linux-kernel - Re: [PATCH V2 RFC 3/3] kvm: Check system load and handle different commit cases accordingly

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Date:	Tue, 30 Oct 2012 13:01:54 +0530
From:	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>
To:	Andrew Jones <drjones@...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>,
	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>,
	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 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..?

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