Re: [PATCH V1 0/6] fast qom tree get
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(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?
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!!!
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.