From mboxrd@z Thu Jan 1 00:00:00 1970 From: Raghavendra K T Subject: Re: [PATCH V3 RFC 2/2] kvm: Handle yield_to failure return code for potential undercommit case Date: Tue, 04 Dec 2012 23:19:19 +0530 Message-ID: <50BE379F.3000405@linux.vnet.ibm.com> References: <20121126120740.2595.33651.sendpatchset@codeblue> <20121126120804.2595.20280.sendpatchset@codeblue> <20121128011228.GH8295@amt.cnet> <50B59CE0.70305@linux.vnet.ibm.com> <20121203195620.GB590@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Peter Zijlstra , "H. Peter Anvin" , Avi Kivity , Gleb Natapov , Ingo Molnar , Rik van Riel , Srikar , "Nikunj A. Dadhania" , KVM , Jiannan Ouyang , Chegu Vinod , "Andrew M. Theurer" , LKML , Srivatsa Vaddagiri , Andrew Jones To: Marcelo Tosatti Return-path: Received: from e28smtp07.in.ibm.com ([122.248.162.7]:44837 "EHLO e28smtp07.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753881Ab2LDRu4 (ORCPT ); Tue, 4 Dec 2012 12:50:56 -0500 Received: from /spool/local by e28smtp07.in.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Tue, 4 Dec 2012 23:20:44 +0530 In-Reply-To: <20121203195620.GB590@amt.cnet> Sender: kvm-owner@vger.kernel.org List-ID: On 12/04/2012 01:26 AM, Marcelo Tosatti wrote: > On Wed, Nov 28, 2012 at 10:40:56AM +0530, Raghavendra K T wrote: >> On 11/28/2012 06:42 AM, Marcelo Tosatti wrote: >>> >>> Don't understand the reasoning behind why 3 is a good choice. >> >> Here is where I came from. (explaining from scratch for >> completeness, forgive me :)) >> In moderate overcommits, we can falsely exit from ple handler even when >> we have preempted task of same VM waiting on other cpus. To reduce this >> problem, we try few times before exiting. >> The problem boils down to: >> what is the probability that we exit ple handler even when we have more >> than 1 task in other cpus. Theoretical worst case should be around 1.5x >> overcommit (As also pointed by Andrew Theurer). [But practical >> worstcase may be around 2x,3x overcommits as indicated by the results >> for the patch series] >> >> So if p is the probability of finding rq length one on a particular cpu, >> and if we do n tries, then probability of exiting ple handler is: >> >> p^(n+1) [ because we would have come across one source with rq length >> 1 and n target cpu rqs with length 1 ] >> >> so >> num tries: probability of aborting ple handler (1.5x overcommit) >> 1 1/4 >> 2 1/8 >> 3 1/16 >> >> We can increase this probability with more tries, but the problem is >> the overhead. >> Also, If we have tried three times that means we would have iterated >> over 3 good eligible vcpus along with many non-eligible candidates. In >> worst case if we iterate all the vcpus, we reduce 1x performance and >> overcommit performance get hit. [ as in results ]. >> >> I have tried num_tries = 1,2,3 and n already ( not 4 yet). So I >> concluded 3 is enough. >> >> Infact I have also run kernbench and hackbench which are giving 5-20% >> improvement. >> >> [ As a side note , I also thought how about having num_tries = f(n) = >> ceil ( log(num_online_cpus)/2 ) But I thought calculation is too much >> overhead and also there is no point in probably making it dependent on >> online cpus ] >> >> Please let me know if you are happy with this rationale/ or correct me >> if you foresee some problem. (Infact Avi, Rik's concern about false >> exiting made me arrive at 'try' logic which I did not have earlier). >> >> I am currently trying out the result for 1.5x overcommit will post the >> result. > > Raghavendra > > Makes sense to me. Thanks. > Hi Marcelo, Thanks for looking into patches.