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Date: Fri, 21 Sep 2012 23:06:58 +0530 From: Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com> To: Chegu Vinod <chegu_vinod@...com> CC: Peter Zijlstra <peterz@...radead.org>, "H. Peter Anvin" <hpa@...or.com>, Marcelo Tosatti <mtosatti@...hat.com>, Ingo Molnar <mingo@...hat.com>, Avi Kivity <avi@...hat.com>, Rik van Riel <riel@...hat.com>, Srikar <srikar@...ux.vnet.ibm.com>, "Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>, KVM <kvm@...r.kernel.org>, Jiannan Ouyang <ouyang@...pitt.edu>, "Andrew M. Theurer" <habanero@...ux.vnet.ibm.com>, LKML <linux-kernel@...r.kernel.org>, Srivatsa Vaddagiri <srivatsa.vaddagiri@...il.com>, Gleb Natapov <gleb@...hat.com>, Andrew Jones <drjones@...hat.com> Subject: Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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