178 lines
6.6 KiB
Markdown
178 lines
6.6 KiB
Markdown
# How to use `mrb_gc_arena_save()`/`mrb_gc_arena_restore()`/`mrb_gc_protect()`
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_This is an English translation of [Matz's blog post][matz blog post]
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written in Japanese._
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_Some parts are updated to reflect recent changes._
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[matz blog post]: http://www.rubyist.net/~matz/20130731.html
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When you are extending mruby using C language, you may encounter
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mysterious "arena overflow error" or memory leak or very slow
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execution speed. This is an error indicating overflow of "GC arena"
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implementing "conservative GC".
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GC (garbage collector) must ensure that object is "alive", in other
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words, that it is referenced by somewhere from program. This can be
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determined by checking if the object can be directly or indirectly
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referenced by root. The local variables, global variables and
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constants etc are root.
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If program execution is performed inside mruby VM, there is nothing to
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worry about because GC can access all roots owned by VM.
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The problem arises when executing C functions. The object referenced
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by C variable is also "alive", but mruby GC cannot aware of this, so
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it might mistakenly recognize the objects referenced by only C
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variables as dead.
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This can be a fatal bug if the GC tries to collect a live object.
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In CRuby, we scan C stack area, and use C variable as root to check
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whether object is alive or not. Of course, because we are accessing C
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stack just as memory region, we never know it is an integer or a
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pointer. We workaround this by assuming that if it looks like a
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pointer, then assume it as a pointer. We call it "conservative".
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By the way, CRuby's "conservative GC" has some problems.
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The biggest problem is we have no way to access to the stack area in
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portable way. Therefore, we cannot use this method if we'd like to
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implement highly portable runtime, like mruby.
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So we came up with an another plan to implement "conservative GC" in mruby.
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Again, the problem is when an object which was created in C function, becomes
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no longer referenced in the Ruby world, and cannot be treated as garbage.
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In mruby, we recognize all objects created in C function are alive.
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Then we have no problem such as confusing a live object as dead.
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This means that because we cannot collect truly dead object, we may
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lose efficiency, but as a trade-off the GC itself is highly portable.
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We can say goodbye to the problem that GC deletes live objects due to
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optimization which sometimes occurs in CRuby.
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According to this idea, we have a table, called "GC arena", which
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remembers objects created in C function.
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The arena is stack structure, when C function execution is returned to mruby
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VM, all objects registered in the arena are popped.
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This works very well, but can cause another problem: "arena overflow error" or
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memory leak.
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As of this writing, mruby automatically extend arena to remember
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objects (See `MRB_GC_FIXED_ARENA` and `MRB_GC_ARENA_SIZE` in
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doc/guides/mrbconf.md).
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If you create many objects in C functions, memory usage will increase, since
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GC never kick in. This memory usage may look like memory leak, but will also
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make execution slower as more memory will need to be allocated.
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With the build time configuration, you can limit the maximum size of
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arena (e.g., 100). Then if you create many objects, arena overflows,
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thus you will get an "arena overflow error".
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To workaround these problems, we have `mrb_gc_arena_save()` and
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`mrb_gc_arena_restore()` functions.
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`int mrb_gc_arena_save(mrb)` returns the current position of the stack
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top of GC arena, and `void mrb_gc_arena_restore(mrb, idx)` sets the
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stack top position to back to given `idx`.
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We can use them like this:
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```c
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int arena_idx = mrb_gc_arena_save(mrb);
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// ...create objects...
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mrb_gc_arena_restore(mrb, arena_idx);
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```
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In mruby, C function calls are surrounded by this save/restore, but we
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can further optimize memory usage by surrounding save/restore, and can
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avoid creating arena overflow bugs.
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Let's take a real example. Here is the source code of `Array#inspect`:
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```c
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static mrb_value
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inspect_ary(mrb_state *mrb, mrb_value ary, mrb_value list)
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{
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mrb_int i;
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mrb_value s, arystr;
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char head[] = { '[' };
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char sep[] = { ',', ' ' };
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char tail[] = { ']' };
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/* check recursive */
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for(i=0; i<RARRAY_LEN(list); i++) {
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if (mrb_obj_equal(mrb, ary, RARRAY_PTR(list)[i])) {
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return mrb_str_new(mrb, "[...]", 5);
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}
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}
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mrb_ary_push(mrb, list, ary);
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arystr = mrb_str_new_capa(mrb, 64);
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mrb_str_cat(mrb, arystr, head, sizeof(head));
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for(i=0; i<RARRAY_LEN(ary); i++) {
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int ai = mrb_gc_arena_save(mrb);
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if (i > 0) {
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mrb_str_cat(mrb, arystr, sep, sizeof(sep));
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}
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if (mrb_array_p(RARRAY_PTR(ary)[i])) {
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s = inspect_ary(mrb, RARRAY_PTR(ary)[i], list);
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}
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else {
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s = mrb_inspect(mrb, RARRAY_PTR(ary)[i]);
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}
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mrb_str_cat(mrb, arystr, RSTRING_PTR(s), RSTRING_LEN(s));
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mrb_gc_arena_restore(mrb, ai);
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}
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mrb_str_cat(mrb, arystr, tail, sizeof(tail));
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mrb_ary_pop(mrb, list);
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return arystr;
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}
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```
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This is a real example, so a little bit complicated, but bear with me.
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The essence of `Array#inspect` is that after stringifying each element
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of array using `inspect` method, we join them together so that we can
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get `inspect` representation of the entire array.
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After the `inspect` representation is created, we no longer require the
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individual string representation. This means that we don't have to register
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these temporal objects into GC arena.
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Therefore, in order to keep the arena size small; the `ary_inspect()` function
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will do the following:
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* save the position of the stack top using `mrb_gc_arena_save()`.
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* get `inspect` representation of each element.
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* append it to the constructing entire `inspect` representation of array.
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* restore stack top position using `mrb_gc_arena_restore()`.
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Please note that the final `inspect` representation of entire array
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was created before the call of `mrb_gc_arena_restore()`. Otherwise,
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required temporal object may be deleted by GC.
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We may have a usecase where after creating many temporal objects, we'd
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like to keep some of them. In this case, we cannot use the same idea
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in `ary_inspect()` like appending objects to existing one.
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Instead, after `mrb_gc_arena_restore()`, we must re-register the objects we
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want to keep in the arena using `mrb_gc_protect(mrb, obj)`.
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Use `mrb_gc_protect()` with caution because it could also lead to an "arena
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overflow error".
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We must also mention that when `mrb_funcall` is called in top level, the return
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value is also registered to GC arena, so repeated use of `mrb_funcall` may
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eventually lead to an "arena overflow error".
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Use `mrb_gc_arena_save()` and `mrb_gc_arena_restore()` or possible use of
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`mrb_gc_protect()` to workaround this.
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