Re: [Qemu-devel] [RFC 0/3] qmp: Return extra information on qom-list-types

[Eduardo Habkost]

On Tue, May 23, 2017 at 04:12:43PM +0200, Markus Armbruster wrote:
> Eduardo Habkost <address@hidden> writes:
> 
> > This series adds 'abstract' and 'parent-types' fields to the
> > output of qom-list-types.
> 
> Peeking at PATCH 3, it looks like 'parent-types' is the transitive
> closure of the single direct parent type (TypeImpl member parent_type).
> Do we need information on interfaces as well?

I think we should, but it is more complex so I plan to do in a
separate patch.

> 
> > For reference, below are the sizes of the output of
> > "qom-list-types abstract=true" on qemu-system-x86_64, before and
> > after applying the patches:
> >
> > * before: 11724 bytes
> > * with 'abstract' field: 20119 bytes
> > * with 'abstract' and 'parent-types': 44383 bytes
> 
> Obvious ways to save space:
> 
> * Make 'abstract' optional, default to false, present only when
>   necessary (79 out of 456 times right now)

Good idea.

> 
> * Pare down 'parent-types' to *direct* parent types.  The indirect ones
>   are redundant.

On the one hand, I assume clients don't care if a given type is a
direct parent or indirect parent, and including only the direct
parent type will require them to make extra queries.

On the other hand, if the client wants to save a few queries it
can use the "implements" argument, already? Not sure.

> 
> A less obvious way:
> 
> * 'parent-types' defines a relation with adjacency lists.  If the
>   relation is tree-shaped, we can send the tree instead, i.e. a tree of
>   device names rather than list of (device name, adjacency list).  New
>   command unless we're willing to break qom-list-types.
> 
> > I'm not sure if extending qom-list-types with this info is the
> > right way to go, or if we should keep qom-list-types as-is and
> > add a new "query-qom-type" QMP command to query info on one QOM
> > type at a time.
> 
> Might lead to more traffic rather than less.  Depends on what
> information the client needs to query.

I think queries that return all available QOM types are likely to
(should?) be cached by clients.  I believe the size will stay
acceptable if we implement the two suggestions above.

-- 
Eduardo