On Thu, Dec 10, 2015 at 5:07 PM, Martin Polednik <mpolednik@redhat.com> wrote:
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.
Note that I think users are more familiar with sockets-cores-threads than NUMA cells, terminology-wise.
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.
There is a reason for report_host_threads_as_cores option. I don't remember it right now, but it had to do with some limitation of some OS or license or something. I don't think we should deprecate it.
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.
YAY... do we have a comparison what libvirt knows / looks at (or they ignore it altogether?) Y.
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 _______________________________________________ Devel mailing list Devel@ovirt.org http://lists.ovirt.org/mailman/listinfo/devel