linux-kernel - Re: [PATCH 2.6.21 review I] [21/25] x86_64: a memcpy that tries to reduce cache pressure

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Date:	Tue, 13 Feb 2007 12:27:14 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andi Kleen <ak@...e.de>
CC:	"Bryan O'Sullivan" <bos@...hscale.com>,
	Roland Dreier <rolandd@...co.com>, patches@...-64.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.21 review I] [21/25] x86_64: a memcpy that tries to
 reduce cache pressure

Andi Kleen a écrit :
> From: "Bryan O'Sullivan" <bos@...hscale.com>
> 
> This copy routine is memcpy-compatible, but on some architectures will use
> cache-bypassing loads to avoid bringing the source data into the cache.
> 
> One case where this is useful is when a device issues a DMA to a memory
> region, and the CPU must copy the DMAed data elsewhere before doing any work
> with it.  Since the source data is read-once, write-never from the CPU's
> perspective, caching the data at those addresses can only evict potentially
> useful data.
> 
> We provide an x86_64 implementation that uses SSE non-temporal loads, and a
> generic version that falls back to plain memcpy.

> +	movq	%r11, 56(%rdi)
> +	addq	%rcx, %rdi
> +	cmpq	%rdx, %rcx	/* is rdx >= 64? */
> +	jbe	.L42
> +	sfence
> +	orl	%edx, %edx
> +	je	.L33

I have three questions/remarks

1) Just curious why sfence is necessary here ?

2) Shouldnt we use this for large buffers, and restrict them to a size 
multiple of 64, to avoid all these conditional branches ?

3) Also, the first 128 bytes of the source buffer will be bring into cache.

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