linux-kernel - Re: [PATCH 2/3] relay: Fix race condition which occurs when reading across CPUs.

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Message-ID: <20080616164609.GM20851@kernel.dk>
Date:	Mon, 16 Jun 2008 18:46:10 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Eduard - Gabriel Munteanu <eduard.munteanu@...ux360.ro>
Cc:	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	Tom Zanussi <tzanussi@...il.com>, penberg@...helsinki.fi,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	righi.andrea@...il.com
Subject: Re: [PATCH 2/3] relay: Fix race condition which occurs when reading  across CPUs.

On Mon, Jun 16 2008, Eduard - Gabriel Munteanu wrote:
> On Mon, 16 Jun 2008 08:22:49 -0400
> Mathieu Desnoyers <compudj@...stal.dyndns.org> wrote:
> 
> > Hi Eduard,
> 
> Hi.
> 
> > Two objections against this. First, taking a spinlock _is_ slow in SMP
> > because it involves synchronized atomic operations.
> 
> In any case, if we use relay in a hot path, we are doing debugging, so
> a couple of atomic operations won't be a big problem. Along with
> setting affinity, this shouldn't be a problem.

Ehm no, that is a completely false claim. Relay speed does matter, a
great deal. We rely heavily on eg blktrace being as ligt weight as
possible to capture millions of events in a very short time frame
without impacting performance too much. Relay is NOT just for debugging.

So I completely agree with Mathieu here, and I'm not a big fan of the
proposed solution.

-- 
Jens Axboe

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