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

[Maxime Coquelin <maxime.coquelin@redhat.com>]

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.

Thanks,
Maxime