linux-kernel - Re: [PATCH] sched: Folding nohz load accounting more accurate

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Date:	Thu, 14 Jun 2012 23:42:37 +0800
From:	Charles Wang <muming.wq@...bao.com>
To:	Doug Smythies <dsmythies@...us.net>
CC:	'Charles Wang' <muming.wq@...il.com>,
	'Peter Zijlstra' <peterz@...radead.org>,
	<linux-kernel@...r.kernel.org>, 'Ingo Molnar' <mingo@...hat.com>,
	'Tao Ma' <tm@....ma>,
	'含黛' <handai.szj@...bao.com>
Subject: Re: [PATCH] sched: Folding nohz load accounting more accurate

于 Thursday, June 14, 2012 12:41 PM, Doug Smythies 写道:

>> On 2012.06.13 00:56 - 0700, Charles Wang wrote:
>
>> Every cpu's load should be the load right on the time executing
>> calculation.
>> This is what my patch expected to do.
>
>> After my patch, it's supposed to let every cpu's load calculation be
>> independent from
>> its idle and other cpus' influence when the calculation is finished.
>> But it seems other problem exists.
>
>> I recorded some trace, show as below:
>
> Charles thanks for your detailed reply.
> I am not sure I agree.
>
> On purpose, I have left out your trace detail from this reply.
> Why? Because I want to take a step back and answer a more fundamental
> question first:
> Is there (or was there) actually a problem that needed to be solved?
>
> I am still not understanding what your expected reported load averages
> for your 16 processes should be. How do you know that the 8 to 10 that
> was being reported was incorrect?
>

I re-create the problem by waiter, using following parameters:
./waiter 16 1800 900 9444 100 1

9444 and 100 is the key point that can make
"processing time+sleeping time" less than 1 tick.

We have 16 processors, and the load of every processor is about 0.25~0.3,
so the actual load should be 16*0.25=4. But the loadavg calculated by the
current logic is only about 1.

> I want to understand so that I can re-create your same reported load
> averages are still to low situation on my test computer, before your patch.
> Then show that it is fixed after your patch.
> I have tried to re-create it and have been unsuccessful. Today, and since
> you
> showed at least 16 CPUs on your system, and I only have 8 CPUs, I tried with
> 8 processes at high duty cycle. The reported load average was always pretty
> close to the actual load (see also attachment).
>
> Perhaps I have some fundamental misunderstanding as to how reported load
> averages should work. I have always used kernels compiled with
> CONFIG_NO_HZ=no (tick based, rather than tickless) as the control reference.
>



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