linux-kernel - Re: [RFC] Documentation about unaligned memory access

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Date:	Thu, 22 Nov 2007 22:18:28 -0800 (PST)
From:	dean gaudet <dean@...tic.org>
To:	Alan Cox <alan@...rguk.ukuu.org.uk>
cc:	Daniel Drake <dsd@...too.org>, linux-kernel@...r.kernel.org,
	davem@...emloft.net, kune@...ne-taler.de, johannes@...solutions.net
Subject: Re: [RFC] Documentation about unaligned memory access

On Fri, 23 Nov 2007, Alan Cox wrote:

> Its usually faster if you don't misalign on x86 as well.

i'm not sure if i agree with "usually"... but i know you (alan) are 
probably aware of the exact requirements of the hw.

for everyone else:

on intel x86 processors an access is unaligned only if it crosses a 
cacheline boundary (64 bytes).  otherwise it's aligned.  the penalty for 
crossing a cacheline boundary varies from ~12 cycles (core2) to many 
dozens of cycles (p4).

on AMD x86 pre-family 10h the boundary is 8 bytes, and on fam 10h it's 16 
bytes.  the penalty is a mere 3 cycles if an access crosses the specified 
boundary.

if you're making <= 4 byte accesses i recommend not worrying about 
alignment on x86.  it's pretty hard to beat the hardware support.

i curse all the RISC and embedded processor designers who pretend 
unaligned accesses are something evil and to be avoided.  in case you're 
worried, MIPS patent 4,814,976 expired in december 2006 :)

-dean
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