Re: [PATCH] virtio-ring: Use threshold for switching to indirectdescriptors

[Avi Kivity]

On 12/05/2011 02:10 AM, Rusty Russell wrote:
> On Sun, 04 Dec 2011 17:16:59 +0200, Avi Kivity <avi@xxxxxxxxxx> wrote:
> > On 12/04/2011 05:11 PM, Michael S. Tsirkin wrote:
> > > > There's also the used ring, but that's a
> > > > mistake if you have out of order completion.  We should have used copying.
> > >
> > > Seems unrelated... unless you want used to be written into
> > > descriptor ring itself?
> > 
> > The avail/used rings are in addition to the regular ring, no?  If you
> > copy descriptors, then it goes away.
>
> There were two ideas which drove the current design:
>
> 1) The Van-Jacobson style "no two writers to same cacheline makes rings
>    fast" idea.  Empirically, this doesn't show any winnage.

Write/write is the same as write/read or read/write.  Both cases have to
send a probe and wait for the result.  What we really need is to
minimize cache line ping ponging, and the descriptor pool fails that
with ooo completion.  I doubt it's measurable though except with the
very fastest storage providers.

> 2) Allowing a generic inter-guest copy mechanism, so we could have
>    genuinely untrusted driver domains.  Yet noone ever did this so it's
>    hardly a killer feature :(

It's still a goal, though not an important one.  But we have to
translate rings anyway, don't, since buffers are in guest physical
addresses, and we're moving into an address space that doesn't map those.

I thought of having a vhost-copy driver that could do ring translation,
using a dma engine for the copy.

> So if we're going to revisit and drop those requirements, I'd say:
>
> 1) Shared device/driver rings like Xen.  Xen uses device-specific ring
>    contents, I'd be tempted to stick to our pre-headers, and a 'u64
>    addr; u64 len_and_flags; u64 cookie;' generic style.  Then use
>    the same ring for responses.  That's a slight space-win, since
>    we're 24 bytes vs 26 bytes now.

Let's cheat and have inline contents.  Take three bits from
len_and_flags to specify additional descriptors as inline data.  Also,
stuff the cookie into len_and_flags as well.

> 2) Stick with physically-contiguous rings, but use them of size (2^n)-1.
>    Makes the indexing harder, but that -1 lets us stash the indices in
>    the first entry and makes the ring a nice 2^n size.

Allocate at lease a cache line for those.  The 2^n size is not really
material, a division is never necessary.

> > > > 16kB worth of descriptors is 1024 entries.  With 4kB buffers, that's 4MB
> > > > worth of data, or 4 ms at 10GbE line speed.  With 1500 byte buffers it's
> > > > just 1.5 ms.  In any case I think it's sufficient.
> > >
> > > Right. So I think that without indirect, we waste about 3 entries
> > > per packet for virtio header and transport etc headers.
> > 
> > That does suck.  Are there issues in increasing the ring size?  Or
> > making it discontiguous?
>
> Because the qemu implementation is broken.  

I was talking about something else, but this is more important.  Every
time we make a simplifying assumption, it turns around and bites us, and
the code becomes twice as complicated as it would have been in the first
place, and the test matrix explodes.

> We can often put the virtio
> header at the head of the packet.  In practice, the qemu implementation
> insists the header be a single descriptor.
>
> (At least, it used to, perhaps it has now been fixed.  We need a
> VIRTIO_NET_F_I_NOW_CONFORM_TO_THE_DAMN_SPEC_SORRY_I_SUCK bit).

We'll run out of bits in no time.

> We currently use small rings: the guest can't negotiate so qemu has to
> offer a lowest-common-denominator value.  The new virtio-pci layout
> fixes this, and lets the guest set the ring size.

Ok good.  Note the figuring out the best ring size needs some info from
the host, but that can be had from other channels.

> > Can you take a peek at how Xen manages its rings?  They have the same
> > problems we do.
>
> Yes, I made some mistakes, but I did steal from them in the first
> place...

There was a bit of second system syndrome there.  And I don't understand
how the ring/pool issue didn't surface during review, it seems so
obvious now but completely eluded me then.

-- 
error compiling committee.c: too many arguments to function

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