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
Tel:+61 2 9698 2322  Fax:+61 2 9699 9174 http://www.aurema.com
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