Re: [PATCH] virtio-net: parameterize min ring num_free for virtio receive

Jason Wang
2019-07-19T02:37:06Z

On 2019/7/18 下午10:43, Michael S. Tsirkin wrote:
> On Thu, Jul 18, 2019 at 10:42:47AM -0400, Michael S. Tsirkin wrote:
>> On Thu, Jul 18, 2019 at 10:01:05PM +0800, Jason Wang wrote:
>>> On 2019/7/18 下午9:04, Michael S. Tsirkin wrote:
>>>> On Thu, Jul 18, 2019 at 12:55:50PM +0000, ? jiang wrote:
>>>>> This change makes ring buffer reclaim threshold num_free configurable
>>>>> for better performance, while it's hard coded as 1/2 * queue now.
>>>>> According to our test with qemu + dpdk, packet dropping happens when
>>>>> the guest is not able to provide free buffer in avail ring timely.
>>>>> Smaller value of num_free does decrease the number of packet dropping
>>>>> during our test as it makes virtio_net reclaim buffer earlier.
>>>>>
>>>>> At least, we should leave the value changeable to user while the
>>>>> default value as 1/2 * queue is kept.
>>>>>
>>>>> Signed-off-by: jiangkidd<jiangkidd@hotmail.com>
>>>> That would be one reason, but I suspect it's not the
>>>> true one. If you need more buffer due to jitter
>>>> then just increase the queue size. Would be cleaner.
>>>>
>>>>
>>>> However are you sure this is the reason for
>>>> packet drops? Do you see them dropped by dpdk
>>>> due to lack of space in the ring? As opposed to
>>>> by guest?
>>>>
>>>>
>>> Besides those, this patch depends on the user to choose a suitable threshold
>>> which is not good. You need either a good value with demonstrated numbers or
>>> something smarter.
>>>
>>> Thanks
>> I do however think that we have a problem right now: try_fill_recv can
>> take up a long time during which net stack does not run at all. Imagine
>> a 1K queue - we are talking 512 packets. That's exceessive.

Yes, we will starve a fast host in this case.

>> napi poll
>> weight solves a similar problem, so it might make sense to cap this at
>> napi_poll_weight.
>>
>> Which will allow tweaking it through a module parameter as a
>> side effect :) Maybe just do NAPI_POLL_WEIGHT.
> Or maybe NAPI_POLL_WEIGHT/2 like we do at half the queue ;). Please
> experiment, measure performance and let the list know
>
>> Need to be careful though: queues can also be small and I don't think we
>> want to exceed queue size / 2, or maybe queue size - napi_poll_weight.
>> Definitely must not exceed the full queue size.

Looking at intel, it uses 16 and i40e uses 32.  It looks to me
NAPI_POLL_WEIGHT/2 is better.

Jiang, want to try that and post a new patch?

Thanks

>>
>> --
>> MST