Re: [Caml-list] OCaml GC [was Is OCaml fast?]

[Benedikt Meurer <benedikt.meurer@googlemail.com>]

On Nov 28, 2010, at 20:40 , Jon Harrop wrote:

> I don’t understand why this would help here though. Wouldn’t that help when a long-lived structure was single large block but, in this case, the long-lived structure is a tree composed of many small heap-allocated blocks and, therefore, they would undergo the same wasteful “allocate in young only to be copied to old” pathological behaviour?

There is no "young" and no "old" in this scheme. There are two different heaps, one for large objects, one for small (probably 2-8k max object size, so not really small in the sense of OCaml). The small area is managed by Immix, which avoids copying of long-lived objects if they are allocated together with other long-lived objects (likely), or if not evacuates a set of probably related objects to a single chunk (this is somewhat dependent on the evacuation strategy, which will be differnt for OCaml compared to Java), further improving locality. There are simple heuristics to ensure that an object is not evacuated too often (it is already unlikely with the base algorithm), so there will be a lot less copying. One difficulty remains however: the pause times. It would probably be necessary to adopt Immix to a (semi-)incremental style to get the same pause times as with the current GC.

Benedikt