Re: [PATCH] migration: Allow user to specify migration available bandwidth

[Peter Xu]

On Fri, Aug 04, 2023 at 02:06:02PM +0200, Markus Armbruster wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Wed, Jul 26, 2023 at 08:21:35AM +0200, Markus Armbruster wrote:
> >> Peter Xu <peterx@redhat.com> writes:
> >> 
> >> > Hi, Markus,
> >> >
> >> > On Tue, Jul 25, 2023 at 01:10:01PM +0200, Markus Armbruster wrote:
> >> 
> >> [...]
> >> 
> >> >> For better or worse, we duplicate full documentation between
> >> >> MigrationParameter, MigrateSetParameters, and MigrationParameters.  This
> >> >> would be the first instance where we reference instead.  I'm not opposed
> >> >> to use references, but if we do, I want them used consistently.
> >> >
> >> > We discussed this over the other "switchover" parameter, but that 
> >> > patchset
> >> > just stranded..
> >> >
> >> > Perhaps I just provide a pre-requisite patch to remove all the comments 
> >> > in
> >> > MigrateSetParameters and MigrationParameters, letting them all point to
> >> > MigrationParameter?
> >> 
> >> Simplifies maintaining the doc commments.  But how does it affect the
> >> documentation generated from it?  Better, neutral, or worse?
> >
> > Probably somewhere neutral.  There are definitely benefits, shorter
> > man/html page in total, and avoid accidentally different docs over the same
> > fields.  E.g., we sometimes have different wordings for different objects:
> >
> >        max-cpu-throttle
> >               maximum cpu throttle percentage.  Defaults to 99.  (Since 3.1)
> >
> >        max-cpu-throttle: int (optional)
> >               maximum cpu throttle percentage.  The default value is 99. 
> > (Since 3.1)
> >
> > This one is fine, but it's just very easy to leak in something that shows
> > differently.  It's good to provide coherent documentation for the same
> > fields over all three objects.
> 
> Yes, but we've been doing okay regardless.
> 
> The drawback of replacing duplicates by references is that readers need
> to follow the references.
> 
> Less onerous when the references can be clicked.
> 
> If we de-duplicate, which copy do we keep, MigrationParameter,
> MigrateSetParameters, or MigrationParameter?  Do we have an idea which
> of these users are likely to read first?

I chose MigrationParameter for no explicit reason, because I can't find
good argument to differenciate them.  Please let me know if you have any
suggestion.

> 
> > When looking at qemu-qmp-ref.7, it can be like this when we can invite the
> > reader to read the other section (assuming we only keep MigrationParameter
> > to keep the documentations):
> >
> >    MigrationParameters (Object)
> >
> >        The object structure to represent a list of migration parameters.
> >        The optional members aren't actually optional.  For detailed
> >        explanation for each of the field, please refer to the documentation
> >        of MigrationParameter.
> >
> > But the problem is we always will generate the Members entry, where now
> > it'll all filled up with "Not documented"..
> >
> >    Members
> >        announce-initial: int (optional)
> >               Not documented
> >
> >        announce-max: int (optional)
> >               Not documented
> >
> >        announce-rounds: int (optional)
> >               Not documented
> >
> >        [...]
> >
> > I think maybe it's better we just list the members without showing "Not
> > documented" every time for the other two objects.  Not sure whether it's an
> > easy way to fix it, or is it a major issue.
> 
> The automatic generation of "Not documented" documentation is a
> stop-gap.  Leaving a member undocumented should be a hard error.  It
> isn't only because we have 511 instances to fix.
> 
> Perhaps a directive to ignore undocumented members could be useful.
> I.e. to suppress the automatic "Not documented" documented now, the
> error later.
> 
> We could write things out in longhand instead, like
> 
>     # @announce-initial: Same as MigrationParameter member
>     #     @announce-initial.

Yes I can definitely do this.

Since I don't really know whether the "put a link" will work at all (at
least man page doesn't really have those, does it?), would this be the way
you suggest us forward?

Note that I am also always happy to simply duplicate the three paragraphs
just like before; that's not something I must do with solving the migration
problem so far, we can decouple the two problems essentially.  But since
we're at it, if you think worthwhile we may have a chance get rid of
duplicated documents here (before code) I can try.

> 
> > For developers, dedup the comment should always be a win, afaict.
> 
> No argument.

Let me explain a bit: I meant the patch author who will reduce writting
duplicated documents, making sure everything match together.  And reviewers
who will read the duplicated content, making sure that everything match
together again.  The two efforts can be avoided.  That's all I meant here
for when I was referring to as "developers" in this context..  Not everyone
as a common sense of developer.

Thanks,

-- 
Peter Xu