On Wed, Aug 23, 2017 at 11:28:24PM -0400, Willem de Bruijn wrote: > >> > * as a generic solution, if we were to somehow overcome the safety > >> > issue, track > >> > the delay and do copy if some threshold is reached could be an answer, > >> > but it's > >> > hard for now.> * so things like the current vhost-net implementation of > >> > deciding whether or not > >> > to do zerocopy beforehand referring the zerocopy tx error ratio is a > >> > point of > >> > practical compromise. > >> > >> The fragility of this mechanism is another argument for switching to tx > >> napi > >> as default. > >> > >> Is there any more data about the windows guest issues when completions > >> are not queued within a reasonable timeframe? What is this timescale and > >> do we really need to work around this. > > > > I think it's pretty large, many milliseconds. > > > > But I wonder what do you mean by "work around". Using buffers within > > limited time frame sounds like a reasonable requirement to me. > > Vhost-net zerocopy delays completions until the skb is really > sent. 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.
There's no issue with out of order. It does not break any guests AFAIK. > > 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. > > >> 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?
