Align inter-thread shared structers on cache-line granularity

Specifically ypipe_t specializations for msg_t and command_t.

Currently only supports platforms where posix_memalign is available.

configure.ac

@@ -481,7 +481,7 @@ AM_CONDITIONAL(ON_LINUX, test "x$libzmq_on_linux" = "xyes")
 
 # Checks for library functions.
 AC_TYPE_SIGNAL
-AC_CHECK_FUNCS(perror gettimeofday clock_gettime memset socket getifaddrs freeifaddrs fork)
+AC_CHECK_FUNCS(perror gettimeofday clock_gettime memset socket getifaddrs freeifaddrs fork posix_memalign)
 AC_CHECK_HEADERS([alloca.h])
 
 LIBZMQ_CHECK_SOCK_CLOEXEC([

src/command.hpp

@@ -146,6 +146,10 @@ namespace zmq
             } done;
 
         } args;
+
+        enum { pad_size = 64 - (sizeof(destination) + sizeof(args)) };
+        unsigned char unused[ pad_size ];
+
     };
 
 }

src/yqueue.hpp

@@ -41,8 +41,16 @@ namespace zmq
     //  T is the type of the object in the queue.
     //  N is granularity of the queue (how many pushes have to be done till
     //  actual memory allocation is required).
-
+#ifdef HAVE_POSIX_MEMALIGN
+    // ALIGN is the memory alignment size to use in the case where we have
+    // posix_memalign available. Default value is 64, this alignment will
+    // prevent two queue chunks from occupying the same CPU cache line on
+    // architectures where cache lines are <= 64 bytes (e.g. most things
+    // except POWER).
+    template <typename T, int N, size_t ALIGN = 64> class yqueue_t
+#else
     template <typename T, int N> class yqueue_t
+#endif
     {
     public:
 
@@ -103,7 +111,13 @@ namespace zmq
                 end_chunk->next = sc;
                 sc->prev = end_chunk;
             } else {
+#ifdef HAVE_POSIX_MEMALIGN
+                void *pv;
+                if (posix_memalign(&pv, ALIGN, sizeof (chunk_t)) == 0)
+                    end_chunk->next = (chunk_t*) pv;
+#else
                 end_chunk->next = (chunk_t*) malloc (sizeof (chunk_t));
+#endif
                 alloc_assert (end_chunk->next);
                 end_chunk->next->prev = end_chunk;
             }