Daniel P. Berrangé <berra...@redhat.com> writes: > On Mon, Jul 04, 2022 at 08:18:54AM +0200, Markus Armbruster wrote: >> Leonardo Bras <leob...@redhat.com> writes: >> >> > Signed-off-by: Leonardo Bras <leob...@redhat.com> >> > --- >> > qapi/migration.json | 5 ++++- >> > migration/migration.c | 1 + >> > monitor/hmp-cmds.c | 4 ++++ >> > 3 files changed, 9 insertions(+), 1 deletion(-) >> > >> > diff --git a/qapi/migration.json b/qapi/migration.json >> > index 7102e474a6..925f009868 100644 >> > --- a/qapi/migration.json >> > +++ b/qapi/migration.json >> > @@ -55,6 +55,9 @@ >> > # @postcopy-bytes: The number of bytes sent during the post-copy phase >> > # (since 7.0). >> > # >> > +# @zero-copy-copied: The number of zero-copy flushes that reported data >> > sent >> > +# using zero-copy that ended up being copied. (since >> > 7.2) >> >> The description feels awkward. What's a "zero-copy flush", and why >> should the user care? I figure what users care about is the number of >> all-zero pages we had to "copy", i.e. send the bulky way. Is this what >> @zero-copy-copied reports? > > MigrationCapability field @zero-copy-send instructs QEMU to try to > avoid copying data between userspace and kernel space when transmitting > RAM region. > > Even if the kernel supports zero copy, it is not guaranteed to happen, > it is merely a request to try. > > QEMU periodically (once per migration iteration) flushes outstanding > zero-copy requests and gets an indication back of whether any copies > took place or not. > > So this counter is a reflection of how many iterations resulted in > zero-copy not being fully honoured.
Aha. Thanks! > IOW, ideally this counter will always be zero. If it is non-zero, > then the magnitude gives a very very very rough guide to what's > going on. If it is '1' then it was just a transient limitation. > If it matches the number of migration iterations, then it is a > more systemic limitation. I think we should rephrase the documentation in terms of migration iterations instead of "flushes", because the latter is a non-obvious term without a definition. > Incidentally, do we report the migration iteration count ? I > thought we did, but i'm not finding it now that I look. I was about to ask exactly that. I'm not sure the value of @zero-copy-copied can be usefully interpreted without.
