Excerpts from Bulat Ziganshin's message of Mon Mar 02 10:14:35 -0600 2009:
let's calculate. if at GC moment your program has allocated 100 mb of memory and only 50 mb was not a garbage, then memory usage will be 150 mb
? A copying collector allocates a piece of memory (say 10mb) which is used as the heap, and only one *half* of it ever has data in it. When a copy occurs, all live data is transferred from one half to the other half. So, if you have a 10mb heap initially, that means you have two sides of the heap, both which are 5mb big. If your program has 2mb of live data at the time of a GC, then those 2mb of data are copied over to the other 5mb side. This copying can be done with pointer arithmetic in a basic manner, so you don't need to allocate any more memory for a copy to occur. So the program never uses more than 10mb heap. GHC might be a little different (so correct me if I'm wrong in the case of GHC.) Copying collection has the advantage that for low-lived allocations, GC moments are pretty quick, because the time spent copying is proportional to the size of the live data. It gets slower if your program has lots of live data at once, not only because collections become more frequent, but because copies will become longer in time as well. Austin