Message-ID: <3D89176B.40FFD09B@digeo.com>
Date: Wed, 18 Sep 2002 17:16:43 -0700
From: Andrew Morton <akpm@digeo.com>
MIME-Version: 1.0
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
CC: "David S. Miller" <davem@redhat.com>, taka@valinux.co.jp,
       neilb@cse.unsw.edu.au, linux-kernel@vger.kernel.org,
       nfs@lists.sourceforge.net
Subject: Re: [PATCH] zerocopy NFS for 2.5.36
References: <20020918.160057.17194839.davem@redhat.com> <1032393277.24895.8.camel@irongate.swansea.linux.org.uk>
Content-Type: text/plain; charset=us-ascii
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2441
Lines: 64

Alan Cox wrote:
> 
> On Thu, 2002-09-19 at 00:00, David S. Miller wrote:
> > It was discussed long ago that csum_and_copy_from_user() performs
> > better than plain copy_from_user() on x86.  I do not remember all
> 
> The better was a freak of PPro/PII scheduling I think
> 
> > details, but I do know that using copy_from_user() is not a real
> > improvement at least on x86 architecture.
> 
> The same as bit is easy to explain. Its totally memory bandwidth limited
> on current x86-32 processors. (Although I'd welcome demonstrations to
> the contrary on newer toys)

Nope.  There are distinct alignment problems with movsl-based
memcpy on PII and (at least) "Pentium III (Coppermine)", which is
tested here:

copy_32 uses movsl.  copy_duff just uses a stream of "movl"s

Time uncached-to-uncached memcpy, source and dest are 8-byte-aligned:

akpm:/usr/src/cptimer> ./cptimer -d -s     
nbytes=10240  from_align=0, to_align=0
    copy_32: copied 19.1 Mbytes in 0.078 seconds at 243.9 Mbytes/sec
__copy_duff: copied 19.1 Mbytes in 0.090 seconds at 211.1 Mbytes/sec

OK, movsl wins.   But now give the source address 8+1 alignment:

akpm:/usr/src/cptimer> ./cptimer -d -s -f 1
nbytes=10240  from_align=1, to_align=0
    copy_32: copied 19.1 Mbytes in 0.158 seconds at 120.8 Mbytes/sec
__copy_duff: copied 19.1 Mbytes in 0.091 seconds at 210.3 Mbytes/sec

The "movl"-based copy wins.  By miles.

Make the source 8+4 aligned:

akpm:/usr/src/cptimer> ./cptimer -d -s -f 4
nbytes=10240  from_align=4, to_align=0
    copy_32: copied 19.1 Mbytes in 0.134 seconds at 142.1 Mbytes/sec
__copy_duff: copied 19.1 Mbytes in 0.089 seconds at 214.0 Mbytes/sec

So movl still beats movsl, by lots.

I have various scriptlets which generate the entire matrix.

I think I ended up deciding that we should use movsl _only_
when both src and dsc are 8-byte-aligned.  And that when you
multiply the gain from that by the frequency*size with which
funny alignments are used by TCP the net gain was 2% or something.

It needs redoing.  These differences are really big, and this
is the kernel's most expensive function.

A little project for someone.

The tools are at http://www.zip.com.au/~akpm/linux/cptimer.tar.gz
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/