Re: "Random" delays in execution

["Jerome Vuarand" <jerome.vuarand@...>]

2008/10/23 Max van Rooij (van Rooij Electronics Design & Software
Engineering) <mvrooij@vanrooij-edse.nl>:
> I've build a (Lua)script interperter on an embedded system. Works like a
> charm, but I have an anoying problem. The script I run is actually 2
> scripts. The first is loaded and run only once. It declares a global 2D
> array of some userdata (alloced in C). The second script is loaded and
> run to completion evry time (in a repeated cycle). This second script
> doesn't use the 2D array, but does have it "visible" in its registry
> (e.g. I could use it, if I want to).
>
> The problem I have is this:
>
> - the 2D array is 20 columns x 10 rows (200 elements)
> - If I allocate userdata to it, the program cycle takes typically 30 to
> 40msec but every n interations (were n could be any were from 15 to 25)
> it "stops" for 1700 to 2500msec!!!
> - If I allocate the same 2D array, but instead of allocate userdata I
> simply write integer numbers to each table entry, the cycle time somehow
> drops to only 20msec, remains constant and I never ever experiance the
> 1700 to 2500msec delays.
>
> After extensive testing, it appears that everything is functionally
> correct, e.g. no memory leaks, out-of-boundaries and the likes. It
> apperas that the garbage collector has somehow have something to do with
> it? Although I dont' know why, since the 2D array is allocated only once
> (and never referenced) for the lifetime of the program. I also never
> observe __gc events on the 2D array, unless, of course, when I quit the
> program.
>
> I want to understand this "random delay" behaviour. Is and why is the GC
> involved? When / were is this invoked in the Lua source code, since I
> only call lua_resume()? I can worsen / lighten the delay effect by
> increasing the nr of columns, making the delay hardly noticeable or so
> long that it appears to hang forever.
>
> Any help is appreciated.

Your slowdown problem is probably due to the garbage collector. If
turning the userdata array into an integer array solves the problem,
it probably means that you are (unexpectedly) reallocating the array
at each cycle. Can you isolate the problem in a minimal example (a few
tens lines of code) so that we can study it and tell you what you are
doing wrong ?