Added some more comments and refactored slightly.
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@ -260,6 +260,7 @@ VALUE secondary_map_mutex = Qnil;
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// Lambda that will GC entries from the secondary map that are no longer present
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// in the primary map.
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VALUE gc_secondary_map = Qnil;
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ID length;
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extern VALUE weak_obj_cache;
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@ -273,14 +274,30 @@ static void SecondaryMap_Init() {
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" secondary.delete_if { |k, v| !weak.key?(v) }\n"
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"}\n");
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secondary_map_mutex = rb_mutex_new();
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length = rb_intern("length");
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}
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// The secondary map is a regular Hash, and will never shrink on its own.
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// The main object cache is a WeakMap that will automatically remove entries
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// when the target object is no longer reachable, but unless we manually
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// remove the corresponding entries from the secondary map, it will grow
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// without bound.
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//
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// To avoid this unbounded growth we periodically remove entries from the
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// secondary map that are no longer present in the WeakMap. The logic of
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// how often to perform this GC is an artbirary tuning parameter that
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// represents a straightforward CPU/memory tradeoff.
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static void SecondaryMap_MaybeGC() {
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size_t weak_len = NUM2ULL(rb_funcall(weak_obj_cache, rb_intern("length"), 0));
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size_t weak_len = NUM2ULL(rb_funcall(weak_obj_cache, length, 0));
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size_t secondary_len = RHASH_SIZE(secondary_map);
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size_t waste = secondary_len - weak_len;
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PBRUBY_ASSERT(secondary_len >= weak_len);
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if (waste > 1000 && waste > secondary_len * 0.2) {
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// GC if we could remove at least 2000 entries or 20% of the table size
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// (whichever is greater). Since the cost of the GC pass is O(N), we
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// want to make sure that we condition this on overall table size, to
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// avoid O(N^2) CPU costs.
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size_t threshold = PBRUBY_MAX(secondary_len * 0.2, 2000);
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if (waste > threshold) {
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rb_funcall(gc_secondary_map, rb_intern("call"), 2, secondary_map, weak_obj_cache);
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}
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}
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@ -106,6 +106,8 @@ extern VALUE cTypeError;
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#define PBRUBY_ASSERT(expr) assert(expr)
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#endif
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#define PBRUBY_MAX(x, y) (((x) > (y)) ? (x) : (y))
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#define UPB_UNUSED(var) (void)var
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#endif // __GOOGLE_PROTOBUF_RUBY_PROTOBUF_H__
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