7061212: use o/s low memory notification in embedded builds

Reviewed-by: dholmes, never, jwilhelm, kvn

hotspot/src/os/linux/vm/os_linux.cpp

@@ -169,7 +169,35 @@ sigset_t SR_sigset;
 /* Used to protect dlsym() calls */
 static pthread_mutex_t dl_mutex;
 
-////////////////////////////////////////////////////////////////////////////////
+#ifdef JAVASE_EMBEDDED
+class MemNotifyThread: public Thread {
+  friend class VMStructs;
+ public:
+  virtual void run();
+
+ private:
+  static MemNotifyThread* _memnotify_thread;
+  int _fd;
+
+ public:
+
+  // Constructor
+  MemNotifyThread(int fd);
+
+  // Tester
+  bool is_memnotify_thread() const { return true; }
+
+  // Printing
+  char* name() const { return (char*)"Linux MemNotify Thread"; }
+
+  // Returns the single instance of the MemNotifyThread
+  static MemNotifyThread* memnotify_thread() { return _memnotify_thread; }
+
+  // Create and start the single instance of MemNotifyThread
+  static void start();
+};
+#endif // JAVASE_EMBEDDED
+
 // utility functions
 
 static int SR_initialize();
@@ -4245,7 +4273,16 @@ jint os::init_2(void)
 }
 
 // this is called at the end of vm_initialization
-void os::init_3(void) { }
+void os::init_3(void)
+{
+#ifdef JAVASE_EMBEDDED
+  // Start the MemNotifyThread
+  if (LowMemoryProtection) {
+    MemNotifyThread::start();
+  }
+  return;
+#endif
+}
 
 // Mark the polling page as unreadable
 void os::make_polling_page_unreadable(void) {
@@ -5368,3 +5405,78 @@ bool os::is_headless_jre() {
     return true;
 }
 
+
+#ifdef JAVASE_EMBEDDED
+//
+// A thread to watch the '/dev/mem_notify' device, which will tell us when the OS is running low on memory.
+//
+MemNotifyThread* MemNotifyThread::_memnotify_thread = NULL;
+
+// ctor
+//
+MemNotifyThread::MemNotifyThread(int fd): Thread() {
+  assert(memnotify_thread() == NULL, "we can only allocate one MemNotifyThread");
+  _fd = fd;
+
+  if (os::create_thread(this, os::os_thread)) {
+    _memnotify_thread = this;
+    os::set_priority(this, NearMaxPriority);
+    os::start_thread(this);
+  }
+}
+
+// Where all the work gets done
+//
+void MemNotifyThread::run() {
+  assert(this == memnotify_thread(), "expected the singleton MemNotifyThread");
+
+  // Set up the select arguments
+  fd_set rfds;
+  if (_fd != -1) {
+    FD_ZERO(&rfds);
+    FD_SET(_fd, &rfds);
+  }
+
+  // Now wait for the mem_notify device to wake up
+  while (1) {
+    // Wait for the mem_notify device to signal us..
+    int rc = select(_fd+1, _fd != -1 ? &rfds : NULL, NULL, NULL, NULL);
+    if (rc == -1) {
+      perror("select!\n");
+      break;
+    } else if (rc) {
+      //ssize_t free_before = os::available_memory();
+      //tty->print ("Notified: Free: %dK \n",os::available_memory()/1024);
+
+      // The kernel is telling us there is not much memory left...
+      // try to do something about that
+
+      // If we are not already in a GC, try one.
+      if (!Universe::heap()->is_gc_active()) {
+        Universe::heap()->collect(GCCause::_allocation_failure);
+
+        //ssize_t free_after = os::available_memory();
+        //tty->print ("Post-Notify: Free: %dK\n",free_after/1024);
+        //tty->print ("GC freed: %dK\n", (free_after - free_before)/1024);
+      }
+      // We might want to do something like the following if we find the GC's are not helping...
+      // Universe::heap()->size_policy()->set_gc_time_limit_exceeded(true);
+    }
+  }
+}
+
+//
+// See if the /dev/mem_notify device exists, and if so, start a thread to monitor it.
+//
+void MemNotifyThread::start() {
+  int    fd;
+  fd = open ("/dev/mem_notify", O_RDONLY, 0);
+  if (fd < 0) {
+      return;
+  }
+
+  if (memnotify_thread() == NULL) {
+    new MemNotifyThread(fd);
+  }
+}
+#endif // JAVASE_EMBEDDED