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Subject: Re: [virtio-dev] RFC: Doorbell suppression, packed-ring mode and hardware offload

On 2019/2/12 äå1:08, Michael S. Tsirkin wrote:
On Mon, Feb 11, 2019 at 02:58:30PM +0000, David Riddoch wrote:
This can result in a very high rate of doorbells with some
drivers, which can become a severe bottleneck (because x86 CPUs can't emit
MMIOs at very high rates).
Interesting. Is there any data you could share to help guide the design?
E.g. what's the highest rate of MMIO writes supported etc?
On an E3-1270 v5 @ 3.60GHz, max rate across all cores is ~18M/s.

On an E5-2637 v4 @ 3.50GHz, max rate on PCIe-local socket is ~14M/s. On
PCIe-remote socket ~8M/s.
Is that mega bytes? Or million writes?
With 32 bit writes are we limited to 2M then?
Sorry, this is million-writes-per-second. The size of the write doesn't
So it's not too bad, you only need one per batch after all.

Also I thought some more about this. In fact on x86/intel specifically
PCI descriptor reads are atomic with cache line granularity
and writes are ordered. So in fact you can safely prefetch
descriptors and if you see them valid, you can go ahead
and use them.

If I understand this correctly, unless driver set avail in order for several descriptors. Device could not assume that if N is avail then [0, N) is also available.

But prefetching indeed improve the performance when I play with vhost kernel implementation. It reduces the overhead of memory accessing, which is similar to PCI.

This makes the descriptor index merely a hint for performance,
which device can play with at will.

Other platforms are not like this so you need the data
but do they have the same problem?

This doesn't just impose a limit on aggregate packet rate: If you hit this
bottleneck then the CPU core is back-pressured by the MMIO writes, and so
instr/cycle takes a huge hit.

The proposed offset/wrap field allows devices to disable doorbells when
appropriate, and determine the latest fill level via a PCIe read.
This kind of looks like a split ring though, does it not?
I don't agree, because the ring isn't split. Split-ring is very painful for
hardware because of the potentially random accesses to the descriptor table
(including pointer chasing) which inhibit batching of descriptor fetches, as
well as causing complexity in implementation.
That's different with IN_ORDER, right?
Yes, IN_ORDER will give some of the benefit of packed ring, and is also a
win because you can merge 'used' writes. (But packed-ring still wins due to
not having to fetch ring and descriptor table separately).

Note, with IN_ORDER, there's no need to read available ring at all even for split ring. And in some cases, no need to update used ring at all.

Packed ring is a huge improvement on both dimensions, but introduces a
specific pain point for hardware offload.

The issue is
we will again need to bounce more cache lines to communicate.
You'll only bounce cache lines if the device chooses to read the idx. A PV
device need not offer this feature. A PCIe device will, but the cost to the
driver of writing an in-memory idx is much smaller than the cost of an MMIO,
which is always a strongly ordered barrier on x86/64.

With vDPA you ideally would have this feature enabled, and the device would
sometimes be PV and sometimes PCIe. The PV device would ignore the new idx
field and so cache lines would not bounce then either.

Ie. The only time cache lines are shared is when sharing with a PCIe device,
which is the scenario when this is a win.
True. OTOH on non-x86 you will need more write barriers :( It would be
natural to say driver can reuse the barrier before flags update, but note
that that would mean hardware still needs to support re-fetching if
flags value is wrong. Such re-fetch is probably rare so fine by me, but it
does add a bit more complexity.
I would prefer to have the write barrier before writing the idx.
Well that's driver overhead for something device might never utilise in
a given workload. If we are optimizing let's optimize for speed.

Note that drivers could take advantage of this feature to avoid read
barriers when consuming descriptors: At the moment there is a virtio_rmb()
per descriptor read. With the proposed feature the driver can do just one
barrier after reading idx.
Oh you want device to write a used index too? Hmm. Isn't this
contrary to your efforts to consume PCIe BW?

I expect that on platforms where write barriers
have a cost read barriers likely have a significant cost too, so this might
be a win with PV devices too.
I'm not sure.  It is pretty easy to replace an rmb with a dependency
which is generally quite cheap in my testing.

But since it's supposed to benefit PV, at this point we already have
implementations so rather than speculate (pun intended), people can
write patches and experiment.

For the proposed avail idx I think Jason (CC'd) was interested in adding
something like this for vhost.

Yes, it's kind of IN_ORDER + split ring I believe.


So I wonder: what if we made a change to spec that would allow prefetch
of ring entries?  E.g. you would be able to read at random and if you
guessed right then you can just use what you have read, no need to
Unless I've misunderstood I think this would imply that the driver would
have to ensure strict ordering for each descriptor it wrote, which would
impose a cost to the driver. At the moment a write barrier is only needed
before writing the flags of the first descriptor in a batch.
On non-x86 right?  OTOH the extra data structure also adds more ordering
Yes on non-x86. The extra data structure only adds an ordering once per
request (when enabled) whereas allowing prefetch would add an ordering per
descriptor. The number of descriptors is always >= the number of requests,
and can be much larger (esp. for a net rx virt-queue).
Question is, is MMIO also slow on these non-x86 platforms?

Prefetch if successful just drastically lowers latency. You can't beat
not needing a read at all.

I suggest the best place to put this would be in the driver area,
immediately after the event suppression structure.
Could you comment on why is that a good place though?
The new field is written by the driver, as are the other fields in the
driver area. Also I expect that devices might be able to read this new idx
together with the interrupt suppression fields in a single read, reducing
PCIe overheads.
OK then you need it in the same aligned 256 byte boundary.
The event suppression structs currently require 4byte align. Are you
suggesting we increase the align required when
VIRTIO_F_RING_PACKED_AVAIL_IDX is negotiated? Sounds good to me, but 8bytes
would presumably suffice.

I am also wondering: what would the analog of this feature be for split
rings? We are burning a feature bit, might as well find a good
use for it.

Thanks for the feedback.


David Riddoch  <driddoch@solarflare.com> -- Chief Architect, Solarflare

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