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!!! 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.