* Peter Xu ([email protected]) wrote: > On Thu, Feb 03, 2022 at 05:45:32PM +0000, Dr. David Alan Gilbert wrote: > > * Peter Xu ([email protected]) wrote: > > > This patch enables postcopy-preempt feature. > > > > > > It contains two major changes to the migration logic: > > > > > > (1) Postcopy requests are now sent via a different socket from precopy > > > background migration stream, so as to be isolated from very high > > > page > > > request delays > > > > > > (2) For huge page enabled hosts: when there's postcopy requests, they > > > can now > > > intercept a partial sending of huge host pages on src QEMU. > > > > > > After this patch, we'll have two "channels" (or say, sockets, because > > > it's only > > > supported on socket-based channels) for postcopy: (1) PRECOPY channel > > > (which is > > > the default channel that transfers background pages), and (2) POSTCOPY > > > channel (which only transfers requested pages). > > > > > > On the source QEMU, when we found a postcopy request, we'll interrupt the > > > PRECOPY channel sending process and quickly switch to the POSTCOPY > > > channel. > > > After we serviced all the high priority postcopy pages, we'll switch back > > > to > > > PRECOPY channel so that we'll continue to send the interrupted huge page > > > again. > > > There's no new thread introduced. > > > > > > On the destination QEMU, one new thread is introduced to receive page > > > data from > > > the postcopy specific socket. > > > > > > This patch has a side effect. After sending postcopy pages, previously > > > we'll > > > assume the guest will access follow up pages so we'll keep sending from > > > there. > > > Now it's changed. Instead of going on with a postcopy requested page, > > > we'll go > > > back and continue sending the precopy huge page (which can be intercepted > > > by a > > > postcopy request so the huge page can be sent partially before). > > > > > > Whether that's a problem is debatable, because "assuming the guest will > > > continue to access the next page" doesn't really suite when huge pages are > > > used, especially if the huge page is large (e.g. 1GB pages). So that > > > locality > > > hint is much meaningless if huge pages are used. > > > > > > If postcopy preempt is enabled, a separate channel is created for it so > > > that it > > > can be used later for postcopy specific page requests. On dst node, a > > > standalone thread is used to receive postcopy requested pages. The > > > thread is > > > created along with the ram listen thread during POSTCOPY_LISTEN phase. > > > > I think this patch could do with being split into two; the first one that > > deals with closing/opening channels; and the second that handles the > > data on the two channels and does the preemption. > > Sounds good, I'll give it a shot on the split. > > > > > Another thought is whether, if in the future we allow multifd + > > postcopy, the multifd code would change - I think it would end up closer > > to using multiple channels taking different pages on each one. > > Right, so potentially the postcopy channels can be multi-threaded too itself. > > We've had a quick discussion on irc, just to recap: I didn't reuse multifd > infra because IMO multifd is designed with below ideas in mind: > > (1) Every multifd thread is equal > (2) Throughput oriented > > However I found that postcopy needs something different when they're mixed up > together with multifd. > > Firstly, we will have some channels sending as much as we could where latency > is not an issue (aka background pages). However it's not suitable for page > requests, so we could also have channels that are servicing page faults fron > dst. In short, there're two types of channels/threads we want, and we may > want > to treat them differently. > > The current model is we only have 1 postcopy channel and 1 precopy channel, > but > it should be easier if we want to make it N post + 1 pre base on this series.
It's not clear to me if we need to be able to do N post + M pre, or whether we have a rule like always at least 1 post, but if there's more pagefaults in the queue then you can steal all of the pre channels. > So far all send() is still done in the migration thread so no new sender > thread > but 1 more receiver thread only. If we want to grow that 1->N for postcopy > channels we may want to move that out too just like what we do with multifd. > Not sure whether there can be something reused around. That's where I haven't > yet explored, but this series should already share a common piece of code on > refactoring of things like tmp huge page on dst node to be able to receive > with > multiple huge pages. Right; it makes me think the multifd+postcopy should just use channels. > This also reminded me that, instead of a new capability, should I simply > expose > a parameter "postcopy-channels=N" to CLI so that we can be prepared with multi > postcopy channels? I'm not sure we know enough yet about what configuration it would have; I'd be tempted to just make it work for the user by enabling both multifd and preemption and then using this new mechanism rather than having to add yet another parameter. Dave > > > > > > Do we need to do anything in psotcopy recovery ? > > Yes. It's a todo (in the cover letter), if the whole thing looks sane I'll add > that together in the non-rfc series. > > Thanks, > > -- > Peter Xu > -- Dr. David Alan Gilbert / [email protected] / Manchester, UK
