Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754853Ab2J3GCe (ORCPT ); Tue, 30 Oct 2012 02:02:34 -0400 Received: from e28smtp05.in.ibm.com ([122.248.162.5]:42612 "EHLO e28smtp05.in.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750720Ab2J3GCb (ORCPT ); Tue, 30 Oct 2012 02:02:31 -0400 Message-ID: <508F6C60.1050202@linux.vnet.ibm.com> Date: Tue, 30 Oct 2012 11:27:52 +0530 From: Raghavendra K T Organization: IBM User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:16.0) Gecko/20121009 Thunderbird/16.0 MIME-Version: 1.0 To: Peter Zijlstra CC: "H. Peter Anvin" , Avi Kivity , Ingo Molnar , Marcelo Tosatti , Rik van Riel , Srikar , "Nikunj A. Dadhania" , KVM , Jiannan Ouyang , Chegu Vinod , "Andrew M. Theurer" , LKML , Srivatsa Vaddagiri , Gleb Natapov , Andrew Jones Subject: Re: [PATCH V2 RFC 3/3] kvm: Check system load and handle different commit cases accordingly References: <20121029140621.15448.92083.sendpatchset@codeblue> <20121029140717.15448.83182.sendpatchset@codeblue> <1351533280.24721.46.camel@twins> In-Reply-To: <1351533280.24721.46.camel@twins> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit x-cbid: 12103006-8256-0000-0000-000004C84E78 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2742 Lines: 87 On 10/29/2012 11:24 PM, Peter Zijlstra wrote: > On Mon, 2012-10-29 at 19:37 +0530, Raghavendra K T wrote: >> +/* >> + * A load of 2048 corresponds to 1:1 overcommit >> + * undercommit threshold is half the 1:1 overcommit >> + * overcommit threshold is 1.75 times of 1:1 overcommit threshold >> + */ >> +#define COMMIT_THRESHOLD (FIXED_1) >> +#define UNDERCOMMIT_THRESHOLD (COMMIT_THRESHOLD >> 1) >> +#define OVERCOMMIT_THRESHOLD ((COMMIT_THRESHOLD << 1) - >> (COMMIT_THRESHOLD >> 2)) >> + >> +unsigned long kvm_system_load(void) >> +{ >> + unsigned long load; >> + >> + load = avenrun[0] + FIXED_1/200; >> + load = load / num_online_cpus(); >> + >> + return load; >> +} > > ARGH.. no that's wrong.. very wrong. > > 1) avenrun[] EXPORT_SYMBOL says it should be removed, that's not a > joke. Okay. > 2) avenrun[] is a global load, do not ever use a global load measure This makes sense. Using a local optimization that leads to near global optimization is the way to go. > > 3) avenrun[] has nothing what so ever to do with runqueue lengths, > someone with a gazillion tasks in D state will get a huge load but the > cpu is very idle. > I used loadavg as an alternative measure. But the above condition poses a concern for that. Okay, now IIUC, usage of *any* global measure is bad? Because I was also thinking to use nrrunning()/ num_online_cpus(), to get an idea of global overcommit sense. (ofcourse since, this involves iteration over per CPU nrrunning, I wanted to calculate this periodically) The overall logic, of having overcommit_threshold, undercommit_threshold, I wanted to use for even dynamic ple_window tuning purpose. so logic was: < undercommit_threshold => 16k ple_window > overcommit_threshold => 4k window. for in between case scale the ple_window accordingly. The alternative was to decide depending on how ple handler succeeded in yield_to. But I thought, that is too sensitive and more overhead. This topic may deserve different thread, but thought I shall table it here. So, Thinking about the alternatives to implement, logic such as (a) if(undercommitted) just go back and spin rather than going for yield_to iteration. (b) if (overcommitted) better to yield rather than spinning logic of current patches.. [ ofcourse, (a) is already met to large extent by your patches..] So I think everything boils down to "how do we measure these two thresholds without much overhead in a compliant way" Ideas welcome.. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/