Re: [PATCH RFC v7 03/14] vhost: use batched get_vq_desc version

Eugenio Perez Martin
2020-06-10T16:19:20Z

On Wed, Jun 10, 2020 at 5:13 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>
> On Wed, Jun 10, 2020 at 02:37:50PM +0200, Eugenio Perez Martin wrote:
> > > +/* This function returns a value > 0 if a descriptor was found, or 0 if none were found.
> > > + * A negative code is returned on error. */
> > > +static int fetch_descs(struct vhost_virtqueue *vq)
> > > +{
> > > + int ret;
> > > +
> > > + if (unlikely(vq->first_desc >= vq->ndescs)) {
> > > + vq->first_desc = 0;
> > > + vq->ndescs = 0;
> > > + }
> > > +
> > > + if (vq->ndescs)
> > > + return 1;
> > > +
> > > + for (ret = 1;
> > > + ret > 0 && vq->ndescs <= vhost_vq_num_batch_descs(vq);
> > > + ret = fetch_buf(vq))
> > > + ;
> >
> > (Expanding comment in V6):
> >
> > We get an infinite loop this way:
> > * vq->ndescs == 0, so we call fetch_buf() here
> > * fetch_buf gets less than vhost_vq_num_batch_descs(vq); descriptors. ret = 1
> > * This loop calls again fetch_buf, but vq->ndescs > 0 (and avail_vq ==
> > last_avail_vq), so it just return 1
>
> That's what
> [PATCH RFC v7 08/14] fixup! vhost: use batched get_vq_desc version
> is supposed to fix.
>

Sorry, I forgot to include that fixup.

With it I don't see CPU stalls, but with that version latency has
increased a lot and I see packet lost:
+ ping -c 5 10.200.0.1
PING 10.200.0.1 (10.200.0.1) 56(84) bytes of data.
From 10.200.0.2 icmp_seq=1 Destination Host Unreachable
From 10.200.0.2 icmp_seq=2 Destination Host Unreachable
From 10.200.0.2 icmp_seq=3 Destination Host Unreachable
64 bytes from 10.200.0.1: icmp_seq=5 ttl=64 time=6848 ms

--- 10.200.0.1 ping statistics ---
5 packets transmitted, 1 received, +3 errors, 80% packet loss, time 76ms
rtt min/avg/max/mdev = 6848.316/6848.316/6848.316/0.000 ms, pipe 4
--

I cannot even use netperf.

If I modify with my proposed version:
+ ping -c 5 10.200.0.1
PING 10.200.0.1 (10.200.0.1) 56(84) bytes of data.
64 bytes from 10.200.0.1: icmp_seq=1 ttl=64 time=7.07 ms
64 bytes from 10.200.0.1: icmp_seq=2 ttl=64 time=0.358 ms
64 bytes from 10.200.0.1: icmp_seq=3 ttl=64 time=5.35 ms
64 bytes from 10.200.0.1: icmp_seq=4 ttl=64 time=2.27 ms
64 bytes from 10.200.0.1: icmp_seq=5 ttl=64 time=0.426 ms

[root@localhost ~]# netperf -H 10.200.0.1 -p 12865 -l 10 -t TCP_STREAM
MIGRATED TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to
10.200.0.1 () port 0 AF_INET
Recv Send Send
Socket Socket Message Elapsed
Size Size Size Time Throughput
bytes bytes bytes secs. 10^6bits/sec

131072 16384 16384 10.01 4742.36
[root@localhost ~]# netperf -H 10.200.0.1 -p 12865 -l 10 -t UDP_STREAM
MIGRATED UDP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to
10.200.0.1 () port 0 AF_INET
Socket Message Elapsed Messages
Size Size Time Okay Errors Throughput
bytes bytes secs # # 10^6bits/sec

212992 65507 10.00 9214 0 482.83
212992 10.00 9214 482.83

I will compare with the non-batch version for reference, but the
difference between the two is noticeable. Maybe it's worth finding a
good value for the if() inside fetch_buf?

Thanks!

> --
> MST
>