On Thu, Aug 24, 2017 at 04:20:39PM -0400, Willem de Bruijn wrote: > >> Traffic shaping can introduce msec timescale latencies. > >> > >> The delay may actually be a useful signal. If the guest does not > >> orphan skbs early, TSQ will throttle the socket causing host > >> queue build up. > >> > >> But, if completions are queued in-order, unrelated flows may be > >> throttled as well. Allowing out of order completions would resolve > >> this HoL blocking. > > > > We can allow out of order, no guests that follow virtio spec > > will break. But this won't help in all cases > > - a single slow flow can occupy the whole ring, you will not > > be able to make any new buffers available for the fast flow > > - what host considers a single flow can be multiple flows for guest > > > > There are many other examples. > > These examples are due to exhaustion of the fixed ubuf_info pool, > right? No - the ring size itself. > We could use dynamic allocation or a resizable pool if these > issues are serious enough. We need some kind of limit on how many requests a guest can queue in the host, or it's an obvious DoS attack vector. We used the ring size for that. > >> > Neither > >> > do I see why would using tx interrupts within guest be a work around - > >> > AFAIK windows driver uses tx interrupts. > >> > >> It does not address completion latency itself. What I meant was > >> that in an interrupt-driver model, additional starvation issues, > >> such as the potential deadlock raised at the start of this thread, > >> or the timer delay observed before packets were orphaned in > >> virtio-net in commit b0c39dbdc204, are mitigated. > >> > >> Specifically, it breaks the potential deadlock where sockets are > >> blocked waiting for completions (to free up budget in sndbuf, tsq, ..), > >> yet completion handling is blocked waiting for a new packet to > >> trigger free_old_xmit_skbs from start_xmit. > > > > This talk of potential deadlock confuses me - I think you mean we would > > deadlock if we did not orphan skbs in !use_napi - is that right? If you > > mean that you can drop skb orphan and this won't lead to a deadlock if > > free skbs upon a tx interrupt, I agree, for sure. > > Yes, that is what I meant. > > >> >> That is the only thing keeping us from removing the HoL blocking in vhost-net zerocopy. > >> > > >> > We don't enable network watchdog on virtio but we could and maybe > >> > should. > >> > >> Can you elaborate? > > > > The issue is that holding onto buffers for very long times makes guests > > think they are stuck. This is funamentally because from guest point of > > view this is a NIC, so it is supposed to transmit things out in > > a timely manner. If host backs the virtual NIC by something that is not > > a NIC, with traffic shaping etc introducing unbounded latencies, > > guest will be confused. > > That assumes that guests are fragile in this regard. A linux guest > does not make such assumptions. Yes it does. Examples above: > > - a single slow flow can occupy the whole ring, you will not > > be able to make any new buffers available for the fast flow > > - what host considers a single flow can be multiple flows for guest it's easier to see if you enable the watchdog timer for virtio. Linux supports that. > There are NICs with hardware > rate limiting, so I'm not sure how much of a leap host os rate > limiting is. I don't know what happens if these NICs hold onto packets for seconds. -- MST _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
