Re: [PATCH V1 0/6] fast qom tree get
Steven Sistare <steven.sistare(a)oracle.com> writes:
On 4/9/2025 3:39 AM, Markus Armbruster wrote:
> Hi Steve, I apologize for the slow response.
>
> Steve Sistare <steven.sistare(a)oracle.com> writes:
>
>> Using qom-list and qom-get to get all the nodes and property values in a
>> QOM tree can take multiple seconds because it requires 1000's of individual
>> QOM requests. Some managers fetch the entire tree or a large subset
>> of it when starting a new VM, and this cost is a substantial fraction of
>> start up time.
>
> "Some managers"... could you name one?
My personal experience is with Oracle's OCI, but likely others could benefit.
Peter Krempa tells us libvirt would benefit.
>> To reduce this cost, consider QAPI calls that fetch more
information in
>> each call:
>> * qom-list-get: given a path, return a list of properties and values.
>> * qom-list-getv: given a list of paths, return a list of properties and
>> values for each path.
>> * qom-tree-get: given a path, return all descendant nodes rooted at that
>> path, with properties and values for each.
>
> Libvirt developers, would you be interested in any of these?
>
>> In all cases, a returned property is represented by ObjectPropertyValue,
>> with fields name, type, value, and error. If an error occurs when reading
>> a value, the value field is omitted, and the error message is returned in the
>> the error field. Thus an error for one property will not cause a bulk fetch
>> operation to fail.
>
> Returning errors this way is highly unusual. Observation; I'm not
> rejecting this out of hand. Can you elaborate a bit on why it's useful?
It is considered an error to read some properties if they are not valid for
the configuration. And some properties are write-only and return an error
if they are read. Examples:
legacy-i8042: <EXCEPTION: Property 'vmmouse.legacy-i8042' is not
readable> (str)
legacy-memory: <EXCEPTION: Property 'qemu64-x86_64-cpu.legacy-memory' is
not readable> (str)
crash-information: <EXCEPTION: No crash occurred> (GuestPanicInformation)
With conventional error handling, if any of these poison pills falls in the
scope of a bulk get operation, the entire operation fails.
I suspect many of these poison pills are design mistakes.
If a property is not valid for the configuration, why does it exist?
QOM is by design dynamic. I wish it wasn't, but as long as it is
dynamic, I can't see why we should create properties we know to be
unusable.
Why is reading crash-information an error when no crash occured? This
is the *normal* case. Errors are for the abnormal.
Anyway, asking you to fix design mistakes all over the place wouldn't be
fair. So I'm asking you something else instead: do you actually need
the error information?
[...]