Re: [virtio-dev] packed ring layout proposal

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 09/16/2016 10:03 PM, Michael S. Tsirkin wrote:
> On Fri, Sep 16, 2016 at 12:11:59PM +0200, Paolo Bonzini wrote:
> > On 16/09/2016 11:46, Stefan Hajnoczi wrote:
> > > > > The idea is to have a r/w descriptor in a ring structure,
> > > > > replacing the used and available ring, index and descriptor
> > > > > buffer.
> > > This is probably obvious but:
> > >
> > > Why is the desc.index field needed?
> > >
> > > The index field would be needed if the driver had to jump over a DESC_HW
> > > descriptor when adding descriptors, but that doesn't seem necessary
> > > since the device is supposed to overwrite descriptors in ascending ring
> > > order when they complete out-of-order.
> >
> > AIUI, when reading completed descriptors, the driver uses the index in
> > order to map the descriptor to the original request.
> >
> > It's more of an id; it's certainly not a linked list as in virtio 1.0 rings.
> >
> > I think we can refine the description of the index, saying that only the
> > first descriptor in a request needs an index; descriptors reached via
> > VRING_DESC_F_NEXT do not need an index.  The advantage is that the
> > device only needs to remember the descriptor of the first descriptor.
> >
> > However...
> >
> > - is there any reason for the device or driver to even look at DESC_HW
> > if VRING_DESC_F_NEXT is set?  DESC_HW only matters on the first buffer.
> >
> > - why not drop VRING_DESC_F_NEXT altogether?
> >
> > Because only two cases are strictly necessary:
> >
> > - single-buffer requests.  These have some very important cases:
> > virtio-rng, virtio-serial, virtio-net with merged rxbufs, virtio-scsi
> > and virtio-vsock events, virtio-balloon, virtio-input all use
> > single-buffer requests exclusively.
> >
> > - indirect requests, where the length of the indirect descriptor can be
> > used to figure out the number of buffers
> >
> > Let's just drop the added complication of multi-buffer direct requests.
> > Yes, this can cause one extra cache miss but 1) as shown above a lot of
> > performance-critical cases use single-buffer requests 2) no one has ever
> > bothered to use multi-buffer direct requests in Linux drivers, at least
> > when indirect descriptors are available.
>
> True. I note that dpdk used direct descriptors exclusively until
> very recently.
>     commit 6dc5de3a6aefba3946fe04368d93994db3f7a5fd
>     Author: Stephen Hemminger <stephen@networkplumber.org>
>     Date:   Fri Mar 4 10:19:19 2016 -0800
>
>         virtio: use indirect ring elements

Indeed, and I'd like to add that this patch doesn't enable
VIRTIO_RING_F_INDIRECT_DESC, so code was here but not used in mainline.

It has been fixed few weeks ago, but the patch didn't land yet into
master, only in virtio-next[0]:

commit b19d3763de3f6c96380b6decf51752acebd2acee
Author: Pierre Pfister <ppfister@cisco.com>
Date:   Wed Sep 7 10:46:18 2016 +0800

    net/virtio: enable indirect descriptors feature


> I'd like to make sure removing these doesn't cause regressions.
> Stephen, could you please confirm that
> 1. with your patch, each packet takes at most 1 slot
> 2. your patch was a win for most workloads
>    (as opposed to e.g. enabling/disabling this depending on workload)

> Also, it does not look like vhost in dpdk supports
> indirect descriptors.

I have done the patch for vhost in dpdk[1] two months ago.
It's not merged because I didn't show noticeable gain using it. Note
that I didn't show noticeable loss either.
I'll try to re-run some benchmarks next week.

Maxime

[0]: git://dpdk.org/next/dpdk-next-virtio
[1]: http://dpdk.org/dev/patchwork/patch/14797/