Re: [Qemu-devel] [PATCH 1/2] vhost: enable any layout feature

[Maxime Coquelin]

On 10/10/2016 04:42 PM, Yuanhan Liu wrote:
> On Mon, Oct 10, 2016 at 02:40:44PM +0200, Maxime Coquelin wrote:
> > > > > At that time, a packet always use 2 descs. Since indirect desc is
> > > > > enabled (by default) now, the assumption is not true then. What's
> > > > > worse, it might even slow things a bit down. That should also be
> > > > > part of the reason why performance is slightly worse than before.
> > > > >
> > > > >         --yliu
> > > >
> > > > I'm not sure I get what you are saying
> > > >
> > > > > commit 1d41d77cf81c448c1b09e1e859bfd300e2054a98
> > > > > Author: Yuanhan Liu <address@hidden>
> > > > > Date:   Mon May 2 17:46:17 2016 -0700
> > > > >
> > > > >    vhost: optimize dequeue for small packets
> > > > >
> > > > >    A virtio driver normally uses at least 2 desc buffers for Tx: the
> > > > >    first for storing the header, and the others for storing the data.
> > > > >
> > > > >    Therefore, we could fetch the first data desc buf before the main
> > > > >    loop, and do the copy first before the check of "are we done yet?".
> > > > >    This could save one check for small packets that just have one data
> > > > >    desc buffer and need one mbuf to store it.
> > > > >
> > > > >    Signed-off-by: Yuanhan Liu <address@hidden>
> > > > >    Acked-by: Huawei Xie <address@hidden>
> > > > >    Tested-by: Rich Lane <address@hidden>
> > > >
> > > > This fast-paths the 2-descriptors format but it's not active
> > > > for indirect descriptors. Is this what you mean?
> > >
> > > Yes. It's also not active when ANY_LAYOUT is actually turned on.
> > > > Should be a simple matter to apply this optimization for indirect.
> > >
> > > Might be.
> >
> > If I understand the code correctly, indirect descs also benefit from this
> > optimization, or am I missing something?
>
> Aha..., you are right!

The interesting thing is that the patch I send on Thursday that removes
header access when no offload has been negotiated[0] seems to reduce
almost to zero the performance seen with indirect descriptors enabled.
I see this with 64 bytes packets using testpmd on both ends.

When I did the patch, I would have expected the same gain with both
modes, whereas I measured +1% for direct and +4% for indirect.

Maxime
[0]: http://dpdk.org/dev/patchwork/patch/16423/