Re: #define HZ 1024 -- negative effects?

[Werner Puschitz]

On Wed, 25 Apr 2001, Dan Maas wrote:

> > Are there any negative effects of editing include/asm/param.h to change
> > HZ from 100 to 1024? Or any other number? This has been suggested as a
> > way to improve the responsiveness of the GUI on a Linux system.
>
> I have also played around with HZ=1024 and wondered how it affects
> interactivity. I don't quite understand why it could help - one thing I've
> learned looking at kernel traces (LTT) is that interactive processes very,
> very rarely eat up their whole timeslice (even hogs like X). So more
> frequent timer interrupts shouldn't have much of an effect...
>
> If you are burning CPU doing stuff like long compiles, then the increased HZ
> might make the system appear more responsive because the CPU hog gets
> pre-empted more often. However, you could get the same result just by
> running the task 'nice'ly...

A tradeoff of having better system responsiveness by having the kernel to
check more often if a running process should be preempted is that the CPU
spends more time in Kernel Mode and less time in User Mode.
And as a consequence, user programs run slower.

Regards,
Werner