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. Berrang? <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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