Re: [SCHED] wrong priority calc - SIMPLE test case

[Paolo Ornati <ornati@fastwebnet.it>]

On Mon, 09 Jan 2006 16:52:17 +0100
Mike Galbraith <efault@gmx.de> wrote:

> >Care to try an experiment?...

Yes.

With my simple proggy things improve a bit:

./a.out 7000 & ./a.out 6537 &
  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 5579 paolo     19   0  2392  288  228 R 51.9  0.1   0:22.38 a.out
 5578 paolo     20   0  2392  288  228 R 43.9  0.1   0:22.50 a.out

As you can see they don't get priority 15/16 anymore :)

DD test: 256 MB, ~8.5s (instead of 8)

In pratice the more CPU they use the more their priority is penalized...


BUT if I start more of them (3/4) I'm able to fool it.

"./a.out 7000 & ./a.out 6537 & ./a.out 6347 & ./a.out 5873"

2 TOP's snapshots:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 5625 paolo     17   0  2392  288  228 R 31.6  0.1   0:10.74 a.out
 5626 paolo     17   0  2392  288  228 R 28.8  0.1   0:09.16 a.out
 5627 paolo     17   0  2392  288  228 R 22.2  0.1   0:07.59 a.out
 5624 paolo     17   0  2392  288  228 R 17.4  0.1   0:08.67 a.out

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 5626 paolo     16   0  2392  288  228 R 30.1  0.1   0:39.95 a.out
 5627 paolo     16   0  2392  288  228 R 24.1  0.1   0:34.93 a.out
 5625 paolo     18   0  2392  288  228 R 23.5  0.1   0:37.53 a.out
 5624 paolo     18   0  2392  288  228 R 21.9  0.1   0:37.60 a.out
 5193 root      15   0  167m  17m 2916 S  0.2  3.5   0:09.67 X
 5638 paolo     18   0  4952 1468  372 R  0.2  0.3   0:00.15 dd

DD test (256MB): real    3m37.122s  (instead of 8s)



REAL LIFE TEST (transcode)

While running only transcode it gets priority 25:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 5857 paolo     25   0  114m  18m 2424 R 90.9  3.7   0:14.28 transcode
 5873 paolo     19   0 49860 4452 1860 S  8.6  0.9   0:01.40 tcdecode
 5308 paolo     16   0 86796  22m  15m R  0.2  4.4   0:06.26 konsole
 5687 paolo     16   0 98648  37m 9348 S  0.2  7.5   0:02.11 perl
 5872 paolo     24   0 21864 1064  600 S  0.2  0.2   0:00.01 tcextract


But if I run also the DD test, "transcode" priority start fluctuating
and can go down to 18/19 (from time to time) interfering with DD:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
 5694 paolo     19   0  114m  18m 2424 R 75.1  3.7   0:42.29 transcode
 5710 paolo     25   0 49856 4452 1860 R  8.0  0.9   0:04.36 tcdecode
 5726 paolo     18   0  4952 1468  372 R  4.0  0.3   0:00.77 dd


This seems to happen because also transcode is reading (not directly but
through pipes) from disk so the massive disk usage of DD interferes
with it, this leads to transcode using less CPU and getting better
priority.

The exact behaviour changes time to time... but seems to confirm my
teory.

I don't know how can "nicksched" keep transcode priority always to 40
even when I'm running the DD test... I should retry and see.


PS: yes, transcode is reading from disk, but SLOWLY... i think that a
good read-ahead should fullfill his needs even when doing the HD
stressing DD test, no?

-- 
	Paolo Ornati
	Linux 2.6.15-sched_trottle on x86_64