LWM is computed rather than explicitly specified by user

doc/zmq_setsockopt.txt

@@ -40,21 +40,6 @@ Default value:: 0
 Applicable socket types:: all
 
 
-ZMQ_LWM: Set low water mark
-~~~~~~~~~~~~~~~~~~~~~~~~~~~
-The 'ZMQ_LWM' option shall set the low water mark for the _message queue_
-associated with the socket.  This option only makes sense when used in
-conjunction with the 'ZMQ_HWM' option. A socket which has reached it's high
-water mark remains in the "emergency" state until the number of outstanding
-messages in it's associated message queue falls below the low water mark, at
-which point normal message processing is resumed.
-
-Option value type:: int64_t
-Option value unit:: messages
-Default value:: 0
-Applicable socket types:: all
-
-
 ZMQ_SWAP: Set disk offload size
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 The 'ZMQ_SWAP' option shall set the disk offload (swap) size for the _message

include/zmq.h

@@ -169,6 +169,7 @@ ZMQ_EXPORT int zmq_term (void *context);
 
 /*  Socket options.                                                           */
 #define ZMQ_HWM 1
+/*  TODO: LWM is obsolete and should be removed in next version.              */
 #define ZMQ_LWM 2
 #define ZMQ_SWAP 3
 #define ZMQ_AFFINITY 4

src/config.hpp

@@ -60,6 +60,9 @@ namespace zmq
         //  unnecessary network stack traversals.
         out_batch_size = 8192,
 
+        //  Maximal delta between high and low watermark.
+        max_wm_delta = 1024,
+
         //  Maximum number of events the I/O thread can process in one go.
         max_io_events = 256,
 

src/options.cpp

@@ -26,7 +26,6 @@
 
 zmq::options_t::options_t () :
     hwm (0),
-    lwm (0),
     swap (0),
     affinity (0),
     rate (100),
@@ -53,14 +52,6 @@ int zmq::options_t::setsockopt (int option_, const void *optval_,
         hwm = *((uint64_t*) optval_);
         return 0;
 
-    case ZMQ_LWM:
-        if (optvallen_ != sizeof (uint64_t)) {
-            errno = EINVAL;
-            return -1;
-        }
-        lwm = *((uint64_t*) optval_);
-        return 0;
-
     case ZMQ_SWAP:
         if (optvallen_ != sizeof (int64_t)) {
             errno = EINVAL;
@@ -155,15 +146,6 @@ int zmq::options_t::getsockopt (int option_, void *optval_, size_t *optvallen_)
         *optvallen_ = sizeof (uint64_t);
         return 0;
 
-    case ZMQ_LWM:
-        if (*optvallen_ < sizeof (uint64_t)) {
-            errno = EINVAL;
-            return -1;
-        }
-        *((uint64_t*) optval_) = lwm;
-        *optvallen_ = sizeof (uint64_t);
-        return 0;
-
     case ZMQ_SWAP:
         if (*optvallen_ < sizeof (int64_t)) {
             errno = EINVAL;

src/options.hpp

@@ -35,7 +35,6 @@ namespace zmq
         int getsockopt (int option_, void *optval_, size_t *optvallen_);
 
         uint64_t hwm;
-        uint64_t lwm;
         int64_t swap;
         uint64_t affinity;
         blob_t identity;

src/pipe.cpp

@@ -233,9 +233,9 @@ bool zmq::writer_t::pipe_full ()
 }
 
 zmq::pipe_t::pipe_t (object_t *reader_parent_, object_t *writer_parent_,
-      uint64_t hwm_, uint64_t lwm_) :
-    reader (reader_parent_, hwm_, lwm_),
-    writer (writer_parent_, hwm_, lwm_)
+      uint64_t hwm_) :
+    reader (reader_parent_, hwm_, compute_lwm (hwm_)),
+    writer (writer_parent_, hwm_, compute_lwm (hwm_))
 {
     reader.set_pipe (this);
     writer.set_pipe (this);
@@ -250,3 +250,32 @@ zmq::pipe_t::~pipe_t ()
     while (read (&msg))
        zmq_msg_close (&msg);
 }
+
+uint64_t zmq::pipe_t::compute_lwm (uint64_t hwm_)
+{
+   //  Following point should be taken into consideration when computing
+   //  low watermark:
+   //
+   //  1. LWM has to be less than HWM.
+   //  2. LWM cannot be set to very low value (such as zero) as after filling
+   //     the queue it would start to refill only after all the messages are
+   //     read from it and thus unnecessarily hold the progress back.
+   //  3. LWM cannot be set to very high value (such as HWM-1) as it would
+   //     result in lock-step filling of the queue - if a single message is read
+   //     from a full queue, writer thread is resumed to write exactly one
+   //     message to the queue and go back to sleep immediately. This would
+   //     result in low performance.
+   //
+   //  Given the 3. it would be good to keep HWM and LWM as far apart as
+   //  possible to reduce the thread switching overhead to almost zero,
+   //  say HWM-LWM should be 500 (max_wm_delta).
+   //
+   //  That done, we still we have to account for the cases where HWM<500 thus
+   //  driving LWM to negative numbers. Let's make LWM 1/2 of HWM in such cases.
+
+    if (hwm_ > max_wm_delta * 2)
+        return hwm_ - max_wm_delta;
+    else
+        return hwm_ / 2;
+}
+

src/pipe.hpp

@@ -150,7 +150,7 @@ namespace zmq
     public:
 
         pipe_t (object_t *reader_parent_, object_t *writer_parent_,
-            uint64_t hwm_, uint64_t lwm_);
+            uint64_t hwm_);
         ~pipe_t ();
 
         reader_t reader;
@@ -158,6 +158,8 @@ namespace zmq
 
     private:
 
+        uint64_t compute_lwm (uint64_t hwm_);
+
         pipe_t (const pipe_t&);
         void operator = (const pipe_t&);
     };

src/session.cpp

@@ -265,8 +265,7 @@ void zmq::session_t::process_attach (i_engine *engine_,
     writer_t *socket_writer = NULL;
 
     if (options.requires_in && !out_pipe) {
-        pipe_t *pipe = new (std::nothrow) pipe_t (owner, this,
-            options.hwm, options.lwm);
+        pipe_t *pipe = new (std::nothrow) pipe_t (owner, this, options.hwm);
         zmq_assert (pipe);
         out_pipe = &pipe->writer;
         out_pipe->set_endpoint (this);
@@ -274,8 +273,7 @@ void zmq::session_t::process_attach (i_engine *engine_,
     }
 
     if (options.requires_out && !in_pipe) {
-        pipe_t *pipe = new (std::nothrow) pipe_t (this, owner,
-            options.hwm, options.lwm);
+        pipe_t *pipe = new (std::nothrow) pipe_t (this, owner, options.hwm);
         zmq_assert (pipe);
         in_pipe = &pipe->reader;
         in_pipe->set_endpoint (this);

src/socket_base.cpp

@@ -194,15 +194,13 @@ int zmq::socket_base_t::connect (const char *addr_)
 
         //  Create inbound pipe, if required.
         if (options.requires_in) {
-            in_pipe = new (std::nothrow) pipe_t (this, peer,
-                options.hwm, options.lwm);
+            in_pipe = new (std::nothrow) pipe_t (this, peer, options.hwm);
             zmq_assert (in_pipe);
         }
 
         //  Create outbound pipe, if required.
         if (options.requires_out) {
-            out_pipe = new (std::nothrow) pipe_t (peer, this,
-                options.hwm, options.lwm);
+            out_pipe = new (std::nothrow) pipe_t (peer, this, options.hwm);
             zmq_assert (out_pipe);
         }
 
@@ -235,16 +233,14 @@ int zmq::socket_base_t::connect (const char *addr_)
 
         //  Create inbound pipe, if required.
         if (options.requires_in) {
-            in_pipe = new (std::nothrow) pipe_t (this, session,
-                options.hwm, options.lwm);
+            in_pipe = new (std::nothrow) pipe_t (this, session, options.hwm);
             zmq_assert (in_pipe);
 
         }
 
         //  Create outbound pipe, if required.
         if (options.requires_out) {
-            out_pipe = new (std::nothrow) pipe_t (session, this,
-                options.hwm, options.lwm);
+            out_pipe = new (std::nothrow) pipe_t (session, this, options.hwm);
             zmq_assert (out_pipe);
         }