linux-kernel - Re: Fast memcpy patch

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Date:	Wed, 23 Nov 2011 21:38:49 +0100
From:	"N. Coesel" <nico@...dev.nl>
To:	Sasha Levin <levinsasha928@...il.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Fast memcpy patch

Sasha,

At 14:04 23-11-2011, Sasha Levin wrote:
>On Wed, 2011-11-23 at 13:51 +0100, N. Coesel wrote:
> > Sasha,
> >
> > At 13:10 23-11-2011, Sasha Levin wrote:
> > >On Wed, 2011-11-23 at 12:25 +0100, N. Coesel wrote:
> > > > Dear readers,
> > > > I noticed the Linux kernel still uses a byte-by-byte copy method for
> > > > memcpy. Since most memory allocations are aligned to the integer size
> > > > of a cpu it is often faster to copy by using the CPU's native word
> > > > size. The patch below does that. The code is already at work in many
> > > > 16 and 32 bit embedded products. It should also work for 64 bit
> > > > platforms. So far I only tested 16 and 32 bit platforms.
> > >
> > >[snip]
> > >
> > >memcpy (along with other mem* functions) are arch specific - for
> > >example, look at arch/x86/lib/memcpy_64.S for the implementation(s) for
> > >x86.
> > >
> > >The code under lib/string.c is simple and should work on all platforms
> > >(and is probably not being used anywhere anymore).
> >
> > Thanks for pointing that out. Currently my primary target is ARM. It
> > seems the memcpy for that arch uses byte-by-byte copying as well with
> > some loop unrolling. I modified the code so it tries to use
> > word-by-word copy if the pointers are aligned on word boundaries, if
> > not it reverts to the old method. For clarity: by word I mean the
> > CPU's native bus width. In case of ARM that's (still) 32 bit.
>
>I don't think we're looking at the same file.
>
>For arm it's arch/arm/lib/copy_template.S, right? Or are you talking
>about something else?

I was looking somewhere else indeed. There are a lot of versions of 
memcpy in the kernel :-) Thanks for pointing me to the right file. 
The asm stuff looks pretty nifty by using the load/store multiple 
instructions. Too bad. I was hoping to get some extra speed with a 
quick fix. I need to copy several MB from a driver.

Nico Coesel


>--
>
>Sasha.

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