Re: [ECOS] memory saving scheme for stacks

[Nick Garnett <nickg at ecoscentric dot com>]

Ãyvind Harboe <oyvind.harboe@zylin.com> writes:

> In my system I have a number of threads that typically do a bit of
> processing and then long periods of sleep.
> 
> Since only one thread can run at any one time, there is lots of 
> unused stack memory at any moment in time. This can be as much as 20% of
> the overall memory in the system. Ignore SMP systems here.
> 
> A scheme to reclaim this memory comes to mind:
> 
> - When a thread is created, it is marked as being special:
> "stacksaving-thread". SST for short.
> - Only one SST thread runs at any one time.
> - All SST share the same stack memory location.
> - When SST thread is switched out, the portion of the stack that is in
> use is saved off to a different memory location.
> - When a SST thread is switched in, the portion of the stack that is in
> use is restored from the saved off location.
> - The threads in my system typically have 200-1000 bytes of stack in use
> at any one time => memory copy shouldn't bring the system to its knees.
> 
> I'm sure this has been done before as I can't see anything that
> indicates that this is impossible.

Wouldn't the save areas have to be as large as the stacks? Given that
the threads could be interrupted and preempted at any point and any
stack depth.

A better approach, would be to combine all these mostly idle threads
into one service thread that simply waits for one of several events
and services then as they arrive. But that does depend on the
structure of your threads.


-- 
Nick Garnett                    eCos Kernel Architect
http://www.ecoscentric.com/     The eCos and RedBoot experts


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