[libvirt-users] dramatic performance slowdown due to THP allocation failure with full pagecache

[Daniel P. Berrange]

On Tue, Nov 14, 2017 at 10:52:03AM -0700, Blair Bethwaite wrote:
> Thanks for the reply Daniel,
> 
> However I think you slightly misunderstood the scenario...
> 
> On 14 November 2017 at 10:32, Daniel P. Berrange <berrange at redhat.com> wrote:
> > IOW, if your application has a certain expectation of performance that can only
> > be satisfied by having the KVM guest backed by huge pages, then you should
> > really change to explicitly reserve huge pages for the guests, and not rely on
> > THP which inherantly can't provide any guarantee in this area.
> 
> We already do this. The problem is not hugepage backing of the guest,
> it is THP allocation inside the guest (or indeed on a bare-metal
> host). The issue in the HPC world is that we support so many different
> applications (some of which are complete black-boxes) that explicit
> hugepage allocation for application memory is generally not viable, so
> we are reliant on THP to avoid TLB thrashing.

Oh well THP usage inside the guest is then not really anything todo with
virt, just a regular Linux questions, so not sure libvirt is the best
place to ask.


Regards,
Daniel
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