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Re: [Qemu-devel] memcpy speed (Re: [PATCH v2] netmap backend (revised))
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] memcpy speed (Re: [PATCH v2] netmap backend (revised)) |
Date: |
Thu, 24 Jan 2013 09:54:19 +0100 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On Wed, Jan 23, 2013 at 06:55:59PM -0800, Luigi Rizzo wrote:
> On Wed, Jan 23, 2013 at 8:03 AM, Luigi Rizzo <address@hidden> wrote:
>
> > > I'm even doubtful that it's always a win on FreeBSD. You have a
> > > threshold to fall back to bcopy() and who knows what the "best" value
> > > for various CPUs is.
> >
> > indeed.
> > With the attached program (which however might be affected by the
> > fact that data is not used after copying) it seems that on a recent
> > linux (using gcc 4.6.2) the fastest is __builtin_memcpy()
> >
> > ./testlock -m __builtin_memcpy -l 64
> >
> > (by a factor of 2 or more) whereas all the other methods have
> > approximately the same speed.
> >
>
> never mind, pilot error. in my test program i had swapped the
> arguments to __builtin_memcpy(). With the correct ones,
> __builtin_memcpy() == bcopy == memcpy on both machines,
> and never faster than the pkt_copy().
Are the bcopy()/memcpy() calls given a length that is a multiple of 64 bytes?
IIUC pkt_copy() assumes 64-byte multiple lengths and that optimization
can matches with memcpy(dst, src, (len + 63) & ~63). Maybe it helps and
at least ensures they are doing equal amounts of byte copying.
Stefan
- [Qemu-devel] [PATCH v2] netmap backend (revised), Luigi Rizzo, 2013/01/22
- Re: [Qemu-devel] [PATCH v2] netmap backend (revised), Anthony Liguori, 2013/01/22
- Re: [Qemu-devel] [PATCH v2] netmap backend (revised), Stefan Hajnoczi, 2013/01/23
- Re: [Qemu-devel] [PATCH v2] netmap backend (revised), Luigi Rizzo, 2013/01/23
- Re: [Qemu-devel] [PATCH v2] netmap backend (revised), Stefan Hajnoczi, 2013/01/23
- [Qemu-devel] memcpy speed (Re: [PATCH v2] netmap backend (revised)), Luigi Rizzo, 2013/01/23
- Re: [Qemu-devel] memcpy speed (Re: [PATCH v2] netmap backend (revised)), Luigi Rizzo, 2013/01/23
- Re: [Qemu-devel] memcpy speed (Re: [PATCH v2] netmap backend (revised)),
Stefan Hajnoczi <=
- Re: [Qemu-devel] memcpy speed (Re: [PATCH v2] netmap backend (revised)), Luigi Rizzo, 2013/01/24
- Re: [Qemu-devel] memcpy speed (Re: [PATCH v2] netmap backend (revised)), Stefan Hajnoczi, 2013/01/25
- Re: [Qemu-devel] memcpy speed (Re: [PATCH v2] netmap backend (revised)), Luigi Rizzo, 2013/01/28
- Re: [Qemu-devel] memcpy speed (Re: [PATCH v2] netmap backend (revised)), Paolo Bonzini, 2013/01/24