Open Stack

On 02/09/2016 04:20 AM, Daniel P. Berrange wrote:

> It not actually a huge waste - in fact it is likely to be beneficial to the
> overall performance of your VM. You need to remember that there is no direct
> relationship between allocation of huge pages to KVM and usage of huge pages
> inside the guest OS - any combination is valid. In particular if you give
> huge pages to KVM, then this has a performance benefit to the guest, even if
> the guest OS doesn't use huge pages itself, because it increases the TLB hit
> rate in the host for memory accesses by the guest. Using huge pages inside
> the guest OS too, increases the TLB hit rate of the guest hardware further
> extending this performance benefit. As such hosts which are intended to use
> huge pages should have all their host memory allocated to the huge page pool,
> except for what is required to run the various host OS services.

It's not very efficient to use 1G hugepages with instances having less than 1G 
of RAM, and since you have to pre-allocate 1G pages at boot time it's hard to 
know how much to allocate to 1G pages vs 2MB pages.

If all your instances have RAM in multiples of 1G then that's not a problem, of 
course.

Chris