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

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Mon, Oct 10, 2016 at 04:54:39PM +0200, Maxime Coquelin wrote:
> 
> 
> 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 <yuanhan.liu@linux.intel.com>
> >>>>>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 <yuanhan.liu@linux.intel.com>
> >>>>>   Acked-by: Huawei Xie <huawei.xie@intel.com>
> >>>>>   Tested-by: Rich Lane <rich.lane@bigswitch.com>
> >>>>
> >>>>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.

Didn't follow that.

> 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.

IIRC, I did a test before (remove those offload code piece), and the
performance was basically the same before and after that. Well, there
might be some small difference, say 1% as you said. But the result has
never been steady.

Anyway, I think your patch is good to have: I just didn't see v2.

	--yliu