[virtio-comment] Re: [PATCH v3 6/8] admin: Add theory of operation for write recording commands

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Nov 08, 2023 at 09:00:00AM +0000, Parav Pandit wrote:
> > From: Michael S. Tsirkin <mst@redhat.com>
> > Sent: Wednesday, November 8, 2023 1:47 PM
> 
> > Only for TX, and I'm pretty sure they had the foresight to test RX not just TX but
> > let's confirm. Parav did you test both directions?
> Rx is the main part to test for the dirty page tracking.
> Tx only is around ring updates, so not very interesting test.
> 
> When page tracking is started, there is impact on Rx as natural throttle by slowing down rx packet rate.

Wait a sec sounds kind of like what we see with SVQ as well?

> And devices have implementation choice to covering the spectrum on amount of drop rate.

What does this mean practically?

> And has implementations evolve, it will improve as well.

Maybe, or maybe not.

> PRI for sure is out of question. Most workload won't use it at all given other limitations of virtio on PCI front and existing PRI behaviors.

Out of curiousity what do you refer to?

> There is also a test with platform IOMMU dirty page tracking done as well but it is very early stage to comment in this forum.

Interesting. That's a very important test I'd say - if platform based
dirty page tracking is just as good we can avoid adding it to virtio.


And I'm rather confused at this point - I was under the impression you
already have a prototype which shows negligeable performance impact
with on device tracking.

-- 
MST


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