linux-kernel - Re: [PATCH v3 0/2] ext4: increase mbcache scalability

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Message-ID: <5228A1AB.2030308@hp.com>
Date:	Thu, 05 Sep 2013 15:22:19 +0000
From:	Thavatchai Makphaibulchoke <thavatchai.makpahibulchoke@...com>
To:	Andreas Dilger <adilger@...ger.ca>
CC:	T Makphaibulchoke <tmac@...com>, Theodore Ts'o <tytso@....edu>,
	Al Viro <viro@...iv.linux.org.uk>,
	"linux-ext4@...r.kernel.org List" <linux-ext4@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org Devel" <linux-fsdevel@...r.kernel.org>,
	aswin@...com, Linus Torvalds <torvalds@...ux-foundation.org>,
	aswin_proj@...ups.hp.com
Subject: Re: [PATCH v3 0/2] ext4: increase mbcache scalability

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
> 

Looks like it's a bit harder to disable mbcache than I thought. I ended up adding code to collect the statics.

With selinux enabled, for new_fserver workload of aim7, there are a total of 0x7e05420100000000 ext4_xattr_cache_find() calls that result in a hit and 0xc100000000000000 calls that are not.  The number does not seem to favor the complete disabling of mbcache in this case.

Thanks,
Mak.


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