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Date: Thu, 5 Aug 2010 22:47:49 +0200
From: Dominik Brodowski <linux@dominikbrodowski.net>
To: linux-kernel@vger.kernel.org, mingo@redhat.com, peterz@infradead.org
Cc: Chris Mason <chris.mason@oracle.com>, josef@redhat.com,
        Valdis.Kletnieks@vt.edu,
        Michael Monnerie <michael.monnerie@is.it-management.at>,
        Christoph Hellwig <hch@infradead.org>, linux-raid@vger.kernel.org,
        xfs@oss.sgi.com, dm-devel@redhat.com
Subject: Performance impact of CONFIG_DEBUG? direct-io test case
Message-ID: <20100805204749.GA6318@comet.dominikbrodowski.net>
Mail-Followup-To: Dominik Brodowski <linux@dominikbrodowski.net>,
	linux-kernel@vger.kernel.org, mingo@redhat.com,
	peterz@infradead.org, Chris Mason <chris.mason@oracle.com>,
	josef@redhat.com, Valdis.Kletnieks@vt.edu,
	Michael Monnerie <michael.monnerie@is.it-management.at>,
	Christoph Hellwig <hch@infradead.org>, linux-raid@vger.kernel.org,
	xfs@oss.sgi.com, dm-devel@redhat.com
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How large is the performance impact of CONFIG_DEBUG? Well, for the test
workload I've been working with lately,

dd if=<device> of=/dev/zero bs=8k count=100000 iflag=direct

where <device> is a dm-crypted LVM volume consisting of several
partitions on a notebook pata harddisk, I get the following results:

1) best results are ~ 28 MB/s

2) Enabling CONFIG_DEBUG_LOCK_ALLOC, which also means CONFIG_LOCKDEP
   being enabled, causes the transfer rate to decrease by ~ 1.2 MB/s

3) Enabling CONFIG_DEBUG_SPINLOCK && CONFIG_DEBUG_MUTEXTES or
   CONFIG_DEBUG_SPINLOCK_SLEEP=y costs about ~ 0.4 MB/s each

4) Enabling all of the following options:
	CONFIG_DEBUG_RT_MUTEXES
	CONFIG_DEBUG_PI_LIST
	CONFIG_PROVE_LOCKING
	CONFIG_LOCK_STAT
	CONFIG_DEBUG_LOCKDEP
   costs another ~ 5 MB/s.

So, for this test case, the performance impact of (some) CONFIG_DEBUG
options is highly significant, here by about 25 %.

Best,
	Dominik
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