On Thu, Apr 22, 2021 at 1:24 PM Roman Mohr <rm...@redhat.com> wrote: > > > On Thu, Apr 22, 2021 at 1:19 PM Roman Mohr <rm...@redhat.com> wrote: > >> >> >> On Wed, Apr 21, 2021 at 1:09 PM Daniel P. Berrangé <berra...@redhat.com> >> wrote: >> >>> On Wed, Apr 21, 2021 at 12:53:49PM +0200, Roman Mohr wrote: >>> > Hi, >>> > >>> > I have a question regarding enabling l3 cache emulation on Domains. Can >>> > this also be enabled without cpu-pinning, or does it need cpu pinning >>> to >>> > emulate the l3 caches according to the cpus where the guest is pinned >>> to? >>> >>> I presume you're referring to >>> >>> <cpu> >>> <cache level='3' mode='emulate|passthrough|none'/> >>> </cpu> >>> >>> There is no hard restriction placed on usage of these modes by QEMU. >>> >>> Conceptually though, you only want to use "passthrough" mode if you >>> have configured the sockets/cores/threads topology to match the host >>> CPUs. In turn you only ever want to set sockets/cores/threads to >>> match the host if you have done CPU pinning such that the topology >>> actually matches the host CPUs that have been pinned to. >>> >>> As a rule of thumb >>> >>> - If letting CPUs float >>> >>> -> Always uses sockets=1, cores=num-vCPUs, threads=1 >>> -> cache==emulate >>> -> Always use 1 guest NUMA node (ie the default) >>> >>> >> Is `emulate` also the default in libvirt? If not, would you see any >> reason, e.g. thinking about migrations, to not set it always if no cpu >> pinning is done? >> > > To answer my own question: I guess something like [1] is a good reason to > not enable l3-cache by default, since it seems to have an impact on VM > density on nodes. >
Hm, seems like this change got only merged for older machine types. So according to the libvirt doc (not setting it means hypervisor default), it is probably set to emulation? > > >> >> >>> >>> - If strictly pinning CPUs 1:1 >>> >>> -> Use sockets=N, cores=M, threads=0 to match the topology >>> of the CPUs that have been pinned to >>> -> cache==passthrough >>> -> Configure virtual NUMA nodes if the CPU pinning or guest >>> RAM needs cross host NUMA nodes. >>> >>> >>> >>> Regards, >>> Daniel >>> -- >>> |: https://berrange.com -o- >>> https://www.flickr.com/photos/dberrange :| >>> |: https://libvirt.org -o- >>> https://fstop138.berrange.com :| >>> |: https://entangle-photo.org -o- >>> https://www.instagram.com/dberrange :| >>> >>> > > [1] https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg04592.html >
