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. 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
