On Mon, Mar 04, 2024 at 09:04:51PM +0000, Daniel P. Berrangé wrote: > On Mon, Mar 04, 2024 at 05:15:05PM -0300, Fabiano Rosas wrote: > > Peter Xu <pet...@redhat.com> writes: > > > > > On Mon, Mar 04, 2024 at 08:53:24PM +0800, Peter Xu wrote: > > >> On Mon, Mar 04, 2024 at 12:42:25PM +0000, Daniel P. Berrangé wrote: > > >> > On Mon, Mar 04, 2024 at 08:35:36PM +0800, Peter Xu wrote: > > >> > > Fabiano, > > >> > > > > >> > > On Thu, Feb 29, 2024 at 12:29:54PM -0300, Fabiano Rosas wrote: > > >> > > > => guest: 128 GB RAM - 120 GB dirty - 1 vcpu in tight loop > > >> > > > dirtying memory > > >> > > > > >> > > I'm curious normally how much time does it take to do the final > > >> > > fdatasync() > > >> > > for you when you did this test. > > > > I measured and it takes ~4s for the live migration and ~2s for the > > non-live. I didn't notice this before because the VM goes into > > postmigrate, so it's paused anyway.
For my case it took me tens of seconds at least, if not go into minutes, which I didn't measure. I could have dirtied harder, or I just had a slower disk. IIUC the worst case is all cache dirty (didn't yet writeback in the kernel), say 100GB, assuming the disk bandwidth 1GB/s (that's the bw of my test machine hard drive of 1M chunk dd for a 10GB file, even without a sync..), IIUC it means it could take 1min or more in reality. > > > > >> > > > > >> > > I finally got a relatively large system today and gave it a quick > > >> > > shot over > > >> > > 128G (100G busy dirty) mapped-ram snapshot with 8 multifd channels. > > >> > > The > > >> > > migration save/load does all fine, so I don't think there's anything > > >> > > wrong > > >> > > with the patchset, however when save completes (I'll need to stop the > > >> > > workload as my disk isn't fast enough I guess..) I'll always hit a > > >> > > super > > >> > > long hang of QEMU on fdatasync() on XFS during which the main thread > > >> > > is in > > >> > > UNINTERRUPTIBLE state. > > >> > > > >> > That isn't very surprising. If you don't have O_DIRECT enabled, then > > >> > all that disk I/O from the migrate is going to be in RAM, and thus the > > >> > fdatasync() is likely to trigger writing out alot of data. > > >> > > > >> > Blocking the main QEMU thread though is pretty unhelpful. That suggests > > >> > the data sync needs to be moved to a non-main thread. > > >> > > >> Perhaps migration thread itself can also be a candidate, then. > > >> > > >> > > > >> > With O_DIRECT meanwhile there should be essentially no hit from > > >> > fdatasync. > > >> > > >> The update of COMPLETED status can be a good place of a marker point to > > >> show such flush done if from the gut feeling of a user POV. If that > > >> makes > > >> sense, maybe we can do that sync before setting COMPLETED. > > > > At the migration completion I believe the multifd threads will have > > already cleaned up and dropped the reference to the channel, it might be > > too late then. > > > > In the multifd threads, we'll be wasting (like we are today) the extra > > syscalls after the first sync succeeds. > > > > >> > > >> No matter which thread does that sync, it's still a pity that it'll go > > >> into > > >> UNINTERRUPTIBLE during fdatasync(), then whoever wants to e.g. attach a > > >> gdb > > >> onto it to have a look will also hang. > > > > > > Or... would it be nicer we get rid of the fdatasync() but leave that for > > > upper layers? QEMU used to support file: migration already, it never > > > manage cache behavior; it does smell like something shouldn't be done in > > > QEMU when thinking about it, at least mapped-ram is nothing special to me > > > from this regard. > > > > > > User should be able to control that either manually (sync), or Libvirt can > > > do that after QEMU quits; after all Libvirt holds the fd itself? It > > > should > > > allow us to get rid of above UNINTERRUPTIBLE / un-debuggable period of > > > QEMU > > > went away. Another side benefit: rather than holding all of QEMU > > > resources > > > (especially, guest RAM) when waiting for a super slow disk flush, Libvirt > > > / > > > upper layer can do that separately after releasing all the QEMU resources > > > first. > > > > I like the idea of QEMU having a self-contained > > implementation. Specially since we'll add O_DIRECT support, which is > > already quite heavy-handed if we're talking about managing cache > > behavior. O_DIRECT is optionally selected by the user by setting the new parameter first, so the user is still in full control - it's still user's decision on how cache should be managed, even if QEMU needs explicit changes to support and expose the new parameter. For fdatasync(), I think it's slightly different in that it doesn't require anything implemented in QEMU, as the snapshot is always in the form of a file, and file is pretty common concept which well supports sync semantics separately. Instead of providing yet another parameter to control it, we can just avoid that datasync. Besides what I already described above as reasons, I think it's also legal if an user wants to temporarily flush a VM into a disk (in paused state), run some RAM-intense loads (which can immediately make use of guest's RAM which is directly freed, but may _not_ always require a page cache flush), then relaunch the VM. In that case keeping some cache around might help already to speedup relaunching to avoid unnecessary swap-ins/swap-outs. > > > > However, it's not trivial to find the right place to add the sync. > > Wherever we put it there will be some implications, such as ensuring the > > sync works even after migration failure, avoiding concurrent cleanup, > > etc. > > > > In any case, I don't think it's correct to have the sync at > > qio_channel_close(), now that we've seen it might block for a long > > time. We could at the very least have a qio_channel_flush()[1] which the > > QIOChannelFile implements with fdatasync(). Then the clients can choose > > when to sync. > > Yes, I agree with de-coupling it. Yes, that decoupling makes sense to me. That definitely answers some of my previous confusions. The following question is whether we should require a qio_channel_flush() by default at anywhere around the end of migration for mapped-ram, in which case I lean towards removing it completely. In all cases, considering the time it could hang qemu (possible in minutes) we may want to change that behavior for 9.0 if possible. Thanks, -- Peter Xu
