[Haskell-cafe] Boxed Mutable Arrays

Gregory Collins greg at gregorycollins.net
Tue Dec 15 12:00:17 EST 2009

Bulat Ziganshin <bulat.ziganshin at gmail.com> writes:

> now i see what you mean. no, i mean trivial transformation. #650 says
> about slow GC. why it's slow? because once you made any update to the
> array, the entire array is marked as updated and scanned on next minor
> GC (which occurs after every 512 kbytes allocated, afaik). let's
> replace big array (say, of 10,000 elements) with array of 100 arrays
> of 100 elements each. now, between minor GCs only some of arrays will
> be changed and entire amount of memory to be scanned will become less

I actually tried this, and modified Data.HashTable to use a two-tiered
chunked dynamic array as you suggest. In some limited testing it was
only about 5% faster, so I gave up on it -- you get some GC time back
but you also pay a significant indirection penalty by adding an extra
cache line miss for every operation.

Gregory Collins <greg at gregorycollins.net>

More information about the Haskell-Cafe mailing list