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

On 2019/2/15 äå12:23, Michael S. Tsirkin wrote:
On Fri, Feb 15, 2019 at 11:59:55AM +0800, Jason Wang wrote:
On 2019/2/14 äå11:41, Michael S. Tsirkin wrote:
I think it's as simple as increase the avail idx by X? Since descriptor were
used in order, device can just read the next X-1 descriptors in this case.

Right so a spec change would be needed, it's not transparent to guest.

With the change, IN_ORDER + split_ring becomes something like submission
queue (descriptor ring) + completion queue (used ring). And used ring access
could be eliminated sometime, e.g for net for TX, we don't care about used
Oh yes but fundamentally this becomes very close to packed ring.

Looks not, it's a two rings vs one ring.

So I'm
not sure yet another option is justified by a small gain in PPS,

10% is done with less than 10 lines of code, I suspect maybe dpdk can see more.

especially considering that it depends on in order and so doesn't
support zero copy.

It looks to me in order is not the blocker, even if we allow out of order, we may still suffer from HOL.

What's more interesting is, this avail idx optimization is not required for
out of order completion which means it could be used for e.g block or SCSI
I don't get the last sentence. This only works for in order right?

Yes, you are right.


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