# MMTk

MMTk starts in rb_objspace_alloc in gc.c. Weirdly it initialises the builder object after the call to calloc to allocate Ruby’s objectspace. I thought mmtk was managing allocations - so what’s objspace used for here?

All the functions in mmtk.h are part of the MMTk Ruby binding layer. the mmtk.h headers are autogenerated using cbindgen from the binding project mmtk-ruby.

it uses an MMTk_Builder type (defined in mmtk.h as a typedef void *), which it creates using mmtk_builder_default (again, defined in mmtk.h).

It then does a few things

• mmtk_builder_set_plan (NoGC, or MarkSweep at the moment)
• sets the heap limit on the builder (either dynamic or fixed depending on the results of rb_mmtk_heap_limit)
• mmtk_init_binding (not sure what this does yet).

## mmtk_builder_default

This is defined in lib/api.rs inside the Ruby bindings and creates a reference to an MMTKBuilder object (using Box), which it then converts into a raw pointer.

MMTkBuilder is a Rust struct in mmtk core (lib/mmtk.rs). It contains a single public member options, which is of type Options (this looks like just a collection of interesting options like which plan we’re using, the number of threads we’re allowed, and some default settings, I’m going to ignore this for now and assume there’s no magic going on in here).

MMTkBuilder has an impl Default - which defines a default function, all this does is call new on the default impl. new just creats a struct of MMTkBuilder, with a default Options instance - so not much interesting happening here.

## mmtk_builder_set_plan

This is again in the Ruby binding layer lib/api.rs. This reads the plan name we set from the command line option (NoGC or MarkSweep), and does what looks like a constant lookup based on the name

let plan_selector: PlanSelector = plan_name_str.parse::<PlanSelector>().unwrap();


butterfly meme: is this metaprogramming?

then it just assigns the plan name to the options in the MMTk_Builder object that we built.

## rb_mmtk_heap_limit

This is fully implemented inside Ruby - specifically in gc.c

switches on mmtk_max_heap_size. where is this from - this can be passed in using --mmtk-make-heap at runtime, otherwise it’s 0.

Ok, so basically - if we’ve defined a fixed heap size, either using the command line argument or the environment variable THIRD_PARTY_HEAP_LIMIT then we set is_dynamic_heap to false, and we calculate the min and the max sizes.

max size is calculated using rb_mmtk_parse_heap_limit - this is basically just trying to convert the string that was passed in, to a size_t.

if nothing was passed then is_dynamic_heap is set to true. At this point min and max are set to the default values (10Mb and 80% of the computers physical memory respectively)

## mmtk_builder_set_{dynamic,fixed}_heap_size

These are functions in the ruby binding layer that set a gc_trigger on the options inside the builder

These use a struct called GCTriggerSelector::FixedHeapSize and DynamicHeapSize respectively.

All this stuff is essentially just setting options on the Ruby heap though - where is the actual heap allocation happening?

## mmtk_init_binding

this function consumes the MMTk_Builder object - but it’s return void. so what’s happening - does it replace the target of the pointer? it’s defined in the Ruby binding layer.

This function takes the pointer to the builder, as well as some binding options and a static C struct of function pointers called upcalls (which is a Rust type that’s abi bound to a C struct defined in gc.c)

Ruby upcalls contains function pointers to init gc worker threads, stop the world, block and resume mutators. It looks like this is where most of the actual work is triggered from. All these functions are defined inside gc.c.

it calls mmtk_init to return a boxed MMTK<Ruby> instance - what is this? and also mmtk_static which is a mutable pointer to the boxed MMTk Ruby instance (created using Box::leak - as per the comments in memory_manager.rs we use leak here to give the boxed MMTk pointer a static lifetime rather than a lifetime that is bound to the box pointer).

It then builds a RubyBinding with the static instance, the options and a borrow of the upcalls. and does something with OnceCell with that binding.

it returns (). So something must be happening with this line of code to give Ruby access back to the RubyBinding

    crate::BINDING
.set(binding)
.unwrap_or_else(|_| panic!("Binding is already initialized"));


Ok. so BINDING is a pub static OnceCell inside the mmtk ruby bindings. This makes it a singleton object. publicly accessible and can be assigned to at most once. But how is this exposed? is it in mmtk.h in the ruby project?

actually does this even need to be exposed to the Ruby side. This is probably just an MMTk object that’s used within the binding and MMTk core right?

Ok, so BINDING is a global, and we set an instance of the Ruby Binding into it, and then unwrap it because reasons.

## mmtk_init

Ok, so now we know how the builder is initialized and builds an MMTk instance we should look into how the MMTk system is actually being initialized.

First, this function takes a parameter, which is an immutable borrow of the builder object, and it returns a boxed pointer of MMTk

RubyBinding is an object that wraps the MMTk instance, the binding options, the upcalls and the plan name - the VMBinding trait.

VMBinding’s job seems to be to define some generic types for the binding, in this case it only really seems to care about VMEdge (type aliased to RubyEdge), and VMMemorySlice (type aliased to RubyMemorySlice).

RubyEdge according to the comments, doesn’t really matter right now as we don’t do edge enqueing (whatever that means). and Ruby memory slice is actually just another type alias for UnimplementedMemorySlice.

aside from some logger shenanigans, this just calls back into builder.build and then boxes the result.

## MMTKBuilder::build

straight up does MMTK::new passing in a reference counted clone of the options struct (using Arc).

## MMTK::new

Ok, now we’re getting somewhere.

firstly, what is SFT_MAP? Apparently it mamages the SFT Table, but what is the SFTTable? Ok - apparently the sft is defined inside mmtk-core. It’s something to do with space specific dynamic dispatch…. I’m just going to ignore this for now and hope it’s not important yet. Good start

Then we build a GCWorkScheduler, passing in the number of configured threads as a parameter (This is 1 for Ruby).

Then we build a plan. VSCode is type annotating this let binding as Box<dyn Plan<VM = VM>> and I don’t know what this means. but it uses a factory method on the plan module: crate::plan::create_plan.

Then we VM_MAP.boot(). What is VM_MAP (a global VMMap that manages the mapping of spaces to virtual memory ranges). I’ve seen spaces referenced before

• this is an MMTk concept, but what is it.

### spaces

What are spaces? They’re mentioned in Cargo.toml which is surprising to me. The main source file seems to be lib/policy/space.rs. I’m pretty sure this is going to be where the heap is allocated.

These look like the main memory spaces. They’re set up and defined by the Plans I think.

# What happens when a Ruby Object is allocated?

Starting in newobj_init0. This code could do with being refactored a bit, but it looks like we calculate which size pool an object belongs in and then we call mmtk_alloc.

We pass in GET_THREAD()->mutator - this is a thread on the actual Ruby rb_thread_struct. It’s been added as part of mmtk. This looks to be just an MMTk wrapper around the current Ruby thread.

We also pass in the slot size we want to allocate (MMTk is still dealing with slot sizes I think, and not specifically object sizes).

** NOTE ** Wait. What does MMTk do with the actual object data? is it still using malloc and heap allocating strings and shit? I think we need to replace ruby_xmalloc etc with mmtk variants.

The allocation size it uses is the size of the slot, plus and prefix and suffix. It stores the size pool size in the prefix and returns the offset address for the actual pointer to the VALUE.

Then we call mmtk_post_alloc which I’m not sure what this does. First lets look at mmtk_alloc.

** mmtk_alloc