Sched_autogroup and niced processes

[Carl-Johan Kjellander]

I've been running seti@home niced to 19 in the background since 1999
without any problems. No noticeable effect even when playing a movie
or a game. But since 2.6.38 the new fix-all-problems automatic
grouping has been messing a bit with me. These are some timed compiles
on my 8 cores.

time make -j12 # with seti@home running
real	4m16.753s
user	10m33.770s
sys	1m39.710s

time make -j12 # without seti@home running
real	2m12.480s
user	10m11.580s
sys	1m39.980s

echo 0 > /proc/sys/kernel/sched_autogroup_enabled
time make -j12 # no autogroup, seti@home running again
real	2m33.276s
user	10m37.540s
sys	1m43.190s

All compiles already had all files cached in RAM.

Now I can take the 10% performance hit, but not the 100% hit of
running stuff super niced in the background. Processes niced to 19
should only use spare cycles and not take up half of the cores even
with autogroup. I would really like to run autogroup since it is a
neat idea, but it can't mess up running niced processes in the
background which have been working fine for 12 years.

/Carl-Johan Kjellander
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