From mboxrd@z Thu Jan 1 00:00:00 1970 From: Raghavendra K T Subject: Re: [PATCH RFC 0/2] kvm: Improving undercommit,overcommit scenarios in PLE handler Date: Fri, 21 Sep 2012 23:06:58 +0530 Message-ID: <505CA5BA.4020801@linux.vnet.ibm.com> References: <20120921115942.27611.67488.sendpatchset@codeblue> <505C691D.4080801@hp.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Peter Zijlstra , "H. Peter Anvin" , Marcelo Tosatti , Ingo Molnar , Avi Kivity , Rik van Riel , Srikar , "Nikunj A. Dadhania" , KVM , Jiannan Ouyang , "Andrew M. Theurer" , LKML , Srivatsa Vaddagiri , Gleb Natapov , Andrew Jones To: Chegu Vinod Return-path: In-Reply-To: <505C691D.4080801@hp.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org 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.