On Thu, Oct 03, 2024 at 10:34:28PM +0200, Maciej S. Szmigiero wrote: > To be clear, these loading threads are mostly blocking I/O threads, NOT > compute threads. > This means that the usual "rule of thumb" that the count of threads should > not exceed the total number of logical CPUs does NOT apply to them. > > They are similar to what glibc uses under the hood to simulate POSIX AIO > (aio_read(), aio_write()), to implement an async DNS resolver > (getaddrinfo_a()) > and what Glib's GIO uses to simulate its own async file operations. > Using helper threads for turning blocking I/O into "AIO" is a pretty common > thing.
Fair enough. Yes I could be over-cautious due to the previous experience on managing all kinds of migration threads. > > To show that these loading threads mostly spend their time sleeping (waiting > for I/O) I made a quick patch at [1] tracing how much time they spend waiting > for incoming buffers and how much time they spend waiting for these buffers > to be loaded into the device. > > The results (without patch [2] described later) are like this: > > 5919@1727974993.403280:vfio_load_state_device_buffer_start (0000:af:00.2) > > 5921@1727974993.407932:vfio_load_state_device_buffer_start (0000:af:00.4) > > 5922@1727974993.407964:vfio_load_state_device_buffer_start (0000:af:00.5) > > 5920@1727974993.408480:vfio_load_state_device_buffer_start (0000:af:00.3) > > 5920@1727974993.666843:vfio_load_state_device_buffer_end (0000:af:00.3) > > wait 43 ms load 217 ms > > 5921@1727974993.686005:vfio_load_state_device_buffer_end (0000:af:00.4) > > wait 75 ms load 206 ms > > 5919@1727974993.686054:vfio_load_state_device_buffer_end (0000:af:00.2) > > wait 69 ms load 210 ms > > 5922@1727974993.689919:vfio_load_state_device_buffer_end (0000:af:00.5) > > wait 79 ms load 204 ms > > Summing up: > 0000:af:00.2 total loading time 283 ms, wait 69 ms load 210 ms > 0000:af:00.3 total loading time 258 ms, wait 43 ms load 217 ms > 0000:af:00.4 total loading time 278 ms, wait 75 ms load 206 ms > 0000:af:00.5 total loading time 282 ms, wait 79 ms load 204 ms > > In other words, these threads spend ~100% of their total runtime waiting > for I/O, 70%-75% of that time waiting for buffers to get loaded into their > target device. > > So having more threads here won't negatively affect the host CPU > consumption since these threads barely use the host CPU at all. > Also, their count is capped at the number of VFIO devices in the VM. > > I also did a quick test with the same config as usual: 4 VFs, 6 multifd > channels, but with patch at [2] simulating forced coupling of loading > threads to multifd receive channel threads. > > With this patch load_state_buffer() handler will return to the multifd > channel thread only when the loading thread finishes loading available > buffers and is about to wait for the next buffers to arrive - just as > loading buffers directly from these channel threads would do. > > The resulting lowest downtime from 115 live migration runs was 1295ms - > that's 21% worse than 1068ms of downtime with these loading threads running > on their own. > > I expect that this performance penalty to get even worse with more VFs > than 4. > > So no, we can't load buffers directly from multifd channel receive threads. 6 channels can be a bit less in this test case with 4 VFs, but indeed adding such dependency on number of multifd threads isn't as good either, I agree. I'm ok as long as VFIO reviewers are fine. > > > PS: I'd suggest if you really need those threads it should still be managed > > by migration framework like the src thread pool. Sorry I'm pretty stubborn > > on this, especially after I notice we have query-migrationthreads API just > > recently.. even if now I'm not sure whether we should remove that API. I > > assume that shouldn't need much change, even if necessary. > > I can certainly make these loading threads managed in a thread pool if that's > easier for you. Yes, if you want to use separate thread it'll be great to match on the src thread model with similar pool. I hope the pool interface you have is easily applicable on both sides. Thanks, -- Peter Xu
