linux-kernel - Re: .. anybody know of any filesystems that depend on the exact VFS 'namehash' implementation?

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Date:	Thu, 01 Mar 2012 16:46:46 -0800
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: .. anybody know of any filesystems that depend on the exact VFS 'namehash' implementation?

Linus Torvalds <torvalds@...ux-foundation.org> writes:

> So I'm doing my normal profiling ("empty kernel build" is my favorite
> one), and link_path_walk() and __d_lookup_rcu() remain some of the
> hottest kernel functions due to their per-character loops.
>
> I can improve __d_lookup_rcu() on my machine by what appears to be
> around 15% by doing things a "unsigned long" at a time (it would be an
> option that only works on little-endian and with cheap unaligned
> accesses, although the big-endian modifications should be pretty
> trivial).

There should be generally better modern general hash algorithms around,
like murmur, cityhash or snoopy. Perhaps even the fnv we have in tree,
but it's somewhat dated by know.

They all have larger code, but if it's really that hot it would be worth
it.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only
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