[Haskell-cafe] Re: Real-time garbage collection for Haskell

Peter Verswyvelen bugfact at gmail.com
Thu Mar 4 04:22:28 EST 2010

A fully concurrent GC running on multiple threads/cores might be
great, but I guess this is difficult to implement and introduces a lot
of overhead.

For simple video games, it might work to always do a full GC per
frame, but don't allow it to take more than T milliseconds. In a sense
the GC function should be able to be paused and resumed, but it could
run on the same thread as the game loop, so no synchronization
overhead would arise (in a sense many video games are already little
cooperative multitasking systems). The game loop should then decide
how to balance the time given to the GC and the memory being
collected. This would cause longer frame times and hence sometimes a
decrease in frame rate, but never long stalls.

Note that the issue also popped up for me in C many years ago. Using
Watcom C/C++ in the nineties, I found out that a call to the free
function could take a very long time. Also for games that could run
many hours or days (e.g. a multi player game server) the C heap could
get very fragmented, causing memory allocations and deallocations to
take ever more time, sometimes even  fail. To make my games run
smoothly I had to implement my own memory manager. To make them run
for a very long time, I had to implement a memory defragmenter. So in
the end, this means a garbage collector :-) Of course this problem is
well known, I just wanted to re-state that for games the typical C
heap is not very well suitable either.

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