l3 cache and cpu pinning

[Roman Mohr]

On Thu, Apr 22, 2021 at 1:24 PM Roman Mohr <rmohr at redhat.com> wrote:

>
>
> On Thu, Apr 22, 2021 at 1:19 PM Roman Mohr <rmohr at redhat.com> wrote:
>
>>
>>
>> On Wed, Apr 21, 2021 at 1:09 PM Daniel P. Berrangé <berrange at 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
>
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