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

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Date:   Wed, 6 Mar 2019 13:43:54 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Nitesh Narayan Lal <nitesh@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, pbonzini@...hat.com, lcapitulino@...hat.com,
        pagupta@...hat.com, wei.w.wang@...el.com, yang.zhang.wz@...il.com,
        riel@...riel.com, david@...hat.com, dodgen@...gle.com,
        konrad.wilk@...cle.com, dhildenb@...hat.com, aarcange@...hat.com,
        alexander.duyck@...il.com
Subject: Re: [RFC][Patch v9 0/6] KVM: Guest Free Page Hinting

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?

So the real reason to want this is to avoid need for writeback on free
pages.

Right?


> >  OTOH Alex's patches cut time down to 5-7 seconds
> > which seems better. 
> I haven't investigated memhog as such so cannot comment on what exactly
> it does and why there was a time difference. I can take a look at it.
> > Want to try testing Alex's patches for comparison?
> Somehow I am not in a favor of doing a hypercall on every page (with
> huge TLB order/MAX_ORDER -1) as I think it will be costly.
> I can try using Alex's host side logic instead of virtio.
> Let me know what you think?
> >
> -- 
> Regards
> Nitesh
>