Re: [RFC][PATCH 4/9] create aggregate kvm_total_used_mmu_pages value

Avi Kivity
2010-06-17T08:23:25Z

On 06/16/2010 07:55 PM, Dave Hansen wrote:
> On Wed, 2010-06-16 at 11:48 +0300, Avi Kivity wrote:
>
>>> +static inline void kvm_mod_used_mmu_pages(struct kvm *kvm, int nr)
>>> +{
>>> + kvm->arch.n_used_mmu_pages += nr;
>>> + kvm_total_used_mmu_pages += nr;
>>>
>>>
>> Needs an atomic operation, since there's no global lock here. To avoid
>> bouncing this cacheline around, make the variable percpu and make
>> readers take a sum across all cpus. Side benefit is that you no longer
>> need an atomic but a local_t, which is considerably cheaper.
>>
> We do have the stuff in:
>
> include/linux/percpu_counter.h
>
> the downside being that they're not precise and they're *HUGE* according
> to the comment. :)
>
> It's actually fairly difficult to do a counter which is precise,
> scalable, and works well for small CPU counts when NR_CPUS is large. Do
> you mind if we just stick with a plain atomic_t for now?
>

Do we really need something precise?

I'm not excited by adding a global atomic. So far nothing in the kvm
hot paths depends on global shared memory (though we have lots of per-vm
shared memory).

Can we perhaps query the kmem_cache for a representation of the amount
of objects?

--
error compiling committee.c: too many arguments to function