l3 cache and cpu pinning

[Roman Mohr]

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.


>
>
>>
>>  - 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-
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>>
>>

[1] https://lists.gnu.org/archive/html/qemu-devel/2017-11/msg04592.html
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