|Version 2 (modified by jberthold, 4 years ago)|
Packing and Unpacking Heap Graph structures
An essential component of Eden as well as other Parallel Haskells for distributed memory is a packing module: traversal/serialisation of a subgraph in the heap, and respective de-serialisation.
In one sentence, packing walks breadth-first through the reachable subgraph from the node to pack, and only copies info-pointer and non-pointers, since pointers can be established from the order inside a packet when unpacking.
The details are a bit more involved, and not much documentation available... You should have a look at the wiki page about Heap closure layout before reading the packing code.
TODO: this subject deserves largely more space, and links to GHC commentary pages.
In the GHC-6.13 port, Pack.c has been created from the previous version (Eden-6.8.3), doing a one-pass review of the old packing code. Packing and unpacking methods for pointer arrays have been added (should not be used on mutable arrays, though). Things to do:
- reformatting in line with GHC customs
- code restructuring,
- directly use an "unpack state", recover from interrupted packing
- send partial structure in separate message
The module can be tested by a primitive duplicate#:: a -> IO a, which packs and unpacks (=> creates a copy of) something in the local heap.
Interesting question is: should the routine be able to pack/unpack _every_ heap object? (up to now unable to pack: MVars, TSOs, all kinds of Stack elements, all STM-related closures)