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

From: "Daniel P. Berrangé" <berrange@redhat.com>
To: Markus Armbruster <armbru@redhat.com>
Cc: Leonardo Bras <leobras@redhat.com>,
	Juan Quintela <quintela@redhat.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Eric Blake <eblake@redhat.com>, Peter Xu <peterx@redhat.com>,
	qemu-devel@nongnu.org
Subject: Re: [PATCH v2 2/3] Add zero-copy-copied migration stat
Date: Mon, 4 Jul 2022 10:06:35 +0100	[thread overview]
Message-ID: <YsKtm2O8+d5d0p/N@redhat.com> (raw)
In-Reply-To: <877d4tz9gx.fsf@pond.sub.org>

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.

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.

Incidentally, do we report the migration iteration count ? I
thought we did, but i'm not finding it now that I look.

With regards,
Daniel
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