linux-kernel - Re: [PATCH RFC v8 02/11] vhost: use batched get_vq

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Date:   Mon, 22 Jun 2020 12:29:28 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Eugenio Perez Martin <eperezma@...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 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.

> > > * 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.

> >
> > >   - netperf UDP_STREAM shows a performance increase of 1.8, almost
> > > doubling performance. This gets lower as frame size increase.
> > >   - rests of the test goes noticeably worse: UDP_RR goes from ~6347
> > > transactions/sec to 5830
> >
> > 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
> >