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Date:   Wed, 1 Jul 2020 12:43:09 +0200
From:   Eugenio Perez Martin <eperezma@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        linux-kernel@...r.kernel.org, kvm list <kvm@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH RFC v8 02/11] vhost: use batched get_vq_desc version

On Tue, Jun 23, 2020 at 6:15 PM Eugenio Perez Martin
<eperezma@...hat.com> wrote:
>
> On Mon, Jun 22, 2020 at 6:29 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> >
> > On Mon, Jun 22, 2020 at 06:11:21PM +0200, Eugenio Perez Martin wrote:
> > > On Mon, Jun 22, 2020 at 5:55 PM Michael S. Tsirkin <mst@...hat.com> wrote:
> > > >
> > > > On Fri, Jun 19, 2020 at 08:07:57PM +0200, Eugenio Perez Martin wrote:
> > > > > On Mon, Jun 15, 2020 at 2:28 PM Eugenio Perez Martin
> > > > > <eperezma@...hat.com> wrote:
> > > > > >
> > > > > > On Thu, Jun 11, 2020 at 5:22 PM Konrad Rzeszutek Wilk
> > > > > > <konrad.wilk@...cle.com> wrote:
> > > > > > >
> > > > > > > On Thu, Jun 11, 2020 at 07:34:19AM -0400, Michael S. Tsirkin wrote:
> > > > > > > > As testing shows no performance change, switch to that now.
> > > > > > >
> > > > > > > What kind of testing? 100GiB? Low latency?
> > > > > > >
> > > > > >
> > > > > > Hi Konrad.
> > > > > >
> > > > > > I tested this version of the patch:
> > > > > > https://lkml.org/lkml/2019/10/13/42
> > > > > >
> > > > > > It was tested for throughput with DPDK's testpmd (as described in
> > > > > > http://doc.dpdk.org/guides/howto/virtio_user_as_exceptional_path.html)
> > > > > > and kernel pktgen. No latency tests were performed by me. Maybe it is
> > > > > > interesting to perform a latency test or just a different set of tests
> > > > > > over a recent version.
> > > > > >
> > > > > > Thanks!
> > > > >
> > > > > I have repeated the tests with v9, and results are a little bit different:
> > > > > * If I test opening it with testpmd, I see no change between versions
> > > >
> > > >
> > > > OK that is testpmd on guest, right? And vhost-net on the host?
> > > >
> > >
> > > Hi Michael.
> > >
> > > No, sorry, as described in
> > > http://doc.dpdk.org/guides/howto/virtio_user_as_exceptional_path.html.
> > > But I could add to test it in the guest too.
> > >
> > > These kinds of raw packets "bursts" do not show performance
> > > differences, but I could test deeper if you think it would be worth
> > > it.
> >
> > Oh ok, so this is without guest, with virtio-user.
> > It might be worth checking dpdk within guest too just
> > as another data point.
> >
>
> Ok, I will do it!
>
> > > > > * If I forward packets between two vhost-net interfaces in the guest
> > > > > using a linux bridge in the host:
> > > >
> > > > And here I guess you mean virtio-net in the guest kernel?
> > >
> > > Yes, sorry: Two virtio-net interfaces connected with a linux bridge in
> > > the host. More precisely:
> > > * Adding one of the interfaces to another namespace, assigning it an
> > > IP, and starting netserver there.
> > > * Assign another IP in the range manually to the other virtual net
> > > interface, and start the desired test there.
> > >
> > > If you think it would be better to perform then differently please let me know.
> >
> >
> > Not sure why you bother with namespaces since you said you are
> > using L2 bridging. I guess it's unimportant.
> >
>
> Sorry, I think I should have provided more context about that.
>
> The only reason to use namespaces is to force the traffic of these
> netperf tests to go through the external bridge. To test netperf
> different possibilities than the testpmd (or pktgen or others "blast
> of frames unconditionally" tests).
>
> This way, I make sure that is the same version of everything in the
> guest, and is a little bit easier to manage cpu affinity, start and
> stop testing...
>
> I could use a different VM for sending and receiving, but I find this
> way a faster one and it should not introduce a lot of noise. I can
> test with two VM if you think that this use of network namespace
> introduces too much noise.
>
> Thanks!
>
> > > >
> > > > >   - netperf UDP_STREAM shows a performance increase of 1.8, almost
> > > > > doubling performance. This gets lower as frame size increase.

Regarding UDP_STREAM:
* with event_idx=on: The performance difference is reduced a lot if
applied affinity properly (manually assigning CPU on host/guest and
setting IRQs on guest), making them perform equally with and without
the patch again. Maybe the batching makes the scheduler perform
better.

> > > > >   - rests of the test goes noticeably worse: UDP_RR goes from ~6347
> > > > > transactions/sec to 5830

* Regarding UDP_RR, TCP_STREAM, and TCP_RR, proper CPU pinning makes
them perform similarly again, only a very small performance drop
observed. It could be just noise.
** All of them perform better than vanilla if event_idx=off, not sure
why. I can try to repeat them if you suspect that can be a test
failure.

* With testpmd and event_idx=off, if I send from the VM to host, I see
a performance increment especially in small packets. The buf api also
increases performance compared with only batching: Sending the minimum
packet size in testpmd makes pps go from 356kpps to 473 kpps. Sending
1024 length UDP-PDU makes it go from 570kpps to 64 kpps.

Something strange I observe in these tests: I get more pps the bigger
the transmitted buffer size is. Not sure why.

** Sending from the host to the VM does not make a big change with the
patches in small packets scenario (minimum, 64 bytes, about 645
without the patch, ~625 with batch and batch+buf api). If the packets
are bigger, I can see a performance increase: with 256 bits, it goes
from 590kpps to about 600kpps, and in case of 1500 bytes payload it
gets from 348kpps to 528kpps, so it is clearly an improvement.

* with testpmd and event_idx=on, batching+buf api perform similarly in
both directions.

All of testpmd tests were performed with no linux bridge, just a
host's tap interface (<interface type='ethernet'> in xml), with a
testpmd txonly and another in rxonly forward mode, and using the
receiving side packets/bytes data. Guest's rps, xps and interrupts,
and host's vhost threads affinity were also tuned in each test to
schedule both testpmd and vhost in different processors.

I will send the v10 RFC with the small changes requested by Stefan and Jason.

Thanks!







> > > >
> > > > OK so it seems plausible that we still have a bug where an interrupt
> > > > is delayed. That is the main difference between pmd and virtio.
> > > > Let's try disabling event index, and see what happens - that's
> > > > the trickiest part of interrupts.
> > > >
> > >
> > > Got it, will get back with the results.
> > >
> > > Thank you very much!
> > >
> > > >
> > > >
> > > > >   - TCP_STREAM goes from ~10.7 gbps to ~7Gbps
> > > > >   - TCP_RR from 6223.64 transactions/sec to 5739.44
> > > >
> >

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