Sluggish system responsiveness on I/O

[Christian <christiand59@web.de>]

Hello lkml!

Im currently testing 2.6.19-rc5-mm1. Everything works really fine except the 
little wart with bad multimedia interactivity with a kernel compiling in the 
background. So I tried to narrow it down as much 
as possible.

I did several find's,dd's and cats in parrallel and watched four instances of 
glxgears and also played a little enemy-territory. The interactivity was very 
good, in fact no loss of interactivity at all. This was contrary to what I 
believed the whole time. The loss of interactivity has nothing to do with 
heavy I/O. In fact it happens only when I run a task which is I/O and CPU 
heavy at the same time. That means a single kernel compile (with -j1) is able 
to harm interactivity with glxgears and enemy-territory, but fully loading my 
three disks does no harm at all.

So I tried to nice the make and see what happens:

nice 5 make -j4: Seems to make no difference. Heavy stuttering in glxgears and 
et
nice 10 make -j4: Somewhat better but still unusable with et

everything above nice 15 is usable. nice 19 has full interactivity, that means 
you can't make out a difference between no load and kernel compile while 
playing enemy-territory.

I suspect that it has something to do with the priority boost for I/O hogs. 
But if this is a "general" scheduler problem, then why aren't more people 
complaining about this?

-Christian