From: "Michael S. Tsirkin" <mst@redhat.com>
To: Willem de Bruijn <willemdebruijn.kernel@gmail.com>
Cc: Koichiro Den <den@klaipeden.com>,
Jason Wang <jasowang@redhat.com>,
virtualization@lists.linux-foundation.org,
Network Development <netdev@vger.kernel.org>
Subject: Re: [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi
Date: Sat, 26 Aug 2017 02:32:37 +0300 [thread overview]
Message-ID: <20170826022744-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <CAF=yD-+1wheMmC+HFKe_B_ULO0Mmh6HMSEbYY5D-HgqxVJee6A@mail.gmail.com>
On Fri, Aug 25, 2017 at 06:44:36PM -0400, Willem de Bruijn wrote:
> >> >> > 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
>
> Oh, right. Though those are due to vring_desc pool exhaustion
> rather than an upper bound on latency of any single packet.
>
> Limiting the number of zerocopy packets in flight to some fraction
> of the ring ensures that fast flows can always grab a slot.
> Running
> out of ubuf_info slots reverts to copy, so indirectly does this. But
> I read it correclty the zerocopy pool may be equal to or larger than
> the descriptor pool. Should we refine the zcopy_used test
>
> (nvq->upend_idx + 1) % UIO_MAXIOV != nvq->done_idx
>
> to also return false if the number of outstanding ubuf_info is greater
> than, say, vq->num >> 1?
We'll need to think about where to put the threshold, but I think it's
a good idea.
Maybe even a fixed number, e.g. max(vq->num >> 1, X) to limit host
resources.
In a sense it still means once you run out of slots zcopt gets disabled possibly permanently.
Need to experiment with some numbers.
--
MST
next prev parent reply other threads:[~2017-08-25 23:32 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-08-19 6:38 [PATCH net-next] virtio-net: invoke zerocopy callback on xmit path if no tx napi Koichiro Den
2017-08-20 20:49 ` Willem de Bruijn
2017-08-21 12:40 ` Koichiro Den
2017-08-22 12:11 ` Willem de Bruijn
2017-08-22 14:04 ` Koichiro Den
2017-08-22 17:19 ` Willem de Bruijn
2017-08-23 14:26 ` Koichiro Den
2017-08-21 12:33 ` Jason Wang
2017-08-21 12:58 ` Koichiro Den
2017-08-21 15:41 ` Willem de Bruijn
2017-08-22 2:50 ` Jason Wang
2017-08-22 3:10 ` Willem de Bruijn
2017-08-22 11:47 ` Jason Wang
2017-08-22 13:42 ` Koichiro Den
2017-08-22 17:16 ` Willem de Bruijn
2017-08-23 14:24 ` Koichiro Den
2017-08-22 17:55 ` Michael S. Tsirkin
2017-08-22 18:01 ` David Miller
2017-08-22 18:28 ` Eric Dumazet
2017-08-22 18:39 ` Michael S. Tsirkin
2017-08-23 14:28 ` Koichiro Den
2017-08-23 14:47 ` Koichiro Den
2017-08-23 15:20 ` Willem de Bruijn
2017-08-23 22:57 ` Michael S. Tsirkin
2017-08-24 3:28 ` Willem de Bruijn
2017-08-24 4:34 ` Michael S. Tsirkin
2017-08-24 13:50 ` Michael S. Tsirkin
2017-08-24 20:20 ` Willem de Bruijn
2017-08-24 20:50 ` Michael S. Tsirkin
2017-08-25 22:44 ` Willem de Bruijn
2017-08-25 23:32 ` Michael S. Tsirkin [this message]
2017-08-26 1:03 ` Willem de Bruijn
2017-08-29 19:35 ` Willem de Bruijn
2017-08-29 19:42 ` Michael S. Tsirkin
2017-08-29 19:53 ` Willem de Bruijn
2017-08-29 20:40 ` Michael S. Tsirkin
2017-08-29 22:55 ` Willem de Bruijn
2017-08-30 1:45 ` Jason Wang
2017-08-30 3:11 ` Willem de Bruijn
2017-09-01 3:08 ` Jason Wang
2017-08-31 14:30 ` Willem de Bruijn
2017-09-01 3:25 ` Jason Wang
2017-09-01 16:15 ` Willem de Bruijn
2017-09-01 16:17 ` Willem de Bruijn
2017-09-04 3:03 ` Jason Wang
2017-09-05 14:09 ` Willem de Bruijn
2017-09-06 3:27 ` Michael S. Tsirkin
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