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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Apr 09, 2025 at 09:58:13AM +0200, Peter Krempa via Devel wrote:
> On Wed, Apr 09, 2025 at 09:39:02 +0200, Markus Armbruster via Devel wrote:
> > Hi Steve, I apologize for the slow response.
> > 
> > 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.
> > 
> > "Some managers"... could you name one?
> 
> libvirt is at ~500 qom-get calls during an average startup ...
> 
> > > 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?
> 
> YES!!!

Not neccessarily, see below... !!!! 

> 
> The getter with value could SO MUCH optimize the startup sequence of a
> VM where libvirt needs to probe CPU flags:
> 
> (note the 'id' field in libvirt's monitor is sequential)
> 
> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"realized"},"id":"libvirt-8"}
> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"hotplugged"},"id":"libvirt-9"}
> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"hotpluggable"},"id":"libvirt-10"}
> 
> [...]
> 
> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"hv-apicv"},"id":"libvirt-470"}
> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"xd"},"id":"libvirt-471"}
> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"sse4_1"},"id":"libvirt-472"}
> buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"unavailable-features"},"id":"libvirt-473"}
> 
> First and last line's timestamps:
> 
> 2025-04-08 14:44:28.882+0000: 1481190: info : qemuMonitorIOWrite:340 : QEMU_MONITOR_IO_WRITE: mon=0x7f4678048360 buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"realized"},"id":"libvirt-8"}
> 
> 2025-04-08 14:44:29.149+0000: 1481190: info : qemuMonitorIOWrite:340 : QEMU_MONITOR_IO_WRITE: mon=0x7f4678048360 buf={"execute":"qom-get","arguments":{"path":"/machine/unattached/device[0]","property":"unavailable-features"},"id":"libvirt-473"}
> 
> Libvirt spent ~170 ms probing cpu flags.

One thing I would point out is that qom-get can be considered an
"escape hatch" to get information when no better QMP command exists.
In this case, libvirt has made the assumption that every CPU feature
is a QOM property.

Adding qom-list-get doesn't appreciably change that, just makes the
usage more efficient.

Considering the bigger picture QMP design, when libvirt is trying to
understand QEMU's CPU feature flag expansion, I would ask why we don't
have something like a "query-cpu" command to tell us the current CPU
expansion, avoiding the need for poking at QOM properties directly.

With regards,
Daniel
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