Optimize stream id sds creation on XADD key * (~20% saved cpu cycles) (#10574)

we can observe that when adding to a stream without ID there is a duplicate work
on sds creation/freeing/sdslen that costs ~11% of the CPU cycles. 

This PR avoids it by not freeing the sds after the first reply. 
The expected reduction in CPU cycles is around 9-10% 

Additionally, we now pre-allocate the sds to the right size, to avoid realloc.
this brought another ~10% improvement

Co-authored-by: Oran Agra <oran@redislabs.com>

1 files changed, 22 insertions, 8 deletions

diff --git a/src/t_stream.c b/src/t_stream.c
index cd7d9723e..399c2091c 100644
--- a/src/t_stream.c
+++ b/src/t_stream.c
@@ -1370,24 +1370,35 @@ void streamLastValidID(stream *s, streamID *maxid)
     streamIteratorStop(&si);
 }
 
+/* Maximum size for a stream ID string. In theory 20*2+1 should be enough,
+ * But to avoid chance for off by one issues and null-term, in case this will
+ * be used as parsing buffer, we use a slightly larger buffer. On the other
+ * hand considering sds header is gonna add 4 bytes, we wanna keep below the
+ * allocator's 48 bytes bin. */
+#define STREAM_ID_STR_LEN 44
+
+sds createStreamIDString(streamID *id) {
+    /* Optimization: pre-allocate a big enough buffer to avoid reallocs. */
+    sds str = sdsnewlen(SDS_NOINIT, STREAM_ID_STR_LEN);
+    sdssetlen(str, 0);
+    return sdscatfmt(str,"%U-%U", id->ms,id->seq);
+}
+
 /* Emit a reply in the client output buffer by formatting a Stream ID
  * in the standard <ms>-<seq> format, using the simple string protocol
  * of REPL. */
 void addReplyStreamID(client *c, streamID *id) {
-    sds replyid = sdscatfmt(sdsempty(),"%U-%U",id->ms,id->seq);
-    addReplyBulkSds(c,replyid);
+    addReplyBulkSds(c,createStreamIDString(id));
 }
 
 void setDeferredReplyStreamID(client *c, void *dr, streamID *id) {
-    sds replyid = sdscatfmt(sdsempty(),"%U-%U",id->ms,id->seq);
-    setDeferredReplyBulkSds(c, dr, replyid);
+    setDeferredReplyBulkSds(c, dr, createStreamIDString(id));
 }
 
 /* Similar to the above function, but just creates an object, usually useful
  * for replication purposes to create arguments. */
 robj *createObjectFromStreamID(streamID *id) {
-    return createObject(OBJ_STRING, sdscatfmt(sdsempty(),"%U-%U",
-                        id->ms,id->seq));
+    return createObject(OBJ_STRING, createStreamIDString(id));
 }
 
 /* Returns non-zero if the ID is 0-0. */
@@ -2025,7 +2036,8 @@ void xaddCommand(client *c) {
             addReplyError(c,"Elements are too large to be stored");
         return;
     }
-    addReplyStreamID(c,&id);
+    sds replyid = createStreamIDString(&id);
+    addReplyBulkCBuffer(c, replyid, sdslen(replyid));
 
     signalModifiedKey(c,c->db,c->argv[1]);
     notifyKeyspaceEvent(NOTIFY_STREAM,"xadd",c->argv[1],c->db->id);
@@ -2050,9 +2062,11 @@ void xaddCommand(client *c) {
     /* Let's rewrite the ID argument with the one actually generated for
      * AOF/replication propagation. */
     if (!parsed_args.id_given || !parsed_args.seq_given) {
-        robj *idarg = createObjectFromStreamID(&id);
+        robj *idarg = createObject(OBJ_STRING, replyid);
         rewriteClientCommandArgument(c, idpos, idarg);
         decrRefCount(idarg);
+    } else {
+        sdsfree(replyid);
     }
 
     /* We need to signal to blocked clients that there is new data on this