Re: [PATCH v6 00/23] migration: File based migration with multifd and mapped-ram

[Peter Xu]

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 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.

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.

Thanks,

-- 
Peter Xu