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+---
+title: tick thread在Multicore OCaml中的作用
+date: 2023-08-15 13:29:21
+tags: [Technique]
+---
+
+Multicore OCaml的程序在启动时会运行一个 tick thread，其实现如下:
+
+```c
+/* The tick thread: posts a SIGPREEMPTION signal periodically */
+
+static void * caml_thread_tick(void * arg)
+{
+  struct timeval timeout;
+  sigset_t mask;
+
+  /* Block all signals so that we don't try to execute an OCaml signal handler*/
+  sigfillset(&mask);
+  pthread_sigmask(SIG_BLOCK, &mask, NULL);
+  while(! caml_tick_thread_stop) {
+    /* select() seems to be the most efficient way to suspend the
+       thread for sub-second intervals */
+    timeout.tv_sec = 0;
+    timeout.tv_usec = Thread_timeout * 1000;
+    select(0, NULL, NULL, NULL, &timeout);
+    /* The preemption signal should never cause a callback, so don't
+     go through caml_handle_signal(), just record signal delivery via
+     caml_record_signal(). */
+    caml_record_signal(SIGPREEMPTION);
+  }
+  return NULL;
+}
+```
+
+这是因为Multicore OCaml的GC目前需要一个进程（或一个Domain）中的所有线程一起参与以避免并发访问。如果一个线程在system call上被阻塞，那么整个Domain就会被卡住，直到该线程可以参与当前的垃圾收集。为了避免这个问题，tick 线程可以代替被阻塞的线程执行垃圾收集操作。
+\ No newline at end of file