[ovirt-devel] CPU sockets, threads, cores and NUMA

[Martin Polednik]

Hello developers,

tl;dr version:
* deprecate report_host_threads_as_cores
* remove cpuSockets, use sum(numaNodes.keys())
* report threadsPerCore for ppc64le / report total number of threads
  for ppc64le
* work on our naming issues

I've been going over our capabilities reporting code in VDSM due to
specific threading requirements on ppc64le platform and noticed few
issues. Before trying to fix something that "works", I'm sending this
mail to start a discussion regarding current and future state of the
code.

First thing is the terminology. What we consider cpu sockets, 
cores and threads are in fact NUMA cells, sum of cores present in NUMA
nodes and the same for threads. I'd like to see the code moving in a
direction that is correct in this sense.

More important are the actual calculations. I believe we should draw
an uncrossable line between cores and threads and not interfere with
it at least on VDSM's side. That would mean deprecating
report_host_threads_as_cores option. The actual algorithm used at
present does calculate the numa cores and numa threads correctly given
that there are no offline CPUs - most likely fine enough. We don't
have to report the actual number of sockets though, as it is reported
in numa* keys.

It does fail to provide us with information that can be used in
ppc64le environment, where for POWER8 we want to run the host without
SMT while VMs would have multiple CPUs assigned. There are various
configurations of so-called subcores in POWER8, where each CPU core
can contain 1, 2 or 4 subcores. This configuration must be taken in
consideration as given e.g. 160 threads overall, it is possible to run
either 20 VMs in smt8 mode, 40 VMs in smt4 mode or 80 VMs in smt2
mode. We have to report either the total number of threads OR just the
threadsPerCore setting, so the users know how many "CPUs" should be
assigned to machines for optimal performance.

As always, I welcome any opinions regarding the proposed ideas. Also
note that all of the changes can be done via deprecation to be fully
backwards compatible - except for the ppc part.

Regards,
mpolednik