Re: CFS review

From: Roman Zippel
Date: Fri Aug 10 2007 - 18:50:03 EST

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.

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

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):

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
4492 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4491 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4490 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4489 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4488 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4487 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4486 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4485 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4484 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4483 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4482 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4481 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4480 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4479 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4478 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4477 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4476 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4475 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4474 roman 20 0 1532 68 16 R 5.0 0.1 0:02.86 l
4473 roman 20 0 1532 296 244 R 5.0 0.2 0:02.86 l

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

bye, Roman
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