Re: CFS review

[Willy Tarreau]

On Sat, Aug 11, 2007 at 12:50:08AM +0200, Roman Zippel wrote:
> Hi,
> 
> On Fri, 10 Aug 2007, Ingo Molnar wrote:
> 
> > achieve that. It probably wont make a real difference, but it's really 
> > easy for you to send and it's still very useful when one tries to 
> > eliminate possibilities and when one wants to concentrate on the 
> > remaining possibilities alone.
> 
> The thing I'm afraid about CFS is its possible unpredictability, which 
> would make it hard to reproduce problems and we may end up with users with 
> unexplainable weird problems. That's the main reason I'm trying so hard to 
> push for a design discussion.

You may be interested by looking at the very early CFS versions. The design
was much more naive and understandable. After that, a lot of tricks have
been added to take into account a lot of uses and corner cases, which may
not help in understanding it globally.

> Just to give an idea here are two more examples of irregular behaviour, 
> which are hopefully easier to reproduce.
> 
> 1. Two simple busy loops, one of them is reniced to 15, according to my 
> calculations the reniced task should get about 3.4% (1/(1.25^15+1)), but I 
> get this:
> 
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>  4433 roman     20   0  1532  300  244 R 99.2  0.2   5:05.51 l
>  4434 roman     35  15  1532   72   16 R  0.7  0.1   0:10.62 l

Could this be caused by typos in some tables like you have found in wmult ?

> OTOH upto nice level 12 I get what I expect.
> 
> 2. If I start 20 busy loops, initially I see in top that every task gets 
> 5% and time increments equally (as it should):
(...)

> But if I renice all of them to -15, the time every task gets is rather 
> random:
> 
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>  4492 roman      5 -15  1532   68   16 R  1.0  0.1   0:07.95 l
>  4491 roman      5 -15  1532   68   16 R  4.3  0.1   0:07.62 l
>  4490 roman      5 -15  1532   68   16 R  3.3  0.1   0:07.50 l
>  4489 roman      5 -15  1532   68   16 R  7.6  0.1   0:07.80 l
>  4488 roman      5 -15  1532   68   16 R  9.6  0.1   0:08.31 l
>  4487 roman      5 -15  1532   68   16 R  3.3  0.1   0:07.59 l
>  4486 roman      5 -15  1532   68   16 R  6.6  0.1   0:07.08 l
>  4485 roman      5 -15  1532   68   16 R 10.0  0.1   0:07.31 l
>  4484 roman      5 -15  1532   68   16 R  8.0  0.1   0:07.30 l
>  4483 roman      5 -15  1532   68   16 R  7.0  0.1   0:07.34 l
>  4482 roman      5 -15  1532   68   16 R  1.0  0.1   0:05.84 l
>  4481 roman      5 -15  1532   68   16 R  1.0  0.1   0:07.16 l
>  4480 roman      5 -15  1532   68   16 R  3.3  0.1   0:07.00 l
>  4479 roman      5 -15  1532   68   16 R  1.0  0.1   0:06.66 l
>  4478 roman      5 -15  1532   68   16 R  8.6  0.1   0:06.96 l
>  4477 roman      5 -15  1532   68   16 R  8.6  0.1   0:07.63 l
>  4476 roman      5 -15  1532   68   16 R  9.6  0.1   0:07.38 l
>  4475 roman      5 -15  1532   68   16 R  1.3  0.1   0:07.09 l
>  4474 roman      5 -15  1532   68   16 R  2.3  0.1   0:07.97 l
>  4473 roman      5 -15  1532  296  244 R  1.0  0.2   0:07.73 l

Do you see this only at -15, or starting with -15 and below ?

Willy

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