Scalability improvements for large amounts of connections

Add signaler_sndbuf_size option to config.hpp which allows the user to
increase the size of the send buffer used by the signalling socketpair.

Implement random backoff for reconnection attempts using a primitive
pseudo-random generation to prevent reconnection storms.

Add wait_before_connect option to config.hpp to allow the user to enable
random delay even on initial connect. Default is false for low latency.
Signed-off-by: Martin Lucina <mato@kotelna.sk>

src/config.hpp

@@ -39,6 +39,10 @@ namespace zmq
         //  using a single system call.
         signal_buffer_size = 8,
 
+        //  Socketpair send buffer size used by signaler. The default value of
+        //  zero means leave it at the system default.
+        signaler_sndbuf_size = 0,
+
         //  Determines how often does socket poll for new commands when it
         //  still has unprocessed messages to handle. Thus, if it is set to 100,
         //  socket will process 100 inbound messages before doing the poll.
@@ -72,6 +76,9 @@ namespace zmq
         //  How long to wait (milliseconds) till reattempting to connect.
         reconnect_period = 100,
 
+        //  Should initial connection attempts be delayed?
+        wait_before_connect = false,
+
         //  Maximal delay to process command in API thread (in CPU ticks).
         //  3,000,000 ticks equals to 1 - 2 milliseconds on current CPUs.
         //  Note that delay is only applied when there is continuous stream of
@@ -87,7 +94,7 @@ namespace zmq
 
         //  Maximal number of non-accepted connections that can be held by
         //  TCP listener object.
-        tcp_connection_backlog = 10,
+        tcp_connection_backlog = 100,
 
         //  Maximum transport data unit size for PGM (TPDU).
         pgm_max_tpdu = 1500

src/signaler.cpp

@@ -75,6 +75,15 @@ zmq::signaler_t::signaler_t ()
     w = WSASocket (AF_INET, SOCK_STREAM, 0, NULL, 0,  0);
     wsa_assert (w != INVALID_SOCKET);
 
+    //  Increase signaler SNDBUF if requested in config.hpp.
+    if (signaler_sndbuf_size) {
+        int sndbuf = signaler_sndbuf_size;
+        socklen_t sndbuf_size = sizeof sndbuf;
+        rc = setsockopt (w, SOL_SOCKET, SO_SNDBUF, (const char *)&sndbuf,
+            sndbuf_size);
+        errno_assert (rc == 0);
+    }
+
     //  Connect to the remote peer.
     rc = connect (w, (sockaddr *) &addr, sizeof (addr));
     wsa_assert (rc != SOCKET_ERROR);
@@ -170,6 +179,14 @@ zmq::signaler_t::signaler_t ()
         flags = 0;
     rc = fcntl (r, F_SETFL, flags | O_NONBLOCK);
     errno_assert (rc != -1);
+
+    //  Increase signaler SNDBUF if requested in config.hpp.
+    if (signaler_sndbuf_size) {
+        int sndbuf = signaler_sndbuf_size;
+        socklen_t sndbuf_size = sizeof sndbuf;
+        rc = setsockopt (w, SOL_SOCKET, SO_SNDBUF, &sndbuf, sndbuf_size);
+        errno_assert (rc == 0);
+    }
 }
 
 zmq::signaler_t::~signaler_t ()
@@ -248,6 +265,14 @@ zmq::signaler_t::signaler_t ()
     errno_assert (rc == 0);
     w = sv [0];
     r = sv [1];
+
+    //  Increase signaler SNDBUF if requested in config.hpp.
+    if (signaler_sndbuf_size) {
+        int sndbuf = signaler_sndbuf_size;
+        socklen_t sndbuf_size = sizeof sndbuf;
+        rc = setsockopt (w, SOL_SOCKET, SO_SNDBUF, &sndbuf, sndbuf_size);
+        errno_assert (rc == 0);
+    }
 }
 
 zmq::signaler_t::~signaler_t ()

src/zmq_connecter.cpp

@@ -19,6 +19,13 @@
 
 #include <new>
 
+#if defined ZMQ_HAVE_WINDOWS
+#include "windows.hpp"
+#else
+#include <sys/types.h>
+#include <unistd.h>
+#endif
+
 #include "zmq_connecter.hpp"
 #include "zmq_engine.hpp"
 #include "zmq_init.hpp"
@@ -31,7 +38,7 @@ zmq::zmq_connecter_t::zmq_connecter_t (class io_thread_t *io_thread_,
     own_t (io_thread_),
     io_object_t (io_thread_),
     handle_valid (false),
-    wait (false),
+    wait (wait_before_connect),
     session (session_),
     options (options_)
 {
@@ -47,10 +54,20 @@ zmq::zmq_connecter_t::~zmq_connecter_t ()
         rm_fd (handle);
 }
 
+int zmq::zmq_connecter_t::get_reconnect_period ()
+{
+#if defined ZMQ_HAVE_WINDOWS
+    return (reconnect_period + (((int)GetCurrentProcessId () * 13)
+        % reconnect_period));
+#else
+    return (reconnect_period + (((int)getpid () * 13) % reconnect_period));
+#endif
+}
+
 void zmq::zmq_connecter_t::process_plug ()
 {
     if (wait)
-        add_timer (reconnect_period, reconnect_timer_id);
+        add_timer (get_reconnect_period (), reconnect_timer_id);
     else
         start_connecting ();
 }
@@ -73,7 +90,7 @@ void zmq::zmq_connecter_t::out_event ()
     if (fd == retired_fd) {
         tcp_connecter.close ();
         wait = true;
-        add_timer (reconnect_period, reconnect_timer_id);
+        add_timer (get_reconnect_period (), reconnect_timer_id);
         return;
     }
 
@@ -122,5 +139,5 @@ void zmq::zmq_connecter_t::start_connecting ()
 
     //  Handle any other error condition by eventual reconnect.
     wait = true;
-    add_timer (reconnect_period, reconnect_timer_id);
+    add_timer (get_reconnect_period (), reconnect_timer_id);
 }

src/zmq_connecter.hpp

@@ -56,6 +56,9 @@ namespace zmq
         //  Internal function to start the actual connection establishment.
         void start_connecting ();
 
+        //  Internal function to return the reconnect backoff delay.
+        int get_reconnect_period ();
+
         //  Actual connecting socket.
         tcp_connecter_t tcp_connecter;