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Re: Promblem with PREF (prefetching) in memcpy

To: "Kevin D. Kissell" <>
Subject: Re: Promblem with PREF (prefetching) in memcpy
From: Alan Cox <>
Date: 04 Oct 2002 14:44:06 +0100
Cc: Carsten Langgaard <>, Dominic Sweetman <>, Ralf Baechle <>,
In-reply-to: <010e01c26ba8$2c9400d0$10eca8c0@grendel>
Original-recipient: rfc822;
References: <><> <><1033734968.31839.5.camel@irongate.swansea.linux> <00fe01c26ba6$04943480$10eca8c0@grendel> <> <010e01c26ba8$2c9400d0$10eca8c0@grendel>
On Fri, 2002-10-04 at 14:15, Kevin D. Kissell wrote:
> Which is excatly the point that Carsten was raising when he started this 
> thread!
> The question is how, i.e. throttle memcpy or thow away a "guard band" of RAM?

The x86 code basically says

        while(over 320 bytes left)
                prefetch ahead
                copy bits
        while(bytes left)
                copy bits

You also have to watch the prefetching in the tcp checksum code. We hit
an x86 bug there also very recently. That one is new but shows up with
the zero copy tcp nfs

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