Re: [PATCH V1 0/6] fast qom tree get

Steven Sistare <steven.sistare(a)oracle.com&gt; writes: > On 4/9/2025 9:34 AM, Markus Armbruster wrote: >> Steven Sistare <steven.sistare(a)oracle.com&gt; 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&gt; 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? > > I don't need the specific error message. > > I could return a boolean meaning "property not available" instead of returning > the exact error message, as long as folks are OK with the output of the qom-tree > script changing for these properties. Let's put aside the qom-tree script for a moment. In your patches, the queries return an object's properties as a list of ObjectPropertyValue, defined as { 'struct': 'ObjectPropertyValue', 'data': { 'name': 'str', 'type': 'str', '*value': 'any', '*error': 'str' } } As far as I understand, exactly one of @value and @error are present. The list has no duplicates, i.e. no two elements have the same value of "name". Say we're interested in property "foo". Three cases: * The list has an element with "name": "foo", and the element has member "value": the property exists and "value" has its value. * The list has an element with "name": "foo", and the element does not have member "value": the property exists, but its value cannot be gotten; member "error" has the error message. * The list has no element with "name": "foo": the property does not exist. If we simply drop ObjectPropertyValue member @error, we lose 'member "error" has the error message'. That's all. If a need for more error information should arise later, we could add member @error. Or something else entirely. Or tell people to qom-get any properties qom-tree-get couldn't get for error information. My point is: dropping @error now does not tie our hands as far as I can tell.

Back to qom-tree. I believe this script is a development aid that exists because qom-get is painful to use for humans. Your qom-tree command would completely obsolete it. I wouldn't worry about it. If you think I'm wrong there, please speak up!