记低版本okhttp超时会导致死锁

一、问题起源

在处理一次生产环境cpu拉满问题时,把日志拉下来看发现很多http请求调用出错,项目使用的是okhttp 3.8.1版本。

二、问题描述

问题出在okhttp3.Dispatcher.finished(Dispatcher.java:201)代码如下:

void finished(AsyncCall call) {
  finished(runningAsyncCalls, call, true);
}
void finished(RealCall call) {
  finished(runningSyncCalls, call, false);
}
private  void finished(Deque calls, T call, boolean promoteCalls) {
  int runningCallsCount;
  Runnable idleCallback;
  synchronized (this) { //201行
    if (!calls.remove(call)) throw new AssertionError("Call wasn't in-flight!");
    if (promoteCalls) promoteCalls();
    runningCallsCount = runningCallsCount();
    idleCallback = this.idleCallback;
  }

  if (runningCallsCount == 0 && idleCallback != null) {
    idleCallback.run();
  }
}

private void promoteCalls() {
  if (runningAsyncCalls.size() >= maxRequests) return; // Already running max capacity.
  if (readyAsyncCalls.isEmpty()) return; // No ready calls to promote.

  for (Iterator i = readyAsyncCalls.iterator(); i.hasNext(); ) {
    AsyncCall call = i.next();

    if (runningCallsForHost(call) < maxRequestsPerHost) {
      i.remove();
      runningAsyncCalls.add(call);
      executorService().execute(call);
    }

    if (runningAsyncCalls.size() >= maxRequests) return; // Reached max capacity.
  }
}

三、分析代码

在OkHttpClient中final Dispatcher dispatcher; 作为成员对象,而我们代码中OkHttpClient作为连接池是单例的,这里是对dispatcher做synchronized。

追踪代码发现,在finished的调用方法中,我们方法中使用的是异步AsyncCall,而这里synchronized方法中的promoteCalls被置为true。所以会调用promoteCalls()方法, 而promoteCalls()方法中会继续调用executorService().execute(call);,就是这里,问题大了,synchronized中执行http请求,那上面代码中的超时不就长时间占用锁了?怪不得进程blocked了。

关于线程的BLOCKED,需要知道:

  • java.lang.Thread.State: BLOCKED:等待监视器锁而被阻塞的线程的线程状态,当进入 synchronized 块/方法或者在调用 wait()被唤醒/超时之后重新进入 synchronized 块/方法, 但是锁被其它线程占有,这个时候被操作系统挂起,状态为阻塞状态。若是有线程长时间处于 BLOCKED 状态,要考虑是否发生了死锁(deadlock)的情况。
  • blocked的线程不会消耗cpu,但频繁的频繁切换线程上下文会导致cpu过高。线程被频繁唤醒,而又由于抢占锁失败频繁地被挂起. 因此也会带来大量的上下文切换, 消耗系统的cpu资源。

四、解决方案

okttp关于这个问题已经有过解答:

Dispatcher no longer has quadratic behaviour by iamdanfox · Pull Request #4581 · square/okhttp · GitHub

[improvement] okhttp 3.12.0 -> 3.13.1, to pick up perf improvements to okhttp3.Dispatcher by iamdanfox · Pull Request #940 · palantir/conjure-java-runtime · GitHub

解决方案就简单多了:升级okhttp到3.14.9,虽然目前最新稳定版本为4.9.3,但是OkHttp 4发布,从Java切换到Kotlin。谨慎一点,还是小版本升级吧。

在3.14.9中,这部分代码被优化为:

private boolean promoteAndExecute() {
  assert (!Thread.holdsLock(this));

  List executableCalls = new ArrayList<>();
  boolean isRunning;
  synchronized (this) {
    for (Iterator i = readyAsyncCalls.iterator(); i.hasNext(); ) {
      AsyncCall asyncCall = i.next();

      if (runningAsyncCalls.size() >= maxRequests) break; // Max capacity.
      if (asyncCall.callsPerHost().get() >= maxRequestsPerHost) continue; // Host max capacity.

      i.remove();
      asyncCall.callsPerHost().incrementAndGet();
      executableCalls.add(asyncCall);
      runningAsyncCalls.add(asyncCall);
    }
    isRunning = runningCallsCount() > 0;
  }

  for (int i = 0, size = executableCalls.size(); i < size; i++) {
    AsyncCall asyncCall = executableCalls.get(i);
    asyncCall.executeOn(executorService());
  }

  return isRunning;
}

执行HTTP请求被移出了synchronized方法了。

你可能感兴趣的:(okhttp)