Julia + MMTk

There has been quite a lot of effort to refactor the GC code inside Julia to support external GCs. The first step to enable using different GC algorithms for Julia was the design and implementation of a GC interface. To drive that interface, we added support for building Julia with MMTk (Memory Management Toolkit). Using Julia + MMTk enables testing different GC implementations, allowing developers to choose a specific implementation when building Julia from source. The connection between Julia and MMTk is done via a binding, which links the language runtime with MMTk core. The MMTk integration lives in src/gc-mmtk, and the Rust binding lives in src/gc-mmtk/mmtk_julia.

Building Julia with MMTk

Julia builds MMTk from the in-tree integration in src/gc-mmtk. To enable MMTk as a third-party GC, set the variable WITH_THIRD_PARTY_GC to mmtk. Then, for example, to use Immix as the GC, set MMTK_PLAN=Immix and build Julia as usual.

There are different configurations supported by the following variables, which can be set in a Make.user file or as an environment variable. Setting MMTK_BUILD_MODE=debug will force a debug build of the binding, which will print some logging information that can be used to find errors that are specific to MMTk.

Variable	Default	Alternative
MMTK_PLAN	StickyImmix	Immix
MMTK_BUILD_MODE	release	debug
MMTK_MOVING	0	1 (not supported yet)

Note that when setting only MMTK_PLAN, then the default is to do a non-moving (MMTK_MOVING=0), release (MMTK_BUILD_MODE=release) build.

The mmtk-julia binding is built from source as part of the Julia build. This requires an installation of the Rust toolchain.

I've got a build error when building Julia with MMTk, what should I do?

If you try to build Julia with MMTk and get an error it is likely due to a change to Julia that has not been yet propagated to the binding or to the code in Julia that is specific to MMTk. Some changes include:

(1) Changing the root objects passed to the GC. Julia passes a set of objects to the GC as roots in the function gc_mark_roots. At the moment, this set needs to be consistent between both the Stock GC and MMTk (in the function jl_gc_scan_vm_specific_roots in src/gc-mmtk/gc-mmtk.c).

(2) Changing how objects are scanned. MMTk uses the same strategy to find references in Julia objects as the stock GC (see gc_mark_outrefs). Changing the logic from this function should be reflected in the Rust function scan_julia_object in src/gc-mmtk/mmtk_julia/src/julia_scanning.rs.

If your case is not included in one of the alternatives above, please create an issue in the Julia repository tagging it with the GC: MMTK label.