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

[Parav Pandit <parav@nvidia.com>]

> From: Michael S. Tsirkin <mst@redhat.com>
> Sent: Friday, November 17, 2023 6:02 PM
> 
> On Fri, Nov 17, 2023 at 12:11:15PM +0000, Parav Pandit wrote:
> >
> >
> > > From: Michael S. Tsirkin <mst@redhat.com>
> > > Sent: Friday, November 17, 2023 5:35 PM
> > > To: Parav Pandit <parav@nvidia.com>
> > >
> > > On Fri, Nov 17, 2023 at 11:45:20AM +0000, Parav Pandit wrote:
> > > >
> > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > Sent: Friday, November 17, 2023 5:04 PM
> > > > >
> > > > > On Fri, Nov 17, 2023 at 11:05:16AM +0000, Parav Pandit wrote:
> > > > > >
> > > > > >
> > > > > > > From: Michael S. Tsirkin <mst@redhat.com>
> > > > > > > Sent: Friday, November 17, 2023 4:30 PM
> > > > > > >
> > > > > > > On Fri, Nov 17, 2023 at 10:03:47AM +0000, Parav Pandit wrote:
> > > > > > > >
> > > > > > > >
> > > > > > > > > From: Zhu, Lingshan <lingshan.zhu@intel.com>
> > > > > > > > > Sent: Friday, November 17, 2023 3:30 PM
> > > > > > > > >
> > > > > > > > > On 11/16/2023 7:59 PM, Michael S. Tsirkin wrote:
> > > > > > > > > > On Thu, Nov 16, 2023 at 06:28:07PM +0800, Zhu, Lingshan
> wrote:
> > > > > > > > > >>
> > > > > > > > > >> On 11/16/2023 1:51 PM, Michael S. Tsirkin wrote:
> > > > > > > > > >>> On Thu, Nov 16, 2023 at 05:29:54AM +0000, Parav Pandit
> wrote:
> > > > > > > > > >>>> We should expose a limit of the device in the
> > > > > > > > > >>>> proposed
> > > > > > > > > WRITE_RECORD_CAP_QUERY command, that how much range it
> > > > > > > > > can
> > > > > track.
> > > > > > > > > >>>> So that future provisioning framework can use it.
> > > > > > > > > >>>>
> > > > > > > > > >>>> I will cover this in v5 early next week.
> > > > > > > > > >>> I do worry about how this can even work though. If
> > > > > > > > > >>> you want a generic device you do not get to dictate
> > > > > > > > > >>> how much memory VM
> > > > > has.
> > > > > > > > > >>>
> > > > > > > > > >>> Aren't we talking bit per page? With 1TByte of
> > > > > > > > > >>> memory to track
> > > > > > > > > >>> -> 256Gbit -> 32Gbit -> 8Gbyte per VF?
> > > > > > > > > >>>
> > > > > > > > > >>> And you happily say "we'll address this in the future"
> > > > > > > > > >>> while at the same time fighting tooth and nail
> > > > > > > > > >>> against adding single bit status registers because scalability?
> > > > > > > > > >>>
> > > > > > > > > >>>
> > > > > > > > > >>> I have a feeling doing this completely theoretical
> > > > > > > > > >>> like this is
> > > > > problematic.
> > > > > > > > > >>> Maybe you have it all laid out neatly in your head
> > > > > > > > > >>> but I suspect not all of TC can picture it clearly
> > > > > > > > > >>> enough based just on spec
> > > > > text.
> > > > > > > > > >>>
> > > > > > > > > >>> We do sometimes ask for POC implementation in linux
> > > > > > > > > >>> / qemu to demonstrate how things work before merging
> code.
> > > > > > > > > >>> We skipped this for admin things so far but I think
> > > > > > > > > >>> it's a good idea to start doing it here.
> > > > > > > > > >>>
> > > > > > > > > >>> What makes me pause a bit before saying please do a
> > > > > > > > > >>> PoC is all the opposition that seems to exist to
> > > > > > > > > >>> even using admin commands in the 1st place. I think
> > > > > > > > > >>> once we finally stop arguing about whether to use
> > > > > > > > > >>> admin commands at all then a PoC will be needed
> > > > > > > before merging.
> > > > > > > > > >> We have POR productions that implemented the approach
> > > > > > > > > >> in my
> > > > > series.
> > > > > > > > > >> They are multiple generations of productions in
> > > > > > > > > >> market and running in customers data centers for years.
> > > > > > > > > >>
> > > > > > > > > >> Back to 2019 when we start working on vDPA, we have
> > > > > > > > > >> sent some samples of production(e.g., Cascade
> > > > > > > > > >> Glacier) and the datasheet, you can find live
> > > > > > > > > >> migration facilities there, includes suspend, vq state and other
> features.
> > > > > > > > > >>
> > > > > > > > > >> And there is an reference in DPDK live migration, I
> > > > > > > > > >> have provided this page
> > > > > > > > > >> before:
> > > > > > > > > >> https://doc.dpdk.org/guides-21.11/vdpadevs/ifc.html,
> > > > > > > > > >> it has been working for long long time.
> > > > > > > > > >>
> > > > > > > > > >> So if we let the facts speak, if we want to see if
> > > > > > > > > >> the proposal is proven to work, I would
> > > > > > > > > >> say: They are POR for years, customers already
> > > > > > > > > >> deployed them for
> > > > > years.
> > > > > > > > > > And I guess what you are trying to say is that this
> > > > > > > > > > patchset we are reviewing here should be help to the
> > > > > > > > > > same standard and there should be a PoC? Sounds reasonable.
> > > > > > > > > Yes and the in-marketing productions are POR, the series
> > > > > > > > > just improves the design, for example, our series also
> > > > > > > > > use registers to track vq state, but improvements than
> > > > > > > > > CG or BSC. So I think they are proven
> > > > > > > to work.
> > > > > > > >
> > > > > > > > If you prefer to go the route of POR and production and
> > > > > > > > proven documents
> > > > > > > etc, there is ton of it of multiple types of products I can
> > > > > > > dump here with open- source code and documentation and more.
> > > > > > > > Let me know what you would like to see.
> > > > > > > >
> > > > > > > > Michael has requested some performance comparisons, not
> > > > > > > > all are ready to
> > > > > > > share yet.
> > > > > > > > Some are present that I will share in coming weeks.
> > > > > > > >
> > > > > > > > And all the vdpa dpdk you published does not have basic
> > > > > > > > CVQ support when I
> > > > > > > last looked at it.
> > > > > > > > Do you know when was it added?
> > > > > > >
> > > > > > > It's good enough for PoC I think, CVQ or not.
> > > > > > > The problem with CVQ generally, is that VDPA wants to shadow
> > > > > > > CVQ it at all times because it wants to decode and cache the
> > > > > > > content. But this problem has nothing to do with dirty
> > > > > > > tracking even though it also
> > > > > mentions "shadow":
> > > > > > > if device can report it's state then there's no need to shadow CVQ.
> > > > > >
> > > > > > For the performance numbers with the pre-copy and device
> > > > > > context of
> > > > > patches posted 1 to 5, the downtime reduction of the VM is 3.71x
> > > > > with active traffic on 8 RQs at 100Gbps port speed.
> > > > >
> > > > > Sounds good can you please post a bit more detail?
> > > > > which configs are you comparing what was the result on each of them.
> > > >
> > > > Common config: 8+8 tx and rx queues.
> > > > Port speed: 100Gbps
> > > > QEMU 8.1
> > > > Libvirt 7.0
> > > > GVM: Centos 7.4
> > > > Device: virtio VF hardware device
> > > >
> > > > Config_1: virtio suspend/resume similar to what Lingshan has,
> > > > largely vdpa stack
> > > > Config_2: Device context method of admin commands
> > >
> > > OK that sounds good. The weird thing here is that you measure "downtime".
> > > What exactly do you mean here?
> > > I am guessing it's the time to retrieve on source and re-program
> > > device state on destination? And this is 3.71x out of how long?
> > Yes. Downtime is the time during which the VM is not responding or receiving
> packets, which involves reprogramming the device.
> > 3.71x is relative time for this discussion.
> 
> Oh interesting. So VM state movement including reprogramming the CPU is
> dominated by reprogramming this single NIC, by a factor of almost 4?
Yes.