Nick Piggin
2007-03-13 11:42                     ` Andrea Arcangeli
2007-03-13 12:02                       ` Eric Dumazet
2007-03-13 12:27                         ` Jakub Jelinek [this message]
2007-03-13 12:08                       ` Nick Piggin
2007-03-14 23:33                         ` Siddha, Suresh B
2007-03-20  2:29                           ` Zhang, Yanmin
2007-04-02  2:59                             ` Zhang, Yanmin
2007-03-13  6:00       ` Eric Dumazet
2007-03-14  0:36       ` Nish Aravamudan
2007-03-14  1:00         ` Eric Dumazet
2007-03-14  1:09           ` Nish Aravamudan
     [not found] <fa.V3M3ZgXL+lFlIyhx43YxCU/JFUk@ifi.uio.no>
     [not found] ` <fa.ciL5lzdfskdJHJPgn+UVCHt/9EM@ifi.uio.no>
     [not found]   ` <fa.2ABbHhyCbp3Fx7hSE/Gr0SuzFvw@ifi.uio.no>
     [not found]     ` <fa.oaZk6Aiqd8gyZNsj7+m+w9MibhU@ifi.uio.no>
     [not found]       ` <fa.RjX9Y4ckjRCle5L+uWNdd0snOio@ifi.uio.no>
     [not found]         ` <fa.XocsudxlGplKh0kloTtA0juPwtA@ifi.uio.no>
2007-02-28  0:20           ` Robert Hancock
2007-02-28  1:32             ` Hiro Yoshioka

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