Re: [RFC][PATCH] O(1) Entitlement Based Scheduler

[John Lee]

On Wed, 25 Feb 2004, Pavel Machek wrote:

> > Usage rate caps are expressed as rational numbers (e.g. "1 / 2") and hard caps 
> > are signified by a "!" suffix.  The rational number indicates the proportion 
> > of a single CPU's capacity that the task may use. The value of the number must 
> > be in the range 0.0 to 1.0 inclusive for soft caps. For hard caps there is an 
> > additional restriction that a value of 0.0 is not permitted.  Tasks with a 
> > soft cap of 0.0 become true background tasks and only get to run when no other 
> > tasks are active.
> 
> Why not use something like percent, parts per milion or whatever?

Fair comment. Fine granularity with percentages would require decimal
points and a function in the kernel to parse that value - maybe I could
get around to doing that. But I suppose ppm could certainly be used. We
just chose rational numbers to start with.

> > When hard capped tasks exceed their cap they are removed from the run queues 
> > and placed in a "sinbin" for a short while until their usage rate decays to
> > within limits.
> 
> How do you solve this one?

The task is removed from the runqueue and a timer is scheduled to put it
back onto the runqueue. The delay period is the required amount of time
for that task's usage to decay to below its cap.

> I want to kill your system.
> 
> I launch task A, "semaphore grabber", that does filesystem
> operations. Those need semaphores. I run it as "true background".
> 
> I wait for A to grab some lock, then I run B, which is while(1);
> 
> A holds lock that can not be unlocked, and your system is dead.
> 
> This may happen randomly, even without me on your system.

Good point. We'll have to rethink background priorities.

John


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