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?
- 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
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