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

[Jason Wang <jasowang@xxxxxxxxxx>]

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@xxxxxxxxxxx>
> > > > 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