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 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. 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. Thanks, -- Peter Xu
