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

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Date:	Fri, 2 Mar 2012 16:17:58 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	"Ted Ts'o" <tytso@....edu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	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: Word-at-a-time dcache name accesses (was Re: .. anybody know of
 any filesystems that depend on the exact VFS 'namehash' implementation?)

On Fri, Mar 2, 2012 at 4:02 PM, Ted Ts'o <tytso@....edu> wrote:
> Stupid question.  Your patch requires unaligned accesses to not have a
> heavy penalty, right?  Wasn't it the case that some generations of x86
> had pretty large penalties for aligned accesses?  Is that something we
> need to worry about?

There are basically no x86's with heavy penalties.

Sure, unaligned accesses are often *slightly* more expensive,
especially if they cross the cache access boundary (which tends to be
8 bytes on older 32-bit cpu's, and generally 16 bytes on more modern
CPUs - so it's not that they are unaligned per se, but that they cross
the bank size). But even then, it's usually not a huge deal (ie it
takes up two read slots instead of just one).

There are x86 chips that are extremely bad at unaligned SSE/MMX
accesses, but not regular words.

                      Linus
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