Re: [ckrm-tech] [RFC 3/5] sched: Add CPU rate hard caps

[Peter Williams <peterw@aurema.com>]

Balbir Singh wrote:
> Peter Williams wrote:
>> Balbir Singh wrote:
>>
>>> Peter Williams wrote:
>>> <snip>
>>>
>>>>>> But you don't need something as complex as CKRM either.  This capping
>>>>>
>>>>> All CKRM^W Resource Groups does is to group unrelated/related tasks 
>>>>> to a
>>>>> group and apply resource limits.
>>>>>
>>>>>
>>>>>> functionality coupled with (the lamented) PAGG patches (should 
>>>>>> have been called TAGG for "task aggregation" instead of PAGG for 
>>>>>> "process aggregation") would allow you to implement a kernel 
>>>>>> module that could apply caps to arbitrary groups of tasks.
>>>>>
>>>>> I do not follow how PAGG + this cap feature can be used to put cap of
>>>>> related/unrelated tasks. Can you provide little more explanation,
>>>>> please.
>>>>
>>>>
>>>> I would have thought it was fairly obvious.  PAGG supplies the task 
>>>> aggregation mechanism, these patches provide per task caps and all 
>>>> that's needed is the code that marries the two.
>>>>
>>>
>>> The problem is that with per-task caps, if I have a resource group A
>>> and I want to limit it to 10%, I need to limit each task in resource
>>> group A to 10% (which makes resource groups not so useful). Is my
>>> understanding correct?
>>
>>
>> Well the general idea is correct but your maths is wrong.  You'd have 
>> to give each of them a cap somewhere between 10% and 10% divided by 
>> the number of tasks in group A.  Exactly where in that range would 
>> vary depending on the CPU demand of each task and would need to be 
>> adjusted dynamically (unless they were very boring tasks whose demands 
>> were constant over time).
>>
> 
> 
> Hmm.. I thought my math was reasonable (but there is always so much to 
> learn)
>  From your formula, if I have 1 task in group A, I need to provide it with
> a cap of b/w 10 to 11%. For two tasks, I need to give them b/w 10 to 10.5%.
> If I have a hundred, it needs to be b/w 10% and 10.01%

Now your arithmetic is failing you.  According to my formula:

1. With one task in group A you give it 10% which is what you get when 
you divide 10% by one.

2. With two tasks in group A you give them each somewhere between 5% 
(which is 10% divided by 2) and 10%.  If they are equally busy you give 
them each 5% and if they are not equally busy you give them you give 
them larger caps.

Another, probably a better but more expensive, formula is to divide the 
10% between them in proportion to their demand.  Being careful not to 
give any of them a zero cap, of course.  I.e. in the two task 10% case 
they each get 5% if they are equally busy but if one is twice as busy as 
the other it gets a 6.6% cap and the other gets 3.3% (approximately).

Peter
-- 
Dr Peter Williams, Chief Scientist         <peterw@aurema.com>
Aurema Pty Limited
Level 2, 130 Elizabeth St, Sydney, NSW 2000, Australia
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