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: <201203041127.IDI56216.FHOQMOVLtOFSJF@I-love.SAKURA.ne.jp>
Date:	Sun, 4 Mar 2012 11:27:01 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	torvalds@...ux-foundation.org, andi@...stfloor.org, hpa@...or.com
Cc:	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?)

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?

Then, I thought we can reconstruct candidates in a hash list.

For example, sort "struct dentry" based on return value of
strcmp(dentry1->d_name.name, dentry2->d_name.name).

For another example, embed another hash value (calculated using some algorithm
that is different from the one used for "struct qstr"->hash) into the leading 1
or 2 or 4 bytes of "struct dentry"->d_name.name (e.g.

  dentry1->d_name.name == { 0x12, 0x56, 0x34, 0x78, 'n', 'a', 'm', 'e', '1' }
  dentry2->d_name.name == { 0xab, 0xcd, 0xef, 0x01, 'n', 'a', 'm', 'e', '2' }

for 32 bits hash case, where 0x12563478 is hash value for name1 and 0xabcdef01
is hash value for name2) and compare the embedded hash (e.g.

  * (u32 *) dentry1->d_name.name == * (u32 *) dentry2->d_name.name

) before starting comparison of char array (e.g.

  memcmp(dentry1->d_name.name + 4, dentry2->d_name.name + 4, len)

).
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