linux-kernel - Performance impact of CONFIG

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Date:	Thu, 5 Aug 2010 22:47:49 +0200
From:	Dominik Brodowski <linux@...inikbrodowski.net>
To:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	peterz@...radead.org
Cc:	Chris Mason <chris.mason@...cle.com>, josef@...hat.com,
	Valdis.Kletnieks@...edu,
	Michael Monnerie <michael.monnerie@...it-management.at>,
	Christoph Hellwig <hch@...radead.org>,
	linux-raid@...r.kernel.org, xfs@....sgi.com, dm-devel@...hat.com
Subject: Performance impact of CONFIG_DEBUG? direct-io test case


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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