Re: [PATCH v2 2/3] Add zero-copy-copied migration stat

[Markus Armbruster]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Mon, Jul 04, 2022 at 08:18:54AM +0200, Markus Armbruster wrote:
>> Leonardo Bras <leobras@redhat.com> writes:
>> 
>> > Signed-off-by: Leonardo Bras <leobras@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.