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SubjectRe: [ckrm-tech] [RFC 3/5] sched: Add CPU rate hard caps
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
Tel:+61 2 9698 2322 Fax:+61 2 9699 9174 http://www.aurema.com
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