PrepareChunkMap (mark-sweep) unnecessarily clearing mark bits of all chunks

[wks]

mmtk-core/src/policy/marksweepspace/native_ms/global.rs

Lines 435 to 437 in 401803c

	for chunk in self.space.chunk_map.all_chunks() {
	side.bzero_metadata(chunk.start(), Chunk::BYTES);
	}

Each PrepareChunkMap work packet for native_ms is supposed to initialize the chunk-level metadata for a single chunk. But the code shown above clears the side mark bits for all blocks. This is totally unnecessary, and is taking up a considerable amount of time during a GC.

The problem becomes more serious when I changed the native MarkSweepSpace to use BlockPageResource instead of the raw FreeListPageResource. The reason may be that BlockPageResource caches blocks in the block queue, and releases chunks lazily, resulting in a higher number of allocated chunks during each GC.

(p.s. ReleaseMutator becomes faster because BlockPageResource::release_block is lock-free, while FreeListPageResource::release_pages needs a mutex to work, which is bad for parallel GC. See: #1145)

It should only clear the side metadata for the single chunk that the work packet is responsible for.

[qinsoon]

Yeah. That code is a clear bug. I guess I simply moved the mark bit clearing code from the plan's prepare to PrepareChunkMap without realizing that PrepareChunkMap is per chunk.