[ODE] Allocation problem?

Geoff Carlton gcarlton at iinet.net.au
Tue Jun 6 18:10:34 MST 2006

I've often wondered whether a different allocator strategy might be more 
reliable.  Here is one strategy I've seen before:
 - Allocate out of blocks of a fixed size (e.g. 1MB).  When a block is 
full, malloc a new block, linked together in an "active" list.
 - Free is no-op.
 - When the operation (WorldStep) is finished, move all blocks into a 
free list ready to start again.
 - Blocks are only deleted at shutdown.

Essentially, its used for this sort of thing where memory is used as 
part of an operation, then completely freed.  It grows to a whatever 
size is needed, and in subsequent operations just references existing 
memory blocks.  It is still relatively fast, and can grow to an 
arbitrary size without the danger of stack overflow. 

Thinking about it, the main disadvantage seems to be memory 
fragmentation, because allocs hop over to different 1MB blocks in 
memory.  Could be solved with a memory consolidation step, where it 
totals the size used at the end of the operation and then replaces all 
the blocks with one of the new total size.  I've never heard of this 
idea before, but it seems like it might work, albeit at increased cost 
as the simulation size is growing.

Actually, could we calculate the maximum size needed based on the number 
of bodies?  Could be a third alternative, keep a buffer around of the 
max theoretical size, resizing it larger at the start of each step if 
needed.  Keeps everything in one contiguous array.

Not that I've had any problems with alloca, but it does seem a little 
unreliable if running big simulations.


Jon Watte (ODE) wrote:
> Yes, dWorldQuickStep() still uses the stack, although less of it than 
> dWorldStep() (last I checked -- many moons ago).
> Cheers,
> 			/ h+
> Jason Perkins wrote:
>> On 6/6/06, Jon Watte (ODE) <hplus-ode at mindcontrol.org> wrote:
>>> My guess is you're on Windows, and using plain dWorldStep(). In that
>>> case, you're running out of stack. This issue is mentioned more
>>> thoroughly in the ODE documentation. Simple solution: switch to
>>> dWorldQuickStep().
>> I'm curious about this...test_crash uses dWorldQuickStep() and it
>> still crashes at that same location. So does dWorldQuickStep() also
>> have a stack space requirement? Just trying to get my head around it.
>> Jason
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