http://d.puremagic.com/issues/show_bug.cgi?id=5623
I've found a way to speed up the GC massively on large heaps without excessive ripple effects. Technically it's still O(N), but with about a hundred fold smaller constant in the case of large heaps with most stuff not scanned. Now, I think the O(N) (where N is the total size of the heap) term has such a small constant that it's for almost all practcal purposes the GC is O(S) (where S is the size of the scanned portion of the heap). It also no longer has any O(N^2) pathological case (which I had discovered while reading the code).
So far all unittests for Phobos, dstats and std.parallelism/parallelfuture pass with this enabled. Please test some other code so we can wring out the corner cases in time for the next release.
Basically all I did was diverge the Pool struct slightly into large and small object sub-varieties. The large object sub-variety is used to allocate objects of at least a page. It only stores gcbits at page-size offsets, and tracks the offsets of B_PAGEPLUS bins from the nearest B_PAGE bin so that they can be found in O(1).
I also added a field to the Pool struct so that the number of free pages in a pool can be tracked in O(1). This should drastically lessen the time it takes to perform large allocations on large heaps. Right now a free memory region is found by a linear search through the pools in the case of large allocations. Unfortunately, I don't see any easy way to fix this. This patch at least allows short circuiting a large number of pools, if there isn't enough free space in the whole pool, let alone contiguous space.
Here are the benchmarks with this patch enabled. Collected a 10 megabyte heap in 0 milliseconds. Collected a 50 megabyte heap in 0 milliseconds. Collected a 250 megabyte heap in 1 milliseconds. Collected a 500 megabyte heap in 0 milliseconds. Collected a 1000 megabyte heap in 1 milliseconds. Collected a 5000 megabyte heap in 3 milliseconds. Collected a 10000 megabyte heap in 6 milliseconds. Collected a 30000 megabyte heap in 16 milliseconds. Collected a 50000 megabyte heap in 26 milliseconds.
