[V1,0/6] fast qom tree get

Message

Steve Sistare March 3, 2025, 9:09 p.m. UTC

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, 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.

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.

Steve Sistare (6):
  qom: qom_resolve_path
  qom: qom-tree-get
  python: use qom-tree-get
  tests/qtest/qom-test: unit test for qom-tree-get
  qom: qom-list-getv
  tests/qtest/qom-test: unit test for qom-list-getv

 python/qemu/utils/qom.py        |  36 +++++------
 python/qemu/utils/qom_common.py |  50 ++++++++++++++++
 qapi/qom.json                   |  93 ++++++++++++++++++++++++++++
 qom/qom-qmp-cmds.c              | 130 ++++++++++++++++++++++++++++++++++++++--
 tests/qtest/qom-test.c          | 116 +++++++++++++++++++++++++++++++++++
 5 files changed, 401 insertions(+), 24 deletions(-)

base-commit: 354925d42252f6f36a9e1e4a6b929aaafb2eaf45