General-Purpose Operating System Prime Mover

[Neal H. Walfield]

At Thu, 12 Jul 2007 16:07:03 +0100,
Sam Mason wrote:
> 
> On Thu, Jul 12, 2007 at 04:41:59PM +0200, Neal H. Walfield wrote:
> > General-purpose systems include both interactive systems, such as PDAs
> > and Desktops, as well as small to medium-sized servers.  Although many
> > applications that run on such systems have timeliness properties,
> > efficient use of available resources is more important than
> > schedulability (i.e., meeting deadlines).
> 
> I tend to use my computer for listening to music and playing videos
> and tend to get annoyed when the music skips or frames are dropped.
> Thus timeliness is as important as overall utilisation for some of my
> workloads.
> 
> If utilisation was all that mattered, we'd still be with cooperative
> multi-threading wouldn't we?

Timeliness is very important, however, the cost of providing hard
real-time guarantees is high.  It lowers resource use efficiency by
making dynamic reallocation more difficult, and completely changes the
way in which programs are written--you have to optimize for the worst
case rather than the average case.

I think that it is desirable to provide an interface that allows
applications to request specific schedules.  This is mostly lacking in
most general-purpose operating systems.  What mechanisms are available
are generally closely guarded.  This is because there is no
middle-ground between requesting a specific schedule and guaranteeing
or rejecting it.  It would be useful is having a way to express:
insofar as it is possible given the available resources and the other
demands and their relative priorities, I would like this schedule.

Thanks,
Neal