Avoid installing the client write handler when possible.

src/networking.c

@@ -166,16 +166,18 @@ int prepareClientToWrite(client *c) {
     /* Only install the handler if not already installed and, in case of
      * slaves, if the client can actually receive writes. */
     if (c->bufpos == 0 && listLength(c->reply) == 0 &&
+        !(c->flags & CLIENT_PENDING_WRITE) &&
         (c->replstate == REPL_STATE_NONE ||
          (c->replstate == SLAVE_STATE_ONLINE && !c->repl_put_online_on_ack)))
     {
-        /* Try to install the write handler. */
-        if (aeCreateFileEvent(server.el, c->fd, AE_WRITABLE,
-                sendReplyToClient, c) == AE_ERR)
-        {
-            freeClientAsync(c);
-            return C_ERR;
-        }
+        /* Here instead of installing the write handler, we just flag the
+         * client and put it into a list of clients that have something
+         * to write to the socket. This way before re-entering the event
+         * loop, we can try to directly write to the client sockets avoiding
+         * a system call. We'll only really install the write handler if
+         * we'll not be able to write the whole reply at once. */
+        c->flags |= CLIENT_PENDING_WRITE;
+        listAddNodeTail(server.clients_pending_write,c);
     }
 
     /* Authorize the caller to queue in the output buffer of this client. */
@@ -739,6 +741,12 @@ void freeClient(client *c) {
         listDelNode(server.clients,ln);
     }
 
+    /* Remove from the list of pending writes if needed. */
+    if (c->flags & CLIENT_PENDING_WRITE) {
+        ln = listSearchKey(server.clients_pending_write,c);
+        listDelNode(server.clients_pending_write,ln);
+    }
+
     /* When client was just unblocked because of a blocking operation,
      * remove it from the list of unblocked clients. */
     if (c->flags & CLIENT_UNBLOCKED) {

src/replication.c

@@ -1857,6 +1857,12 @@ void replicationCacheMaster(client *c) {
     serverAssert(ln != NULL);
     listDelNode(server.clients,ln);
 
+    /* Remove from the list of clients with pending writes as well. */
+    if (c->flags & CLIENT_PENDING_WRITE) {
+        ln = listSearchKey(server.clients_pending_write,c);
+        if (ln) listDelNode(server.clients_pending_write,ln);
+    }
+
     /* Save the master. Server.master will be set to null later by
      * replicationHandleMasterDisconnection(). */
     server.cached_master = server.master;

src/server.c

@@ -1274,6 +1274,34 @@ int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
     return 1000/server.hz;
 }
 
+/* This function is called just before entering the event loop, in the hope
+ * we can just write the replies to the client output buffer without any
+ * need to use a syscall in order to install the writable event handler,
+ * get it called, and so forth. */
+void handleClientsWithPendingWrites(void) {
+    listIter li;
+    listNode *ln;
+
+    listRewind(server.clients_pending_write,&li);
+    while((ln = listNext(&li))) {
+        client *c = listNodeValue(ln);
+        c->flags &= ~CLIENT_PENDING_WRITE;
+        listDelNode(server.clients_pending_write,ln);
+
+        /* Try to write buffers to the client socket. */
+        sendReplyToClient(server.el,c->fd,c,0);
+
+        /* If there is nothing left, do nothing. Otherwise install
+         * the write handler. */
+        if ((c->bufpos || listLength(c->reply)) &&
+            aeCreateFileEvent(server.el, c->fd, AE_WRITABLE,
+                sendReplyToClient, c) == AE_ERR)
+        {
+            freeClientAsync(c);
+        }
+    }
+}
+
 /* This function gets called every time Redis is entering the
  * main loop of the event driven library, that is, before to sleep
  * for ready file descriptors. */
@@ -1317,6 +1345,9 @@ void beforeSleep(struct aeEventLoop *eventLoop) {
 
     /* Write the AOF buffer on disk */
     flushAppendOnlyFile(0);
+
+    /* Handle writes with pending output buffers. */
+    handleClientsWithPendingWrites();
 }
 
 /* =========================== Server initialization ======================== */
@@ -1781,6 +1812,7 @@ void initServer(void) {
     server.clients_to_close = listCreate();
     server.slaves = listCreate();
     server.monitors = listCreate();
+    server.clients_pending_write = listCreate();
     server.slaveseldb = -1; /* Force to emit the first SELECT command. */
     server.unblocked_clients = listCreate();
     server.ready_keys = listCreate();

src/server.h

@@ -260,6 +260,8 @@ typedef long long mstime_t; /* millisecond time type. */
 #define CLIENT_READONLY (1<<17)    /* Cluster client is in read-only state. */
 #define CLIENT_PUBSUB (1<<18)      /* Client is in Pub/Sub mode. */
 #define CLIENT_PREVENT_PROP (1<<19)  /* Don't propagate to AOF / Slaves. */
+#define CLIENT_PENDING_WRITE (1<<20) /* Client has output to send but a write
+                                        handler is yet not installed. */
 
 /* Client block type (btype field in client structure)
  * if CLIENT_BLOCKED flag is set. */
@@ -714,6 +716,7 @@ struct redisServer {
     int cfd_count;              /* Used slots in cfd[] */
     list *clients;              /* List of active clients */
     list *clients_to_close;     /* Clients to close asynchronously */
+    list *clients_pending_write; /* There is to write or install handler. */
     list *slaves, *monitors;    /* List of slaves and MONITORs */
     client *current_client; /* Current client, only used on crash report */
     int clients_paused;         /* True if clients are currently paused */