On Tue, Feb 25, 2014 at 02:53:58PM +0800, Jason Wang wrote: > We used to stop the handling of tx when the number of pending DMAs > exceeds VHOST_MAX_PEND. This is used to reduce the memory occupation > of both host and guest. But it was too aggressive in some cases, since > any delay or blocking of a single packet may delay or block the guest > transmission. Consider the following setup: > > +-----+ +-----+ > | VM1 | | VM2 | > +--+--+ +--+--+ > | | > +--+--+ +--+--+ > | tap0| | tap1| > +--+--+ +--+--+ > | | > pfifo_fast htb(10Mbit/s) > | | > +--+--------------+---+ > | bridge | > +--+------------------+ > | > pfifo_fast > | > +-----+ > | eth0|(100Mbit/s) > +-----+ > > - start two VMs and connect them to a bridge > - add an physical card (100Mbit/s) to that bridge > - setup htb on tap1 and limit its throughput to 10Mbit/s > - run two netperfs in the same time, one is from VM1 to VM2. Another is > from VM1 to an external host through eth0. > - result shows that not only the VM1 to VM2 traffic were throttled but > also the VM1 to external host through eth0 is also throttled somehow. > > This is because the delay added by htb may lead the delay the finish > of DMAs and cause the pending DMAs for tap0 exceeds the limit > (VHOST_MAX_PEND). In this case vhost stop handling tx request until > htb send some packets. The problem here is all of the packets > transmission were blocked even if it does not go to VM2. > > We can solve this issue by relaxing it a little bit: switching to use > data copy instead of stopping tx when the number of pending DMAs > exceed the VHOST_MAX_PEND. This is safe because: > > - The number of pending DMAs were still limited by VHOST_MAX_PEND > - The out of order completion during mode switch can make sure that > most of the tx buffers were freed in time in guest. > > So even if about 50% packets were delayed in zero-copy case, vhost > could continue to do the transmission through data copy in this case. > > Test result: > > Before this patch: > VM1 to VM2 throughput is 9.3Mbit/s > VM1 to External throughput is 40Mbit/s > > After this patch: > VM1 to VM2 throughput is 9.3Mbit/s > Vm1 to External throughput is 93Mbit/s Would like to see CPU utilization #s as well. > Simple performance test on 40gbe shows no obvious changes in > throughput after this patch. > > The patch only solve this issue when unlimited sndbuf. We still need a > solution for limited sndbuf. > > Cc: Michael S. Tsirkin <mst@xxxxxxxxxx> > Cc: Qin Chuanyu <qinchuanyu@xxxxxxxxxx> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx> I think this needs some thought. In particular I think this works because VHOST_MAX_PEND is much smaller than the ring size. Shouldn't max_pend then be tied to the ring size if it's small? Another question is about stopping vhost: ATM it's waiting for skbs to complete. Should we maybe hunt down skbs queued and destroy them instead? I think this happens when a device is removed. Thoughts? > --- > drivers/vhost/net.c | 17 +++++++---------- > 1 file changed, 7 insertions(+), 10 deletions(-) > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c > index a0fa5de..3e96e47 100644 > --- a/drivers/vhost/net.c > +++ b/drivers/vhost/net.c > @@ -345,7 +345,7 @@ static void handle_tx(struct vhost_net *net) > .msg_flags = MSG_DONTWAIT, > }; > size_t len, total_len = 0; > - int err; > + int err, num_pends; > size_t hdr_size; > struct socket *sock; > struct vhost_net_ubuf_ref *uninitialized_var(ubufs); > @@ -366,13 +366,6 @@ static void handle_tx(struct vhost_net *net) > if (zcopy) > vhost_zerocopy_signal_used(net, vq); > > - /* If more outstanding DMAs, queue the work. > - * Handle upend_idx wrap around > - */ > - if (unlikely((nvq->upend_idx + vq->num - VHOST_MAX_PEND) > - % UIO_MAXIOV == nvq->done_idx)) > - break; > - > head = vhost_get_vq_desc(&net->dev, vq, vq->iov, > ARRAY_SIZE(vq->iov), > &out, &in, > @@ -405,9 +398,13 @@ static void handle_tx(struct vhost_net *net) > break; > } > > + num_pends = likely(nvq->upend_idx >= nvq->done_idx) ? > + (nvq->upend_idx - nvq->done_idx) : > + (nvq->upend_idx + UIO_MAXIOV - > + nvq->done_idx); > + > zcopy_used = zcopy && len >= VHOST_GOODCOPY_LEN > - && (nvq->upend_idx + 1) % UIO_MAXIOV != > - nvq->done_idx > + && num_pends <= VHOST_MAX_PEND > && vhost_net_tx_select_zcopy(net); > > /* use msg_control to pass vhost zerocopy ubuf info to skb */ > -- > 1.8.3.2 -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
