linux-ext4 - ext2 lock contention

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Date:	Fri, 15 Sep 2006 20:33:46 +0000
From:	Dave Kleikamp <shaggy@...tin.ibm.com>
To:	Valerie Henson <val_henson@...ux.intel.com>
Cc:	ext4 development <linux-ext4@...r.kernel.org>
Subject: ext2 lock contention

Val,
Do you remember this patch:
[PATCH] Make EXT2_DEBUG work again
http://www.kernel.org/git/gitweb.cgi?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=21730eed11de42f22afcbd43f450a1872a0b5ea1

The second part causes a 30% performance degradation running dbench with
16 threads.

> Second, debug versions of ext2_count_free_{inodes/blocks} reacquires
> superblock lock.  Moved lock into callers.

The problem is that before your patch, the superblock lock was taken
only when EXT2_DEBUG was enabled, but with your patch, the superblock
lock is held during the entire call to ext2_statfs() with our without
EXT2_DEBUG defined.

Was there a reason the lock needed to be held across both calls?  Since
the patch didn't mention fixing the non-debug case, I assume the lack of
the lock in the non-debug case was okay.
-- 
David Kleikamp
IBM Linux Technology Center

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