[Xen-devel] Re: [PATCH] add timeouts to mini-os

[Grzegorz Milos <gm281@xxxxxxxxx>]

> Which functionalities would you like to see that are currently  
> missing?
The timers I was thinking about would follow Xen/Linux model. For  
each timer you would need to specify the following:
- a timeout (timestamp)
- a handler function
- if the timer is one-shot or periodic

Such timers are more flexible, but the handler function is limited in  
functionality by the fact that you are running in the interrupt  
context. Since Mini-OS doesn't maintain much state (that could be  
modified by a timeout timer), the only thing you would be likely  
doing is waking up a thread/set of threads. It therefore follows that  
your implementation does the same thing, but it's shorter/cleaner. In  
particular there is no need for timer heap or a similar datastructure.
> (There are few shortcomings of my implementation that I'm well  
> aware of
> myself. For instance, I think it would be more efficient to unlink  
> blocked
> threads from the ready queue and keep the ones with pending  
> timeouts in a
> "timeout" queue, sorted by ascending expiry date.  That way, it  
> wouldn't be
> necessary to walk the entire ready queue all the time to see if any  
> timeouts
> have expired: it would suffice to just look at the head of the  
> timeout queue.
> However, my goal was to keep things simple and to not change more than
> absolutely necessary.)
That's fine. I decided against separate lists for runnable/blocked  
threads for exactly the same reasons. Mini-OS is not about  
scalability, keeping the things as simple as possible is the main  
priority.
Cheers
Gregor

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