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Berrang=C3=A9 writes: > On Mon, Mar 04, 2024 at 08:35:36PM +0800, Peter Xu wrote: >> Fabiano, >>=20 >> On Thu, Feb 29, 2024 at 12:29:54PM -0300, Fabiano Rosas wrote: >> > =3D> guest: 128 GB RAM - 120 GB dirty - 1 vcpu in tight loop dirtying = memory >>=20 >> I'm curious normally how much time does it take to do the final fdatasyn= c() >> for you when you did this test. I haven't looked at the fdatasync() in isolation. I'll do some measurements soon. >>=20 >> I finally got a relatively large system today and gave it a quick shot o= ver >> 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 wro= ng >> 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 if we move the fsync to the same spot as the multifd thread sync instead of having a big one at the end? Not sure how that looks with concurrency in the mix. I'll have to experiment a bit.