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. 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
