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-one: given a path, return a list of properties and values.
* qom-list-get: 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-one 0.402 0.230 0.029 0.143
qom-list-get 0.200 0.132 0.015 0.053
qom-tree-get 0.143 0.123 0.012 0.008
qom-tree-get is the fastest, reducing elapsed time by a factor of 14X,
and reducing QEMU CPU time by 132X.
qom-list-get 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).
To keep things simple, provide only qom-list-get.
Changes in V4:
* renamed qom-list-getv -> qom-list-get
* expanded the qtest to verify a value
* cosmetic changes in the docs
* expanded the qom-list-get commit message
Changes in V3:
* dropped qom-tree-get
* modified the qom-tree script to use qom-list-getv
* cosmetic changes in the docs and code.
Changes in V2:
* removed "qom: qom_resolve_path", which was pulled separately
* dropped the error member
* fixed missing _list_tree in qom.py
* updated 10.0 to 10.1
Steve Sistare (3):
qom: qom-list-get
python: use qom-list-get
tests/qtest/qom-test: unit test for qom-list-getv
python/qemu/utils/qom.py | 43 ++++++++-------
python/qemu/utils/qom_common.py | 53 ++++++++++++++++++
qapi/qom.json | 50 +++++++++++++++++
qom/qom-qmp-cmds.c | 53 ++++++++++++++++++
tests/qtest/qom-test.c | 116 +++++++++++++++++++++++++++++++++++++++-
5 files changed, 296 insertions(+), 19 deletions(-)
--
1.8.3.1
