Re: [RFC][Patch v9 0/6] KVM: Guest Free Page Hinting

[Michael S. Tsirkin]

On Wed, Mar 06, 2019 at 07:59:57PM +0100, David Hildenbrand wrote:
> On 06.03.19 19:43, Michael S. Tsirkin wrote:
> > On Wed, Mar 06, 2019 at 01:30:14PM -0500, Nitesh Narayan Lal wrote:
> >>>> Here are the results:
> >>>>
> >>>> Procedure: 3 Guests of size 5GB is launched on a single NUMA node with
> >>>> total memory of 15GB and no swap. In each of the guest, memhog is run
> >>>> with 5GB. Post-execution of memhog, Host memory usage is monitored by
> >>>> using Free command.
> >>>>
> >>>> Without Hinting:
> >>>>                  Time of execution    Host used memory
> >>>> Guest 1:        45 seconds            5.4 GB
> >>>> Guest 2:        45 seconds            10 GB
> >>>> Guest 3:        1  minute               15 GB
> >>>>
> >>>> With Hinting:
> >>>>                 Time of execution     Host used memory
> >>>> Guest 1:        49 seconds            2.4 GB
> >>>> Guest 2:        40 seconds            4.3 GB
> >>>> Guest 3:        50 seconds            6.3 GB
> >>> OK so no improvement.
> >> If we are looking in terms of memory we are getting back from the guest,
> >> then there is an improvement. However, if we are looking at the
> >> improvement in terms of time of execution of memhog then yes there is none.
> > 
> > Yes but the way I see it you can't overcommit this unused memory
> > since guests can start using it at any time.  You timed it carefully
> > such that this does not happen, but what will cause this timing on real
> > guests?
> 
> Whenever you overcommit you will need backup swap.

Right and the point of hinting is that pages can just be
discarded and not end up in swap.


Point is you should be able to see the gain.

Hinting patches cost some CPU so we need to know whether
they cost too much. How much is too much? When the cost
is bigger than benefit. But we can't compare CPU cycles
to bytes. So we need to benchmark everything in terms of
cycles.

> There is no way
> around it. It just makes the probability of you having to go to disk
> less likely.


Right and let's quantify this. Does this result in net gain or loss?


> If you assume that all of your guests will be using all of their memory
> all the time, you don't have to think about overcommiting memory in the
> first place. But this is not what we usually have.

Right and swap is there to support overcommit. However it
was felt that hinting can be faster since it avoids IO
involved in swap.

> > 
> > So the real reason to want this is to avoid need for writeback on free
> > pages.
> > 
> > Right?
> 
> -- 
> 
> Thanks,
> 
> David / dhildenb