Introduction of a new string encoding: EMBSTRmemopt

Previously two string encodings were used for string objects:

1) REDIS_ENCODING_RAW: a string object with obj->ptr pointing to an sds
stirng.

2) REDIS_ENCODING_INT: a string object where the obj->ptr void pointer
is casted to a long.

This commit introduces a experimental new encoding called
REDIS_ENCODING_EMBSTR that implements an object represented by an sds
string that is not modifiable but allocated in the same memory chunk as
the robj structure itself.

The chunk looks like the following:

+--------------+-----------+------------+--------+----+
| robj data... | robj->ptr | sds header | string | \0 |
+--------------+-----+-----+------------+--------+----+
                     |                       ^
                     +-----------------------+

The robj->ptr points to the contiguous sds string data, so the object
can be manipulated with the same functions used to manipulate plan
string objects, however we need just on malloc and one free in order to
allocate or release this kind of objects. Moreover it has better cache
locality.

This new allocation strategy should benefit both the memory usage and
the performances. A performance gain between 60 and 70% was observed
during micro-benchmarks, however there is more work to do to evaluate
the performance impact and the memory usage behavior.

1 files changed, 6 insertions, 7 deletions

diff --git a/src/rdb.c b/src/rdb.c
index 9190ea514..9db9af0e1 100644
--- a/src/rdb.c
+++ b/src/rdb.c
@@ -296,7 +296,7 @@ int rdbSaveStringObject(rio *rdb, robj *obj) {
     if (obj->encoding == REDIS_ENCODING_INT) {
         return rdbSaveLongLongAsStringObject(rdb,(long)obj->ptr);
     } else {
-        redisAssertWithInfo(NULL,obj,obj->encoding == REDIS_ENCODING_RAW);
+        redisAssertWithInfo(NULL,obj,sdsEncodedObject(obj));
         return rdbSaveRawString(rdb,obj->ptr,sdslen(obj->ptr));
     }
 }
@@ -757,7 +757,7 @@ robj *rdbLoadObject(int rdbtype, rio *rdb) {
             /* If we are using a ziplist and the value is too big, convert
              * the object to a real list. */
             if (o->encoding == REDIS_ENCODING_ZIPLIST &&
-                ele->encoding == REDIS_ENCODING_RAW &&
+                sdsEncodedObject(ele) &&
                 sdslen(ele->ptr) > server.list_max_ziplist_value)
                     listTypeConvert(o,REDIS_ENCODING_LINKEDLIST);
 
@@ -831,9 +831,8 @@ robj *rdbLoadObject(int rdbtype, rio *rdb) {
             if (rdbLoadDoubleValue(rdb,&score) == -1) return NULL;
 
             /* Don't care about integer-encoded strings. */
-            if (ele->encoding == REDIS_ENCODING_RAW &&
-                sdslen(ele->ptr) > maxelelen)
-                    maxelelen = sdslen(ele->ptr);
+            if (sdsEncodedObject(ele) && sdslen(ele->ptr) > maxelelen)
+                maxelelen = sdslen(ele->ptr);
 
             znode = zslInsert(zs->zsl,score,ele);
             dictAdd(zs->dict,ele,&znode->score);
@@ -865,10 +864,10 @@ robj *rdbLoadObject(int rdbtype, rio *rdb) {
             /* Load raw strings */
             field = rdbLoadStringObject(rdb);
             if (field == NULL) return NULL;
-            redisAssert(field->encoding == REDIS_ENCODING_RAW);
+            redisAssert(sdsEncodedObject(field));
             value = rdbLoadStringObject(rdb);
             if (value == NULL) return NULL;
-            redisAssert(field->encoding == REDIS_ENCODING_RAW);
+            redisAssert(sdsEncodedObject(value));
 
             /* Add pair to ziplist */
             o->ptr = ziplistPush(o->ptr, field->ptr, sdslen(field->ptr), ZIPLIST_TAIL);