linux-kernel - Re: Word-at-a-time dcache name accesses (was Re: .. anybody know ofany filesystems that depend on the exact VFS 'namehash' implementation?)

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Message-ID: <20120304043107.GC22197@one.firstfloor.org>
Date:	Sun, 4 Mar 2012 05:31:07 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	torvalds@...ux-foundation.org, andi@...stfloor.org, hpa@...or.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	viro@...iv.linux.org.uk
Subject: Re: Word-at-a-time dcache name accesses (was Re: .. anybody know ofany filesystems that depend on the exact VFS 'namehash' implementation?)

On Sun, Mar 04, 2012 at 11:27:01AM +0900, Tetsuo Handa wrote:
> A passer-by's mumble.
> 
> We are discussing about systems with huge "struct dentry" instances where
> selecting a hash list based on "struct qstr"->hash cannot effectively narrow
> down candidates to compare, aren't we?

This is effectively a simple bloom filter. 

Would be only worth it if longer hash chains are common. And if that's the case
we should probably have either a larger table or a better hash (or both)

-Andi
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