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Date: Thu, 7 Feb 2002 23:09:16 +0100 (CET)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: <mingo@elte.hu>
To: yodaiken <yodaiken@fsmlabs.com>
Cc: Martin Wirth <Martin.Wirth@dlr.de>,
        linux-kernel <linux-kernel@vger.kernel.org>, akpm <akpm@zip.com.au>,
        torvalds <torvalds@transmet.com>, rml <rml@tech9.net>,
        nigel <nigel@nrg.org>
Subject: Re: [RFC] New locking primitive for 2.5
In-Reply-To: <20020207125601.A21354@hq.fsmlabs.com>
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On Thu, 7 Feb 2002, yodaiken wrote:

> So what's the difference between combi_spin and combi_mutex?
> combi_spin becomes
> 	if not mutex locked, spin
> 	else sleep
> Bizzare

no, the real optimization is that when spin meets spin, they will not
mutex. If a mutex-user has it then spins turn into mutex, but that (is
supposed to) happen rarely.

i think one example *could* be to turn inode->i_sem into a combi-lock. Eg.
generic_file_llseek() could use the spin variant.

this is a real performance problem, i've seen scheduling storms in
dbench-type runs due to llseek taking the inode semaphore.

whether combi-locks truly bring performance benefits remains to be seen,
but the patch definitely needs to provide some working example and some
hard numbers for some real workload.

	Ingo

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