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Added some more comments and refactored slightly.
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haberman committed Mar 25, 2021
1 parent f0d6fcb commit e1ac393
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Showing 2 changed files with 21 additions and 2 deletions.
21 changes: 19 additions & 2 deletions ruby/ext/google/protobuf_c/protobuf.c
Original file line number Diff line number Diff line change
Expand Up @@ -260,6 +260,7 @@ VALUE secondary_map_mutex = Qnil;
// Lambda that will GC entries from the secondary map that are no longer present
// in the primary map.
VALUE gc_secondary_map = Qnil;
ID length;

extern VALUE weak_obj_cache;

Expand All @@ -273,14 +274,30 @@ static void SecondaryMap_Init() {
" secondary.delete_if { |k, v| !weak.key?(v) }\n"
"}\n");
secondary_map_mutex = rb_mutex_new();
length = rb_intern("length");
}

// The secondary map is a regular Hash, and will never shrink on its own.
// The main object cache is a WeakMap that will automatically remove entries
// when the target object is no longer reachable, but unless we manually
// remove the corresponding entries from the secondary map, it will grow
// without bound.
//
// To avoid this unbounded growth we periodically remove entries from the
// secondary map that are no longer present in the WeakMap. The logic of
// how often to perform this GC is an artbirary tuning parameter that
// represents a straightforward CPU/memory tradeoff.
static void SecondaryMap_MaybeGC() {
size_t weak_len = NUM2ULL(rb_funcall(weak_obj_cache, rb_intern("length"), 0));
size_t weak_len = NUM2ULL(rb_funcall(weak_obj_cache, length, 0));
size_t secondary_len = RHASH_SIZE(secondary_map);
size_t waste = secondary_len - weak_len;
PBRUBY_ASSERT(secondary_len >= weak_len);
if (waste > 1000 && waste > secondary_len * 0.2) {
// GC if we could remove at least 2000 entries or 20% of the table size
// (whichever is greater). Since the cost of the GC pass is O(N), we
// want to make sure that we condition this on overall table size, to
// avoid O(N^2) CPU costs.
size_t threshold = PBRUBY_MAX(secondary_len * 0.2, 2000);
if (waste > threshold) {
rb_funcall(gc_secondary_map, rb_intern("call"), 2, secondary_map, weak_obj_cache);
}
}
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2 changes: 2 additions & 0 deletions ruby/ext/google/protobuf_c/protobuf.h
Original file line number Diff line number Diff line change
Expand Up @@ -106,6 +106,8 @@ extern VALUE cTypeError;
#define PBRUBY_ASSERT(expr) assert(expr)
#endif

#define PBRUBY_MAX(x, y) (((x) > (y)) ? (x) : (y))

#define UPB_UNUSED(var) (void)var

#endif // __GOOGLE_PROTOBUF_RUBY_PROTOBUF_H__

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