How to use mrb_gc_arena_save()/mrb_gc_arena_restore()/mrb_gc_protect()

This is an English translation of [Matz’s blog post][matz blog post] written in Japanese. Some parts are updated to reflect recent changes. [matz blog post]: http://www.rubyist.net/~matz/20130731.html

When you are extending mruby using C language, you may encounter mysterious “arena overflow error” or memory leak or very slow execution speed. This is an error indicating overflow of “GC arena” implementing “conservative GC”.

GC (garbage collector) must ensure that object is “alive”, in other words, that it is referenced by somewhere from program. This can be determined by checking if the object can be directly or indirectly referenced by root. The local variables, global variables and constants etc are root.

If program execution is performed inside mruby VM, there is nothing to worry about because GC can access all roots owned by VM.

The problem arises when executing C functions. The object referenced by C variable is also “alive”, but mruby GC cannot aware of this, so it might mistakenly recognize the objects referenced by only C variables as dead.

This can be a fatal bug if the GC tries to collect a live object.

In CRuby, we scan C stack area, and use C variable as root to check whether object is alive or not. Of course, because we are accessing C stack just as memory region, we never know it is an integer or a pointer. We workaround this by assuming that if it looks like a pointer, then assume it as a pointer. We call it “conservative”.

By the way, CRuby’s “conservative GC” has some problems.

The biggest problem is we have no way to access to the stack area in portable way. Therefore, we cannot use this method if we’d like to implement highly portable runtime, like mruby.

So we came up with an another plan to implement “conservative GC” in mruby.

Again, the problem is when an object which was created in C function, becomes no longer referenced in the Ruby world, and cannot be treated as garbage.

In mruby, we recognize all objects created in C function are alive. Then we have no problem such as confusing a live object as dead.

This means that because we cannot collect truly dead object, we may lose efficiency, but as a trade-off the GC itself is highly portable. We can say goodbye to the problem that GC deletes live objects due to optimization which sometimes occurs in CRuby.

According to this idea, we have a table, called “GC arena”, which remembers objects created in C function.

The arena is stack structure, when C function execution is returned to mruby VM, all objects registered in the arena are popped.

This works very well, but can cause another problem: “arena overflow error” or memory leak.

As of this writing, mruby automatically extend arena to remember objects (See MRB_GC_FIXED_ARENA and MRB_GC_ARENA_SIZE in doc/guides/mrbconf.md).

If you create many objects in C functions, memory usage will increase, since GC never kick in. This memory usage may look like memory leak, but will also make execution slower as more memory will need to be allocated.

With the build time configuration, you can limit the maximum size of arena (e.g., 100). Then if you create many objects, arena overflows, thus you will get an “arena overflow error”.

To workaround these problems, we have mrb_gc_arena_save() and mrb_gc_arena_restore() functions.

int mrb_gc_arena_save(mrb) returns the current position of the stack top of GC arena, and void mrb_gc_arena_restore(mrb, idx) sets the stack top position to back to given idx.

We can use them like this:

```c int arena_idx = mrb_gc_arena_save(mrb);

// …create objects… mrb_gc_arena_restore(mrb, arena_idx);

```

In mruby, C function calls are surrounded by this save/restore, but we can further optimize memory usage by surrounding save/restore, and can avoid creating arena overflow bugs.

Let’s take a real example. Here is the source code of Array#inspect:

```c static mrb_value inspect_ary(mrb_state *mrb, mrb_value ary, mrb_value list) { mrb_int i; mrb_value s, arystr; char head[] = { ‘[’ }; char sep[] = { ‘,’, ‘ ‘ }; char tail[] = { ‘]’ };

/* check recursive */ for(i=0; i<RARRAY_LEN(list); i++) { if (mrb_obj_equal(mrb, ary, RARRAY_PTR(list)[i])) { return mrb_str_new(mrb, “[…]”, 5); } }

mrb_ary_push(mrb, list, ary);

arystr = mrb_str_new_capa(mrb, 64); mrb_str_cat(mrb, arystr, head, sizeof(head));

for(i=0; i<RARRAY_LEN(ary); i++) { int ai = mrb_gc_arena_save(mrb);

if (i > 0) {
  mrb_str_cat(mrb, arystr, sep, sizeof(sep));
}
if (mrb_array_p(RARRAY_PTR(ary)[i])) {
  s = inspect_ary(mrb, RARRAY_PTR(ary)[i], list);
}
else {
  s = mrb_inspect(mrb, RARRAY_PTR(ary)[i]);
}
mrb_str_cat(mrb, arystr, RSTRING_PTR(s), RSTRING_LEN(s));
mrb_gc_arena_restore(mrb, ai);   }

mrb_str_cat(mrb, arystr, tail, sizeof(tail)); mrb_ary_pop(mrb, list);

return arystr; } ```

This is a real example, so a little bit complicated, but bear with me. The essence of Array#inspect is that after stringifying each element of array using inspect method, we join them together so that we can get inspect representation of the entire array.

After the inspect representation is created, we no longer require the individual string representation. This means that we don’t have to register these temporal objects into GC arena.

Therefore, in order to keep the arena size small; the ary_inspect() function will do the following:

• save the position of the stack top using mrb_gc_arena_save().
• get inspect representation of each element.
• append it to the constructing entire inspect representation of array.
• restore stack top position using mrb_gc_arena_restore().

Please note that the final inspect representation of entire array was created before the call of mrb_gc_arena_restore(). Otherwise, required temporal object may be deleted by GC.

We may have a usecase where after creating many temporal objects, we’d like to keep some of them. In this case, we cannot use the same idea in ary_inspect() like appending objects to existing one. Instead, after mrb_gc_arena_restore(), we must re-register the objects we want to keep in the arena using mrb_gc_protect(mrb, obj). Use mrb_gc_protect() with caution because it could also lead to an “arena overflow error”.

We must also mention that when mrb_funcall is called in top level, the return value is also registered to GC arena, so repeated use of mrb_funcall may eventually lead to an “arena overflow error”.

Use mrb_gc_arena_save() and mrb_gc_arena_restore() or possible use of mrb_gc_protect() to workaround this.