[PATCH for-4.17 v1 0/2] xenctrl.ml: improve scalability of domain_getinfolist

["Edwin Török" <edvin.torok@citrix.com>]

Pau has performed some performance tests by booting 1000 mirage
unikernels test VMs and shutting them down.
We've noticed on the flamegraphs that a lot of time is spent in Xenctrl
`domain_getinfolist`, 17.7% of overall time
(This needs to be multiplied by 16 because Dom0 100% usage = 16 vCPUs)
In particular time is spent in camlXenctrl___getlist_339
as can be seen from this flamegraph, and it also creates a very deep
call stack:
https://cdn.jsdelivr.net/gh/edwintorok/xen@xenctrl-coverletter/docs/tmp/perf-merge-boot.svg?x=948.9&y=2213

After some algorithmic improvements to the code now the function barely
shows up at all on a flamegraph, taking only 0.02%.
The function is called camlXenctrl___getlist_343, but that is just due
to the changed arguments, still the same function:
https://cdn.jsdelivr.net/gh/edwintorok/xen@xenctrl-coverletter/docs/tmp/perf-xen-boot-1150.svg?x=1188.0&y=1941&s=infolist

It was calling the Xen hypercall ~500*1000 times for 1000 VMs, and
instead it is now calling it "only" 1000 times.

I would suggest to try to take this in 4.17 given the massive
improvement in scalability (number of VMs on a Xen host).

There are further improvements possible here, but they'll be in xenopsd
(part of XAPI) to avoid calling domain_getinfolist and just use
domain_getinfo: the only reason it needs use infolist is that it does
the lookup by VM UUID and not by domid, but it could have a small cache
of UUID->domid mappings and then call just domain_getinfo (or get the
mapping from xenstore if not in the cache), but it looks like that
improvement is not even needed if this function barely registers on a
flamegraph now.

P.S.: the mirage test VM is a very old PV version, at some point we'll
repeat the test with a Solo5 based PVH one.

Edwin Török (2):
  xenctrl.ml: make domain_getinfolist tail recursive
  xenctrl: use larger chunksize in domain_getinfolist

 tools/ocaml/libs/xc/xenctrl.ml | 25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

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2.34.1