Re: [PATCH V2 0/5] fast qom tree get

Steve Sistare <steven.sistare(a)oracle.com&gt; 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. > > 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, 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 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. How badly do you need the additional performance qom-tree-get can give you in certain cases? I'm asking because I find qom-list-getv *much* simpler.