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Subject: Re: [virtio-comment] Re: [PATCH v3 6/8] admin: Add theory of operation for write recording commands
On Thu, Nov 23, 2023 at 05:19:06AM -0800, Si-Wei Liu wrote: > If anything relevant I would more like to see performance comparison with > platform dirty tracking via IOMMUFD, but that's perhaps too early stage at > this point to conclude anything given there's very limited availability (in > terms of supporting software, I know some supporting hardware has been > around for a few years) and none of the potential software optimizations is > in place at this point to make a fair comparison for. Exactly. I suggested shadow as a kind of fallback since it is instead available. > Granted device > assisted tracking has its own set of limitations e.g. loose coupling or > integration with platform features, lack of nested and PASID support et al. > However, state of the art for platform dirty tracking is not perfect either, > far off being highly optimized for all types of workload or scenarios. At > least to me the cost of page table walk to scan all PTEs across all levels > is not easily negligible - given no PML equivalent here, are we sure the > whole range scan can be as efficient and scalable as memory size / # of PTEs > grows? How large it may impact the downtime with this rudimentary dirty > scan? No data point was given thus far. If chances are that there could be > major improvement from device tracking for those general use cases to > supplement what platform cannot achieve efficiently enough, it's not too > good to kill off the possibility entirely at this early stage. Maybe a PoC > or some comparative performance data can help prove the theory? > > On the other hand, the device assisted tracking has at least one advantage > that platform cannot simply offer - throttle down device for convergence, > inherently or explicitly whenever needed. I think earlier Micheal suggested > something to make the core data structure used for logging more efficient > and compact, working like PML but using a queue or an array, and the entry > of which may contain a list of discrete pages or contiguous PFN ranges. Hmm no, what I really meant is a way for device to store all or parts of this structure in host RAM as opposed to all of it in on-device memory as it has to be now. > On > top of this one may add parallelism to distribute load to multiple queues, > or add zero copy to speed up dirty sync to userspace - things virtio queues > are pretty good at doing. After all, nothing can be perfect to begin with, > and every complex feature would need substantial time to improve and evolve. > It does so for shadow virtqueue from where it gets started to where it is > now, even so there's still a lot of optimization work not done yet. There > must be head room here for device page tracking or platform tracking, too. > > Regards, > -Siwei > > > > > > Shadow virtqueue can be used with a save/load model for device state > > recovery for sure. > > > > > > But it seems the shadow virtqueue itself is not the major factor but > > > > the time spent on programming vendor specific mappings for example. > > > Yep. The slowness on mapping part is mostly due to the artifact of > > > software-based implementation. IMHO for live migration p.o.v it's better > > > to not involve any mapping operation in the down time path at all. > > Yes. > > > > Thanks > > > > > -Siwei > > > > Thanks > > > > > > > > > The time didn't come from pci side or boot side. > > > > > > > > > > For pci side of things you would want to compare the pci vs non pci device based VM boot time. > > > > > > > > > This publicly archived list offers a means to provide input to the > > > > > > > > OASIS Virtual I/O Device (VIRTIO) TC. > > > > > > > > > > > > > > > > In order to verify user consent to the Feedback License terms and > > > > > > > > to minimize spam in the list archive, subscription is required > > > > > > > > before posting. > > > > > > > > > > > > > > > > Subscribe: virtio-comment-subscribe@lists.oasis-open.org > > > > > > > > Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org > > > > > > > > List help: virtio-comment-help@lists.oasis-open.org > > > > > > > > List archive: https://lists.oasis-open.org/archives/virtio-comment/ > > > > > > > > Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf > > > > > > > > List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists > > > > > > > > Committee: https://www.oasis-open.org/committees/virtio/ > > > > > > > > Join OASIS: https://www.oasis-open.org/join/ > > > > > > > > > > This publicly archived list offers a means to provide input to the > > > OASIS Virtual I/O Device (VIRTIO) TC. > > > > > > In order to verify user consent to the Feedback License terms and > > > to minimize spam in the list archive, subscription is required > > > before posting. > > > > > > Subscribe: virtio-comment-subscribe@lists.oasis-open.org > > > Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org > > > List help: virtio-comment-help@lists.oasis-open.org > > > List archive: https://lists.oasis-open.org/archives/virtio-comment/ > > > Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf > > > List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists > > > Committee: https://www.oasis-open.org/committees/virtio/ > > > Join OASIS: https://www.oasis-open.org/join/ > > > > > > > This publicly archived list offers a means to provide input to the > > > > OASIS Virtual I/O Device (VIRTIO) TC. > > > > > > > > In order to verify user consent to the Feedback License terms and > > > > to minimize spam in the list archive, subscription is required > > > > before posting. > > > > > > > > Subscribe: virtio-comment-subscribe@lists.oasis-open.org > > > > Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org > > > > List help: virtio-comment-help@lists.oasis-open.org > > > > List archive: https://lists.oasis-open.org/archives/virtio-comment/ > > > > Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf > > > > List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists > > > > Committee: https://www.oasis-open.org/committees/virtio/ > > > > Join OASIS: https://www.oasis-open.org/join/ > >
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