Memory fragmentation

[Joel Sherrill]

Angelo Fraietta wrote:

> About a year and a half ago, I found that there I was getting memory 
> fragmentation, which caused RTEMS to lock up.
> 
> I had a class of about eight bytes in size, and allocated and 
> deallocated an object of this class 100 times every second. There was 
> only ever  two objects active at any one time.
> 
> After about three days, RTEMS locked up and had to be rebooted. I 
> overcame this by overloading operators new and delete in the class and 
> made a memory manager for the class.  I have run it for two weeks 
> without a problem now.

I think this was a good solution in your case.  It sounds like the C++
equivalent of not really freeing something to the heap and reusing them.
Very close to preallocating a set of objects and having
allocate and deallocate routines which did not touch the heap at all.

I would have thought that instead of locking up, it would have
eventually reused some of those blocks.  But if there were other
new/delete's then it might ahve simply failed at some point.

Also did you do free's from interrupt handlers -- that is currently
forbidden.  That would have a chance of a weird failure.

> I am bring this up now because I am writing up my thesis and has come to 
> mind.
> 
> Has memory fragmentation been resolved or considered for RTEMS apart 
> from partitioning?

Not really.  I suspect in your case, the problem is that the heap
is a first fit algorithm while your application might be better
served with a best fit one.

But in general, I think your solution was the appropriate embedded
systems one.  You ended up solving not only a fragmentation problem
but probably a performance one as well.  It has to be less expensive
computationally to avoid malloc.  NOTE: free on RTEMS is deterministic
for the C Program Heap since you can't block waiting on memory from
it.

-- 
Joel Sherrill, Ph.D.             Director of Research & Development
joel at OARcorp.com                 On-Line Applications Research
Ask me about RTEMS: a free RTOS  Huntsville AL 35805
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