Re: [External] Re: [PATCH v3 6/7] migration/multifd: Add zero pages and zero bytes counter to migration status interface.

[Markus Armbruster <armbru@redhat.com>]

Hao Xiang <hao.xiang@bytedance.com> writes:

> On Wed, Feb 28, 2024 at 1:52 AM Markus Armbruster <armbru@redhat.com> wrote:
>>
>> Hao Xiang <hao.xiang@bytedance.com> writes:
>>
>> > This change extends the MigrationStatus interface to track zero pages
>> > and zero bytes counter.
>> >
>> > Signed-off-by: Hao Xiang <hao.xiang@bytedance.com>
>>
>> [...]
>>
>> > diff --git a/qapi/migration.json b/qapi/migration.json
>> > index a0a85a0312..171734c07e 100644
>> > --- a/qapi/migration.json
>> > +++ b/qapi/migration.json
>> > @@ -63,6 +63,10 @@
>> >  #     between 0 and @dirty-sync-count * @multifd-channels.  (since
>> >  #     7.1)
>> >  #
>> > +# @zero-pages: number of zero pages (since 9.0)
>> > +#
>> > +# @zero-bytes: number of zero bytes sent (since 9.0)
>> > +#
>>
>> Awfully terse.  How are these two related?
>
> Sorry I forgot to address the same feedback from the last version.

Happens :)

> zero-pages are the number of pages being detected as all "zero" and
> hence the payload isn't sent over the network. zero-bytes is basically
> zero-pages * page_size. It's the number of bytes migrated (but not
> actually sent through the network) because they are all "zero". These
> two are related to the existing interface below. normal and
> normal-bytes are the same representation of pages who are not all
> "zero" and are actually sent through the network.
>
> # @normal: number of normal pages (since 1.2)
> #
> # @normal-bytes: number of normal bytes sent (since 1.2)

We also have

  # @duplicate: number of duplicate (zero) pages (since 1.2)
  #
  # @skipped: number of skipped zero pages. Always zero, only provided for
  #     compatibility (since 1.5)

Page skipping was introduced in 1.5, and withdrawn in 1.5.3 and 1.6.
@skipped was formally deprecated in 8.1.  It'll soon be gone, no need to
worry about it now.

That leaves three values related to pages sent: @normal (and
@normal-bytes), @duplicate (but no @duplicate-bytes), and @zero-pages
(and @zero-bytes).

I unwittingly created a naming inconsistency between @normal,
@duplicate, and @zero-pages when I asked you to rename @zero to
@zero-pages.

The meaning of the three values is not obvious, and the doc comments
don't explain them.  Can you, or anybody familiar with migration,
explain them to me?

MigrationStats return some values as bytes, some as pages, and some as
both.  I hate that.  Can we standardize on bytes?

>>
>> >  # Features:
>> >  #
>> >  # @deprecated: Member @skipped is always zero since 1.5.3
>>
>> [...]
>>