Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenariosin PLE handler

[Raghavendra K T]

On 09/21/2012 06:48 PM, Chegu Vinod wrote:
> On 9/21/2012 4:59 AM, Raghavendra K T wrote:
> > In some special scenarios like #vcpu <= #pcpu, PLE handler may
> > prove very costly,
>
> Yes.
> > because there is no need to iterate over vcpus
> > and do unsuccessful yield_to burning CPU.
> >
> > An idea to solve this is:
> > 1) As Avi had proposed we can modify hardware ple_window
> > dynamically to avoid frequent PL-exit.
>
> Yes. We had to do this to get around some scaling issues for large
> (>20way) guests (with no overcommitment)

Do you mean you already have some solution tested for this?

> As part of some experimentation we even tried "switching off" PLE too :(

Honestly,
Your this experiment and Andrew Theurer's observations were the
motivation for this patch.

> > (IMHO, it is difficult to
> > decide when we have mixed type of VMs).
>
> Agree.
>
> Not sure if the following alternatives have also been looked at :
>
> - Could the behavior associated with the "ple_window" be modified to be
> a function of some [new] per-guest attribute (which can be conveyed to
> the host as part of the guest launch sequence). The user can choose to
> set this [new] attribute for a given guest. This would help avoid the
> frequent exits due to PLE (as Avi had mentioned earlier) ?

Ccing Drew also. We had a good discussion on this idea last time.
(sorry that I forgot to include in patch series)

May be a good idea when we know the load in advance..

> - Can the PLE feature ( in VT) be "enhanced" to be made a per guest
> attribute ?
>
>
> IMHO, the approach of not taking a frequent exit is better than taking
> an exit and returning back from the handler etc.

I entirely agree on this point. (though have not tried above
approaches). Hope to see more expert opinions pouring in.

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