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Date: Wed, 04 Sep 2013 15:33:50 +0000
From: Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@hp.com>
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To: Andreas Dilger <adilger@dilger.ca>
CC: T Makphaibulchoke <tmac@hp.com>, "Theodore Ts'o" <tytso@mit.edu>,
        Al Viro <viro@zeniv.linux.org.uk>,
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        aswin@hp.com, Linus Torvalds <torvalds@linux-foundation.org>,
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Subject: Re: [PATCH v3 0/2] ext4: increase mbcache scalability
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On 09/04/2013 08:00 PM, Andreas Dilger wrote:

> 
> In the past, I've raised the question of whether mbcache is even
> useful on real-world systems.  Essentially, this is providing a
> "deduplication" service for ext2/3/4 xattr blocks that are identical.
> The question is how often this is actually the case in modern use?
> The original design was for allowing external ACL blocks to be
> shared between inodes, at a time when ACLs where pretty much the
> only xattrs stored on inodes.
> 
> The question now is whether there are common uses where all of the
> xattrs stored on multiple inodes are identical?  If that is not the
> case, mbcache is just adding overhead and should just be disabled
> entirely instead of just adding less overhead.
> 
> There aren't good statistics on the hit rate for mbcache, but it
> might be possible to generate some with systemtap or similar to
> see how often ext4_xattr_cache_find() returns NULL vs. non-NULL.
> 
> Cheers, Andreas
> 

Thanks Andreas for the comments.  Since I'm not familiar with systemtap, I'm thinking probably the quickest and simplest way is to re-run aim7 and swing bench with mbcache disabled for comparison. Please let me know if you have any other benchmark suggestion or if you think systemtap on ext4_xattr_cache_find() would give a more accurate measurement.

Thanks,
Mak.