add sysctl vm.max_map_count for mmap.

@@ -554,6 +554,7 @@ int SrsServer::initialize_st()
     
     // @remark, st alloc segment use mmap, which only support 32757 threads,
     // if need to support more, for instance, 100k threads, define the macro MALLOC_STACK.
+    // TODO: FIXME: maybe can use "sysctl vm.max_map_count" to refine.
     if (_srs_config->get_max_connections() > 32756) {
         ret = ERROR_ST_EXCEED_THREADS;
         srs_error("st mmap for stack allocation must <= %d threads, "