Merge pull request #7146 from oranagra/optimize_deferred_reply

optimize memory usage of deferred replies

src/networking.c

@@ -436,6 +436,36 @@ void addReplyStatusFormat(client *c, const char *fmt, ...) {
     sdsfree(s);
 }
 
+/* Sometimes we are forced to create a new reply node, and we can't append to
+ * the previous one, when that happens, we wanna try to trim the unused space
+ * at the end of the last reply node which we won't use anymore. */
+void trimReplyUnusedTailSpace(client *c) {
+    listNode *ln = listLast(c->reply);
+    clientReplyBlock *tail = ln? listNodeValue(ln): NULL;
+
+    /* Note that 'tail' may be NULL even if we have a tail node, becuase when
+     * addDeferredMultiBulkLength() is used */
+    if (!tail) return;
+
+    /* We only try to trim the space is relatively high (more than a 1/4 of the
+     * allocation), otherwise there's a high chance realloc will NOP.
+     * Also, to avoid large memmove which happens as part of realloc, we only do
+     * that if the used part is small.  */
+    if (tail->size - tail->used > tail->size / 4 &&
+        tail->used < PROTO_REPLY_CHUNK_BYTES)
+    {
+        size_t old_size = tail->size;
+        tail = zrealloc(tail, tail->used + sizeof(clientReplyBlock));
+        /* If realloc was a NOP, we got the same value which has internal frag */
+        if (tail == listNodeValue(ln)) return;
+        /* take over the allocation's internal fragmentation (at least for
+         * memory usage tracking) */
+        tail->size = zmalloc_usable(tail) - sizeof(clientReplyBlock);
+        c->reply_bytes += tail->size - old_size;
+        listNodeValue(ln) = tail;
+    }
+}
+
 /* Adds an empty object to the reply list that will contain the multi bulk
  * length, which is not known when this function is called. */
 void *addReplyDeferredLen(client *c) {
@@ -443,6 +473,7 @@ void *addReplyDeferredLen(client *c) {
      * ready to be sent, since we are sure that before returning to the
      * event loop setDeferredAggregateLen() will be called. */
     if (prepareClientToWrite(c) != C_OK) return NULL;
+    trimReplyUnusedTailSpace(c);
     listAddNodeTail(c->reply,NULL); /* NULL is our placeholder. */
     return listLast(c->reply);
 }