> > Hi Ciara, > > > Thanks for your feedback. The limitation is only placed on phy port queues > on the VP (vhost -> phy) path. VV path and PV path are not > > affected. > > Yes, you are right. VM to VM traffic is copied on transmit to the second VM. > > > > I would much rather put a requirement on tenants that their virtio drivers > > > need to allocate enough virtio packet buffers if they want their VM to use > > > zero-copy vhostuser ports. Or is the critical resource owned and > managed by > > > Qemu and we'd need a patch on Qemu to overcome this limit? > > Can you comment on that? Can a user also reduce the problem by > configuring > a) a larger virtio Tx queue size (up to 1K) in Qemu, or
Is this possible right now without modifying QEMU src? I think the size is hardcoded to 256 at the moment although it may become configurable in the future. If/when it does, we can test and update the docs if it does solve the problem. I don’t think we should suggest modifying the QEMU src as a workaround now. > b) a larger mempool for packets in Tx direction inside the guest (driver?) Using the DPDK driver in the guest & generating traffic via testpmd I modified the number of descriptors given to the virtio device from 512 (default) to 2048 & 4096 but it didn't resolve the issue unfortunately. > > > > > > > And what about increased packet drop risk due to shortened tx queues? > > > > I guess this could be an issue. If I had some data to back this up I would > include it in the documentation and mention the risk. > > If the risk is unacceptable to the user they may choose to not enable the > feature. It's disabled by default so shouldn't introduce an issue for > > the standard case. > > Yes, but it would be good to understand the potential drawback for a better > judgement of the trade-off between better raw throughput and higher loss > risk. I ran RFC2544 0% packet loss tests for ZC on & off (64B PVP) and observed the following: Max rate (Mpps) with 0% loss ZC Off 2599518 ZC On 1678758 As you suspected, there is a trade-off. I can mention this in the docs. Thanks, Ciara > > Regards, Jan _______________________________________________ dev mailing list [email protected] https://mail.openvswitch.org/mailman/listinfo/ovs-dev
