Date:	Wed, 31 Oct 2012 17:54:45 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Andrew Jones <drjones@...hat.com>, Rik van Riel <riel@...hat.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, Avi Kivity <avi@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Marcelo Tosatti <mtosatti@...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>,
	Chegu Vinod <chegu_vinod@...com>,
	"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>
Subject: Re: [PATCH V2 RFC 3/3] kvm: Check system load and handle different
 commit cases accordingly

On 10/30/2012 02:37 PM, Andrew Jones wrote:
> On Tue, Oct 30, 2012 at 01:01:54PM +0530, Raghavendra K T wrote:
>> On 10/30/2012 12:04 PM, Andrew Jones wrote:
>>> On Tue, Oct 30, 2012 at 11:27:52AM +0530, Raghavendra K T wrote:
>>>> 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..
>>>>
>>>
>>> What happened to Avi's preempt notifier idea for determining
>>> under/overcommit? If nobody has picked that up yet, then I'll go ahead and
>>> try to prototype it.
>>
>> Hi Drew,
>>
>> I had assumed my priority order as
>> 1) this patch series 2) dynamic ple window 3) preempt notifiers.
>>
>> But I do not have any problem on re-prioritizing / helping on these
>> as far as we are clear on what we are looking into.
>>
>> I was thinking about preempt notifier idea as a tool to refine
>> candidate VCPUs. But you are right, Avi, also told we can use
>> bitmap/counter itself as an indicator to decide whether we go ahead
>> with yield_to at all.
>>
>> IMO, only patch(3) has some conflict because of various approach we can
>> try.May be we should attack the problem via all 3 solutions at once and
>> decide?
>>
>> To be frank, within each of the approach, trying/analyzing all the
>>   possibilities made the things slow.. (my end).
>>
>> Suggestions..?
>>
>
> I agree, it's a complex problem that needs lots of trial+error work. We
> should definitely work in parallel on multiple ideas. I'll go ahead and
> dig into the preempt notifiers.
>

Okay. Thank you. I will concentrate on dynamic_ple window.. But I think
implementation need some overlapping details from preempt notifier.

For dynamic ple window, To summarize, what we thought of
doing,

( I hope we have to keep the ple window between 4k - 16k throughout)

 From preempt notifiers:

(1) from the preempt notifier check the overcommit case, if so increase 
the ple window
questions:
How do we say we are overcommitted?
  - is it number of preemption we keep track vs total vcpus. I think so.
But we have to convert into some formula.. we shall decrease the ple 
window by some factor (unless we hit 4k)

(2) How can say we are undercommitted:
    Perhaps there is very less number of vcpus that are scheduled out 
currently. we tend to set ple window closer to max (16k).

 From yield_to failures:

if yield_to fails with ESRCH, it potentially indicate undercommit and
we can again use logic of increasing ple window.

Did we miss anything?

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