[gmpi] Re: Req 76,78

  • From: Tim Hockin <thockin@xxxxxxxxxx>
  • To: gmpi@xxxxxxxxxxxxx
  • Date: Wed, 9 Feb 2005 11:14:19 -0800

On Wed, Feb 09, 2005 at 07:55:19PM +0100, Didier Dambrin wrote:
> let's say someone made an app that was doing lengthy stuff, and always 
> ping-pong-ing between 2 processes. If the OS really directly switch from 
> one OS to another, wouldn't that leave an app a way to litterally freeze 
> the OS?

It's a lot more complicated than just passing control.  Scheduling
heuristics have been fine-tuned over years to get a good balance of
reliability vs speed.

If you fully exhaust your time slice, then you will be rescheduled and
other tasks will have a chance to run.  If you don't exhaust your
timeslice, you have a much better chance of being run before something
that DID fully exhaust it's timeslice.

If your priority is high enough, then yes, you can lock up the system
(well, you get all the user CPU time).

With Linux, at least, (assuming no RT priorities) all processes get their
priority recalculated periodically based on how long they run, how often
they block, etc.  What you end up with is that your 2 ping-ponging tasks
get most of the CPU time, but more importantly, they get the least
scheduling latency.

Really, task scheduling is as much an art as a science.  I point you at
kernel/sched.c in the Linux kernel for some *fun* reading :)

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