Linux Virtualization - Jul 2019 - [PATCH] virtio-net: parameterize min ring num_free for virtio receive

On 2019/7/18 ??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 at 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

On Thu, Jul 18, 2019 at 10:01:05PM +0800, Jason Wang
wrote:
> 
> On 2019/7/18 ??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 at 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.  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.

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.

-- 
MST

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 ??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 at 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.  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.
> 
> -- 
> MST