Name: Anonymous 2015-07-28 19:56
A bit of background is in order. GHC allocates memory from the operating system in units of aligned megabytes. So it owns a collection of megabytes that might be anywhere in the address space. For every pointer it sees, the GC asks the question “is this a pointer into the heap?”, and the memory manager should return true for any of the memory that GHC has allocated, and false otherwise. This is the job of the HEAP_ALLOCED() macro.
On a 32-bit machine, answering the question is easy: a megabyte is 20 bits, so we only need a 12-bit lookup table, and even using a full byte for each entry, that’s just 4KB.
On a 64-bit machine, it’s much harder. Even taking advantage of the fact that only 48 bits are available address space on x86_64 architecture machines, that still leaves 28 bits, which is a 256MB table (32MB using bits instead of bytes). This is not likely to be an acceptable memory overhead.
https://simonmar.github.io/posts/2015-07-28-optimising-garbage-collection-overhead-in-sigma.html
On a 32-bit machine, answering the question is easy: a megabyte is 20 bits, so we only need a 12-bit lookup table, and even using a full byte for each entry, that’s just 4KB.
On a 64-bit machine, it’s much harder. Even taking advantage of the fact that only 48 bits are available address space on x86_64 architecture machines, that still leaves 28 bits, which is a 256MB table (32MB using bits instead of bytes). This is not likely to be an acceptable memory overhead.
https://simonmar.github.io/posts/2015-07-28-optimising-garbage-collection-overhead-in-sigma.html