linux-ext4 - Re: [PATCH] x86_64/lib: improve the performance of memmove

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Date:	Thu, 16 Sep 2010 08:48:25 +0200 (CEST)
From:	"Andi Kleen" <andi@...stfloor.org>
To:	miaox@...fujitsu.com
Cc:	"Andi Kleen" <andi@...stfloor.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>, "Theodore Ts'o" <tytso@....edu>,
	"Chris Mason" <chris.mason@...cle.com>,
	"Linux Kernel" <linux-kernel@...r.kernel.org>,
	"Linux Btrfs" <linux-btrfs@...r.kernel.org>,
	"Linux Ext4" <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH] x86_64/lib: improve the performance of memmove

> When the dest and the src do overlap and the memory area is large, memmove
> of
> x86_64 is very inefficient, and it led to bad performance, such as btrfs's
> file
> deletion performance. This patch improved the performance of memmove on
> x86_64
> by using __memcpy_bwd() instead of byte copy when doing large memory area
> copy
> (len > 64).


I still don't understand why you don't simply use a backwards
string copy (with std) ? That should be much simpler and
hopefully be as optimized for kernel copies on recent CPUs.

-Andi


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