Re: [Xen-devel] reported memory usage does not match real memory usage

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.02.2020 15:22, Olaf Hering wrote:
> Am Wed, 12 Feb 2020 11:53:25 +0100
> schrieb Olaf Hering <olaf@xxxxxxxxx>:
> 
>> Is there a formula to calculate that amount of extra memory, is this 
>> behavior documented somewhere?

See e.g. sh_min_allocation() and whatever its HAP counterpart is. But
this and hence ...

> With the script below, the formula may look like this:
> - each vcpu needs 1MB extra memory
> - each GB of a HVM domU memory needs 8MB extra memory
> - each HVM domU needs 2MB extra memory

... this is really an implementation details.

> I assume these 8MB per GB is needed for the EPT page tables.
> 
> In case this extra memory is indeed some obvious static value,
> it would be better to allocate it from the value specified in
> 'memory=' to make sure a domU uses (almost) exactly the value
> that was configured.

Well, the "memory=" is, afaik, supposed to (largely) represent
what the guest is going to "see" (not all of it may in fact be
visible through the E820 map the guest sees at boot, but that's
a different aspect). Along with it there also is
"shadow_memory=", but the implementation has always been growing
this value if it was too small, to avoid rendering guests
unstable. So switching to a model like you propose would likely
require a new guest config option (and then how to deal with
conflicts between old and new options would need to be spelled
out).

Jan

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