netdev - Re: [PATCH RFC 0/4] vsock/virtio: optimizations to increase the throughput

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Date:   Mon, 8 Apr 2019 10:23:18 +0100
From:   Stefan Hajnoczi <stefanha@...hat.com>
To:     Stefano Garzarella <sgarzare@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
        Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [PATCH RFC 0/4] vsock/virtio: optimizations to increase the
 throughput

On Fri, Apr 05, 2019 at 09:49:17AM +0200, Stefano Garzarella wrote:
> On Thu, Apr 04, 2019 at 02:04:10PM -0400, Michael S. Tsirkin wrote:
> > On Thu, Apr 04, 2019 at 06:47:15PM +0200, Stefano Garzarella wrote:
> > > On Thu, Apr 04, 2019 at 11:52:46AM -0400, Michael S. Tsirkin wrote:
> > > > I simply love it that you have analysed the individual impact of
> > > > each patch! Great job!
> > > 
> > > Thanks! I followed Stefan's suggestions!
> > > 
> > > > 
> > > > For comparison's sake, it could be IMHO benefitial to add a column
> > > > with virtio-net+vhost-net performance.
> > > > 
> > > > This will both give us an idea about whether the vsock layer introduces
> > > > inefficiencies, and whether the virtio-net idea has merit.
> > > > 
> > > 
> > > Sure, I already did TCP tests on virtio-net + vhost, starting qemu in
> > > this way:
> > >   $ qemu-system-x86_64 ... \
> > >       -netdev tap,id=net0,vhost=on,ifname=tap0,script=no,downscript=no \
> > >       -device virtio-net-pci,netdev=net0
> > > 
> > > I did also a test using TCP_NODELAY, just to be fair, because VSOCK
> > > doesn't implement something like this.
> > 
> > Why not?
> > 
> 
> I think because originally VSOCK was designed to be simple and
> low-latency, but of course we can introduce something like that.
> 
> Current implementation directly copy the buffer from the user-space in a
> virtio_vsock_pkt and enqueue it to be transmitted.
> 
> Maybe we can introduce a buffer per socket where accumulate bytes and
> send it when it is full or when a timer is fired . We can also introduce
> a VSOCK_NODELAY (maybe using the same value of TCP_NODELAY for
> compatibility) to send the buffer immediately for low-latency use cases.
> 
> What do you think?

Today virtio-vsock implements a 1:1 sendmsg():packet relationship
because it's simple.  But there's no need for the guest to enqueue
multiple VIRTIO_VSOCK_OP_RW packets when a single large packet could
combine all payloads for a connection.  This is not the same as
TCP_NODELAY but related.

I think it's worth exploring TCP_NODELAY and send_pkt_list merging.
Hopefully it won't make the code much more complicated.

Stefan

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