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

From: Markus Armbruster <armbru@redhat.com>
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: "Daniel P. Berrangé" <berrange@redhat.com>,
	"Leonardo Bras" <leobras@redhat.com>,
	"Juan Quintela" <quintela@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, 04 Jul 2022 14:04:41 +0200	[thread overview]
Message-ID: <87k08tw0bq.fsf@pond.sub.org> (raw)
In-Reply-To: <YsLRmYjFpdGw0AjK@work-vm> (David Alan Gilbert's message of "Mon,  4 Jul 2022 12:40:09 +0100")

"Dr. David Alan Gilbert" <dgilbert@redhat.com> writes:

> * Daniel P. Berrangé (berrange@redhat.com) wrote:
>> 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)

since 7.1, unless you're planning for really tortuous review :)

>> > 
>> > 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.
>
> Yes we do; it's dirty-sync-count

Please rephrase the documentation of @zero-copy-copied in terms of
@dirty-sync-count.  Here's my attempt.

# @zero-copy-copied: Number of times dirty RAM synchronization could not
#                    avoid copying zero pages.  This is between 0 and
#                    @dirty-sync-count.  (since 7.1)