Re: [PATCH V2 0/5] fast qom tree get

[Steven Sistare <steven.sistare@oracle.com>]

On 7/4/2025 8:26 AM, Markus Armbruster wrote:
> Steve Sistare <steven.sistare@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.
>>
>> 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.
>>
>> In all cases, a returned property is represented by ObjectPropertyValue,
>> with fields name, type, and value.  If an error occurs when reading a value
>> the value field is omitted.  Thus an error for one property will not cause a
>> bulk fetch operation to fail.
>>
>> To evaluate each method, I modified scripts/qmp/qom-tree to use the method,
>> verified all methods produce the same output, and timed each using:
>>
>>    qemu-system-x86_64 -display none \
>>      -chardev socket,id=monitor0,path=/tmp/vm1.sock,server=on,wait=off \
>>      -mon monitor0,mode=control &
>>
>>    time qom-tree -s /tmp/vm1.sock > /dev/null
>>
>> I only measured once per method, but the variation is low after a warm up run.
>> The 'real - user - sys' column is a proxy for QEMU CPU time.
>>
>> method               real(s)   user(s)   sys(s)  (real - user - sys)(s)
>> qom-list / qom-get   2.048     0.932     0.057   1.059
>> qom-list-get         0.402     0.230     0.029   0.143
>> qom-list-getv        0.200     0.132     0.015   0.053
>> qom-tree-get         0.143     0.123     0.012   0.008
>>
>> qom-tree-get is the clear winner, reducing elapsed time by a factor of 14X,
>> and reducing QEMU CPU time by 132X.
>>
>> qom-list-getv is slower when fetching the entire tree, but can beat
>> qom-tree-get when only a subset of the tree needs to be fetched (not shown).
>> qom-list-get is shown for comparison only, and is not included in this series.
> 
> How badly do you need the additional performance qom-tree-get can give
> you in certain cases?
> 
> I'm asking because I find qom-list-getv *much* simpler.

I would be content with qom-list-getv, so I will drop qom-tree-get.
qom-list-getv needs ObjectPropertyValue and qom_list_add_property_value
from the qom-tree-get patch, so I will respond to those comments.

- Steve