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