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

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

On Tue, Oct 11, 2016 at 08:39:54AM +0200, Maxime Coquelin wrote:
> 
> 
> On 10/11/2016 08:04 AM, Yuanhan Liu wrote:
> >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.
> 
> I waited to gather some comments/feedback before sending the v2.
> I'll send it today or tomorrow.

Interesting, I saw a deadlock then: I haven't looked at the code
carefully once you said there is a v2, thus I'm waiting for it.
However, you are waitting for my review. :)

Anyway, I will take time to look at it shortly.

	--yliu