Peter Xu <pet...@redhat.com> writes: > 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.
Ok, I'll remove it for 9.0 then. And I guess I'll also remove the flush completely since there are no other users except for migration.
