multithreading - 线程停在AbstractFuture上的等待条件下

Question

因此我们正在使用Guava v18。
我们将对此内容进行大量讨论:

"catalina-exec-ajp2" daemon prio=10 tid=0x00007fb45c001800 nid=0x18aa waiting on condition [0x00007fb435151000]
   java.lang.Thread.State: WAITING (parking)
    at sun.misc.Unsafe.park(Native Method)
    - parking to wait for  <0x000000075ff0cfe8> (a com.google.common.util.concurrent.AbstractFuture$Sync)
    at java.util.concurrent.locks.LockSupport.park(LockSupport.java:186)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:834)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:994)
    at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1303)
    at com.google.common.util.concurrent.AbstractFuture$Sync.get(AbstractFuture.java:285)
    at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:116)
    at com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly(Uninterruptibles.java:137)
    at com.google.common.cache.LocalCache$LoadingValueReference.waitForValue(LocalCache.java:3557)
    at com.google.common.cache.LocalCache$Segment.waitForLoadingValue(LocalCache.java:2302)
    at com.google.common.cache.LocalCache$Segment.get(LocalCache.java:2191)
    at com.google.common.cache.LocalCache.get(LocalCache.java:3937)
    at com.google.common.cache.LocalCache.getOrLoad(LocalCache.java:3941)
    at com.google.common.cache.LocalCache$LocalLoadingCache.get(LocalCache.java:4824)
    at com.google.common.cache.LocalCache$LocalLoadingCache.getUnchecked(LocalCache.java:4830)
    at com.xxx.yyy.repository.cache.acl.guava.GuavaCacheWrapper.get(GuavaCacheWrapper.java:21)
    at com.xxx.yyy.repository.cache.CachedCpaRepository.findAll(CachedCpaRepository.java:32)

我们的 Guava 配置为:

  @SuppressWarnings("unchecked")
  @Override
  protected <K, R> CacheWrapper<K, R> newCacheConfiguration(Fetcher fetcher, String cacheName, int maxElementsInMemory,
      int timeToIdleSeconds, int timeToLiveSeconds)
  {
    Cache existingCache = manager.getCache(cacheName);
    CacheWrapper<K, R> cache;
    if(null == existingCache) 
    {
      cache = new GuavaCacheWrapper<>(CacheBuilder.newBuilder()
        .maximumSize(maxElementsInMemory)
        .expireAfterAccess(timeToIdleSeconds, SECONDS)
        .refreshAfterWrite(timeToLiveSeconds, SECONDS)
        .recordStats()
        .build( CacheLoader.asyncReloading(fetcher.getAs(CacheLoader.class), executors)));
      manager.addCache(cacheName, cache);
    }
    else 
    {
      cache = (CacheWrapper<K, R>) ((GuavaCache) existingCache).getNativaGuavaCache();
    }
    return cache;
  }

我对可以检查出导致问题的原因一无所知。您能提出我可以检查的内容吗？

Answer 1

如果您在LoadingCache上遇到阻塞，则意味着您正在从多个线程中加载同一键的值:第一个线程实际加载了该值，而下一个线程等待该加载完成。

也许您的 key 数量很少，并且处理该值是一项昂贵的操作？您可以记录您要加载的 key 以及加载的持续时间。

更新

实际加载的线程是调用CacheLoader的那些线程，但是您无法从线程转储中找到哪个线程与等待线程相对应(如果有多个加载的线程)。

我创建了一个测试来重现这种情况:

public class ExpensiveCache {
    private final LoadingCache<String, String> cache = CacheBuilder.newBuilder().build(new ExpensiveCacheLoader());

    public static void main(String... args) {
        new ExpensiveCache().run();
    }

    private void run() {
        // Start 2 threads concurrently accessing the same key in the cache
        for (int i = 0; i < 2; i++) {
            new Thread(() -> cache.getUnchecked("key")).start();
        }
    }

    private static class ExpensiveCacheLoader extends CacheLoader<String, String> {
        @Override
        public String load(String key) throws Exception {
            // Make the computation "expensive" by sleeping for 60s
            Thread.sleep(60_000);
            return key;
        }
    }
}

您确实在线程转储的其他任何地方都找不到条件的ID:

"Thread-2" #12 prio=5 os_prio=31 tid=0x00007fbb6384e000 nid=0x5703 waiting on condition [0x00000001291c5000]
   java.lang.Thread.State: WAITING (parking)
        at sun.misc.Unsafe.park(Native Method)
        - parking to wait for  <0x000000076b183208> (a com.google.common.util.concurrent.AbstractFuture$Sync)
        at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:997)
        at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1304)
        at com.google.common.util.concurrent.AbstractFuture$Sync.get(AbstractFuture.java:285)
        at com.google.common.util.concurrent.AbstractFuture.get(AbstractFuture.java:116)
        at com.google.common.util.concurrent.Uninterruptibles.getUninterruptibly(Uninterruptibles.java:137)
        at com.google.common.cache.LocalCache$LoadingValueReference.waitForValue(LocalCache.java:3557)
        at com.google.common.cache.LocalCache$Segment.waitForLoadingValue(LocalCache.java:2302)
        at com.google.common.cache.LocalCache$Segment.lockedGetOrLoad(LocalCache.java:2289)
        at com.google.common.cache.LocalCache$Segment.get(LocalCache.java:2197)
        at com.google.common.cache.LocalCache.get(LocalCache.java:3937)
        at com.google.common.cache.LocalCache.getOrLoad(LocalCache.java:3941)
        at com.google.common.cache.LocalCache$LocalLoadingCache.get(LocalCache.java:4824)
        at com.google.common.cache.LocalCache$LocalLoadingCache.getUnchecked(LocalCache.java:4830)
        at io.github.fpavageau.ExpensiveCache.lambda$run$0(ExpensiveCache.java:16)
        at io.github.fpavageau.ExpensiveCache$$Lambda$1/1108411398.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:745)

"Thread-1" #11 prio=5 os_prio=31 tid=0x00007fbb6384d000 nid=0x5503 waiting on condition [0x00000001290c2000]
   java.lang.Thread.State: TIMED_WAITING (sleeping)
        at java.lang.Thread.sleep(Native Method)
        at io.github.fpavageau.ExpensiveCache$ExpensiveCacheLoader.load(ExpensiveCache.java:23)
        at io.github.fpavageau.ExpensiveCache$ExpensiveCacheLoader.load(ExpensiveCache.java:20)
        at com.google.common.cache.LocalCache$LoadingValueReference.loadFuture(LocalCache.java:3527)
        at com.google.common.cache.LocalCache$Segment.loadSync(LocalCache.java:2319)
        at com.google.common.cache.LocalCache$Segment.lockedGetOrLoad(LocalCache.java:2282)
        - locked <0x000000076b199a50> (a com.google.common.cache.LocalCache$StrongEntry)
        at com.google.common.cache.LocalCache$Segment.get(LocalCache.java:2197)
        at com.google.common.cache.LocalCache.get(LocalCache.java:3937)
        at com.google.common.cache.LocalCache.getOrLoad(LocalCache.java:3941)
        at com.google.common.cache.LocalCache$LocalLoadingCache.get(LocalCache.java:4824)
        at com.google.common.cache.LocalCache$LocalLoadingCache.getUnchecked(LocalCache.java:4830)
        at io.github.fpavageau.ExpensiveCache.lambda$run$0(ExpensiveCache.java:16)
        at io.github.fpavageau.ExpensiveCache$$Lambda$1/1108411398.run(Unknown Source)
        at java.lang.Thread.run(Thread.java:745)