Unpacker: key_cache option.

Heavily inspired from ruby/json#675.

When parsing documents with the same structure repeatedly, a lot
of time can be saved by keeping a small cache of map keys encountered.

Using the existing `bench/bench.rb`, comparing with and without the
cache shows a 30% performance improvement:

```
Calculating -------------------------------------
       unpack-pooled    960.380k (± 1.4%) i/s -      4.865M in   5.066600s
    unpack-key-cache      1.245M (± 1.6%) i/s -      6.232M in   5.009060s

Comparison:
       unpack-pooled:   960379.8 i/s
    unpack-key-cache:  1244517.6 i/s - 1.30x  (± 0.00) faster
```

However, on the same benchmark, but with the cache filled with other
keys, the performance is notably degraded:

```
Calculating -------------------------------------
       unpack-pooled    926.849k (± 2.1%) i/s -      4.639M in   5.007333s
    unpack-key-cache    822.266k (± 2.4%) i/s -      4.113M in   5.004645s

Comparison:
       unpack-pooled:   926849.2 i/s
    unpack-key-cache:   822265.6 i/s - 1.13x  (± 0.00) slower
```

So this feature is powerful but situational.

doclib/msgpack/unpacker.rb

@@ -19,6 +19,8 @@ class Unpacker
     # Supported options:
     #
     # * *:symbolize_keys* deserialize keys of Hash objects as Symbol instead of String
+    # * *:freeze* freeze the deserialized objects. Can allow string deduplication and some allocation elision.
+    # * *:key_cache* Enable caching of map keys, this can improve performance significantly if the same map keys are frequently encountered, but also degrade performance if that's not the case.
     # * *:allow_unknown_ext* allow to deserialize ext type object with unknown type id as ExtensionValue instance. Otherwise (by default), unpacker throws UnknownExtTypeError.
     #
     # See also Buffer#initialize for other options.

ext/msgpack/buffer.h

@@ -473,4 +473,131 @@ static inline VALUE msgpack_buffer_read_top_as_symbol(msgpack_buffer_t* b, size_
     return rb_str_intern(msgpack_buffer_read_top_as_string(b, length, true, utf8));
 }
 
+// Hash keys are likely to be repeated, and are frozen.
+// As such we can re-use them if we keep a cache of the ones we've seen so far,
+// and save much more expensive lookups into the global fstring table.
+// This cache implementation is deliberately simple, as we're optimizing for compactness,
+// to be able to fit easily embeded inside msgpack_unpacker_t.
+// As such, binary search into a sorted array gives a good tradeoff between compactness and
+// performance.
+#define MSGPACK_KEY_CACHE_CAPACITY 63
+
+typedef struct msgpack_key_cache_t msgpack_key_cache_t;
+struct msgpack_key_cache_t {
+    int length;
+    VALUE entries[MSGPACK_KEY_CACHE_CAPACITY];
+};
+
+static inline VALUE build_interned_string(const char *str, const long length)
+{
+# ifdef HAVE_RB_ENC_INTERNED_STR
+    return rb_enc_interned_str(str, length, rb_utf8_encoding());
+# else
+    VALUE rstring = rb_utf8_str_new(str, length);
+    return rb_funcall(rb_str_freeze(rstring), s_uminus, 0);
+# endif
+}
+
+static inline VALUE build_symbol(const char *str, const long length)
+{
+    return rb_str_intern(build_interned_string(str, length));
+}
+
+static void rvalue_cache_insert_at(msgpack_key_cache_t *cache, int index, VALUE rstring)
+{
+    MEMMOVE(&cache->entries[index + 1], &cache->entries[index], VALUE, cache->length - index);
+    cache->length++;
+    cache->entries[index] = rstring;
+}
+
+static inline int rstring_cache_cmp(const char *str, const long length, VALUE rstring)
+{
+    long rstring_length = RSTRING_LEN(rstring);
+    if (length == rstring_length) {
+        return memcmp(str, RSTRING_PTR(rstring), length);
+    } else {
+        return (int)(length - rstring_length);
+    }
+}
+
+static VALUE rstring_cache_fetch(msgpack_key_cache_t *cache, const char *str, const long length)
+{
+    int low = 0;
+    int high = cache->length - 1;
+    int mid = 0;
+    int last_cmp = 0;
+
+    while (low <= high) {
+        mid = (high + low) >> 1;
+        VALUE entry = cache->entries[mid];
+        last_cmp = rstring_cache_cmp(str, length, entry);
+
+        if (last_cmp == 0) {
+            return entry;
+        } else if (last_cmp > 0) {
+            low = mid + 1;
+        } else {
+            high = mid - 1;
+        }
+    }
+
+    VALUE rstring = build_interned_string(str, length);
+
+    if (cache->length < MSGPACK_KEY_CACHE_CAPACITY) {
+        if (last_cmp > 0) {
+            mid += 1;
+        }
+
+        rvalue_cache_insert_at(cache, mid, rstring);
+    }
+    return rstring;
+}
+
+static VALUE rsymbol_cache_fetch(msgpack_key_cache_t *cache, const char *str, const long length)
+{
+    int low = 0;
+    int high = cache->length - 1;
+    int mid = 0;
+    int last_cmp = 0;
+
+    while (low <= high) {
+        mid = (high + low) >> 1;
+        VALUE entry = cache->entries[mid];
+        last_cmp = rstring_cache_cmp(str, length, rb_sym2str(entry));
+
+        if (last_cmp == 0) {
+            return entry;
+        } else if (last_cmp > 0) {
+            low = mid + 1;
+        } else {
+            high = mid - 1;
+        }
+    }
+
+    VALUE rsymbol = build_symbol(str, length);
+
+    if (cache->length < MSGPACK_KEY_CACHE_CAPACITY) {
+        if (last_cmp > 0) {
+            mid += 1;
+        }
+
+        rvalue_cache_insert_at(cache, mid, rsymbol);
+    }
+    return rsymbol;
+}
+
+static inline VALUE msgpack_buffer_read_top_as_interned_symbol(msgpack_buffer_t* b, msgpack_key_cache_t *cache, size_t length)
+{
+    VALUE result = rsymbol_cache_fetch(cache, b->read_buffer, length);
+    _msgpack_buffer_consumed(b, length);
+    return result;
+}
+
+static inline VALUE msgpack_buffer_read_top_as_interned_string(msgpack_buffer_t* b, msgpack_key_cache_t *cache, size_t length)
+{
+    VALUE result = rstring_cache_fetch(cache, b->read_buffer, length);
+    _msgpack_buffer_consumed(b, length);
+    return result;
+}
+
 #endif

ext/msgpack/unpacker.c

@@ -130,11 +130,20 @@ void msgpack_unpacker_mark_stack(msgpack_unpacker_stack_t* stack)
     }
 }
 
+void msgpack_unpacker_mark_key_cache(msgpack_key_cache_t *cache)
+{
+    int index;
+    for (index = 0; index < cache->length; index++) {
+        rb_gc_mark(cache->entries[index]);
+    }
+}
+
 void msgpack_unpacker_mark(msgpack_unpacker_t* uk)
 {
     rb_gc_mark(uk->last_object);
     rb_gc_mark(uk->reading_raw);
     msgpack_unpacker_mark_stack(&uk->stack);
+    msgpack_unpacker_mark_key_cache(&uk->key_cache);
     /* See MessagePack_Buffer_wrap */
     /* msgpack_buffer_mark(UNPACKER_BUFFER_(uk)); */
     rb_gc_mark(uk->buffer_ref);
@@ -374,15 +383,32 @@ static inline int read_raw_body_begin(msgpack_unpacker_t* uk, int raw_type)
     size_t length = uk->reading_raw_remaining;
     if(length <= msgpack_buffer_top_readable_size(UNPACKER_BUFFER_(uk))) {
         int ret;
-        if ((uk->optimized_symbol_ext_type && uk->symbol_ext_type == raw_type) || (uk->symbolize_keys && is_reading_map_key(uk))) {
+        if ((uk->optimized_symbol_ext_type && uk->symbol_ext_type == raw_type)) {
             VALUE symbol = msgpack_buffer_read_top_as_symbol(UNPACKER_BUFFER_(uk), length, raw_type != RAW_TYPE_BINARY);
             ret = object_complete_symbol(uk, symbol);
+        } else if (is_reading_map_key(uk) && raw_type == RAW_TYPE_STRING) {
+           /* don't use zerocopy for hash keys but get a frozen string directly
+            * because rb_hash_aset freezes keys and it causes copying */
+            VALUE key;
+            if (uk->symbolize_keys) {
+                if (uk->use_key_cache) {
+                    key = msgpack_buffer_read_top_as_interned_symbol(UNPACKER_BUFFER_(uk), &uk->key_cache, length);
+                } else {
+                    key = msgpack_buffer_read_top_as_symbol(UNPACKER_BUFFER_(uk), length, true);
+                }
+                ret = object_complete_symbol(uk, key);
+            } else {
+                if (uk->use_key_cache) {
+                    key = msgpack_buffer_read_top_as_interned_string(UNPACKER_BUFFER_(uk), &uk->key_cache, length);
+                } else {
+                    key = msgpack_buffer_read_top_as_string(UNPACKER_BUFFER_(uk), length, true, true);
+                }
+
+                ret = object_complete(uk, key);
+            }
         } else {
             bool will_freeze = uk->freeze;
             if(raw_type == RAW_TYPE_STRING || raw_type == RAW_TYPE_BINARY) {
-               /* don't use zerocopy for hash keys but get a frozen string directly
-                * because rb_hash_aset freezes keys and it causes copying */
-                will_freeze = will_freeze || is_reading_map_key(uk);
                 VALUE string = msgpack_buffer_read_top_as_string(UNPACKER_BUFFER_(uk), length, will_freeze, raw_type == RAW_TYPE_STRING);
                 ret = object_complete(uk, string);
             } else {

ext/msgpack/unpacker.h

@@ -50,6 +50,7 @@ struct msgpack_unpacker_stack_t {
 struct msgpack_unpacker_t {
     msgpack_buffer_t buffer;
     msgpack_unpacker_stack_t stack;
+    msgpack_key_cache_t key_cache;
 
     VALUE self;
     VALUE last_object;
@@ -66,10 +67,12 @@ struct msgpack_unpacker_t {
 
     /* options */
     int symbol_ext_type;
-    bool symbolize_keys;
-    bool freeze;
-    bool allow_unknown_ext;
-    bool optimized_symbol_ext_type;
+
+    bool use_key_cache: 1;
+    bool symbolize_keys: 1;
+    bool freeze: 1;
+    bool allow_unknown_ext: 1;
+    bool optimized_symbol_ext_type: 1;
 };
 
 #define UNPACKER_BUFFER_(uk) (&(uk)->buffer)
@@ -101,6 +104,11 @@ static inline void msgpack_unpacker_set_symbolized_keys(msgpack_unpacker_t* uk,
     uk->symbolize_keys = enable;
 }
 
+static inline void msgpack_unpacker_set_key_cache(msgpack_unpacker_t* uk, bool enable)
+{
+    uk->use_key_cache = enable;
+}
+
 static inline void msgpack_unpacker_set_freeze(msgpack_unpacker_t* uk, bool enable)
 {
     uk->freeze = enable;

ext/msgpack/unpacker_class.c

@@ -34,6 +34,7 @@ static VALUE eUnknownExtTypeError;
 static VALUE mTypeError;  // obsoleted. only for backward compatibility. See #86.
 
 static VALUE sym_symbolize_keys;
+static VALUE sym_key_cache;
 static VALUE sym_freeze;
 static VALUE sym_allow_unknown_ext;
 
@@ -128,6 +129,9 @@ VALUE MessagePack_Unpacker_initialize(int argc, VALUE* argv, VALUE self)
     if(options != Qnil) {
         VALUE v;
 
+        v = rb_hash_aref(options, sym_key_cache);
+        msgpack_unpacker_set_key_cache(uk, RTEST(v));
+
         v = rb_hash_aref(options, sym_symbolize_keys);
         msgpack_unpacker_set_symbolized_keys(uk, RTEST(v));
 
@@ -413,6 +417,7 @@ void MessagePack_Unpacker_module_init(VALUE mMessagePack)
     eUnknownExtTypeError = rb_define_class_under(mMessagePack, "UnknownExtTypeError", eUnpackError);
 
     sym_symbolize_keys = ID2SYM(rb_intern("symbolize_keys"));
+    sym_key_cache = ID2SYM(rb_intern("key_cache"));
     sym_freeze = ID2SYM(rb_intern("freeze"));
     sym_allow_unknown_ext = ID2SYM(rb_intern("allow_unknown_ext"));