linux-kernel - Re: [PATCH 2.6.20-rc4 0/4] futexes functionalities and improvements

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Message-ID: <45A73E90.7050805@bull.net>
Date:	Fri, 12 Jan 2007 08:53:52 +0100
From:	Pierre Peiffer <pierre.peiffer@...l.net>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Andrew Morton <akpm@...l.org>, Ulrich Drepper <drepper@...hat.com>,
	Ingo Molnar <mingo@...e.hu>, Jakub Jelinek <jakub@...hat.com>
Subject: Re: [PATCH 2.6.20-rc4 0/4] futexes functionalities and improvements

Andrew Morton a écrit :
 > OK.  Unfortunately patches 2-4 don't apply without #1 present and the fix
 > is not immediately obvious, so we'll need a respin+retest, please.

Ok, I'll provide updated patches for -mm ASAP.

> On Thu, 11 Jan 2007 09:47:28 -0800
> Ulrich Drepper <drepper@...hat.com> wrote:

>> if the patches allow this, I'd like to see parts 2, 3, and 4 to be in
>> -mm ASAP.  Especially the 64-bit variants are urgently needed.  Just
>> hold off adding the plist use, I am still not convinced that
>> unconditional use is a good thing, especially with one single global list.

Just to avoid any misunderstanding (I (really) understand your point about 
performance issue), but:

* the problem I mention about several futexes hashed on the same key, and thus 
with all potential waiters listed on the same list, is _not_ a new problem which 
comes with this patch: it already exists today, with simple list.

* the measures of performance done with pthread_broadcast (and thus with 
futex_requeue) is a good choice (well, may be not realistic, when considering 
real applications (*)) to put in evidence the performance impact, rather than 
threads making FUTEX_WAIT/FUTEX_WAKE: what is expensive with plist is the 
plist_add operation (which occurs in FUTEX_WAIT), not plist_del (which occurs 
during FUTEX_WAKE => thus, no big impact should be noticed here). Any measure 
will be difficult to do with only FUTEX_WAIT/WAKE.

=> futex_requeue does as many plist_del/plist_add operations as the number of 
threads waiting (minus 1), and thus has a direct impact on the time needed to 
wake everybody (or to wake the first thread to be more precise).

(*) I'll try the volano bench, if I have time.

-- 
Pierre
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