Concurrent Java 2 - Unstructured lock vs Structured lock

Unstructured Lock

Java提供另一種比較彈性的lock機制: Lock interface。

Lock object跟每個object的intrinsic lock沒什不同，也是有wait/notify，但有個tryLock的method，會return一個boolean，這讓嘗試要acquire lock的thread不會被block住，可以去做其他的事，之後再來try。

但是麻煩的就是獲得lock的人一定要想辦法在對的地方呼叫unlock，即便是exception發生 (永遠在finally clause做unlock)，由於不會block structure自動acquire/release lock，所以稱為unstructured lock。

Lock l = ...;
     l.lock();
     try {
         // access the resource protected by this lock
     } finally {
         l.unlock();
     }

ReentrantLock

這是一個implement Lock interface的class。

ReadWriteLock

這比較特別，他能提供concurrent read lock，也就是多個thread 能夠同時acquire這個read lock的時候，可以concurrent執行被這個read lock "保護"的code，但是只有一個thread能acquire write lock，而且一旦write lock被acquire，則acuire read lock的threads全部被block，直到write lock被unlock。


Performance Comparison: Structured vs Unstructured
我們來比較在四核心上，structured vs unstructured對linked list的 read/write performance比較:

CoarseList是使用ReentrantLock ，RWCoarseList是使用ReentrantReadWriteLock ，而SyncList是使用intrinsic lock，以SyncList當baseline











(1) CoarseList vs. SyncList (Large Random)

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1.0839688041594453x improvement in add throughput

0.9205057152753724x improvement in contains throughput

0.8414179104477613x improvement in remove throughput

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可以看到overall ReentrantLock的overhead比intrinsic lock大一些。

(2) RWCoarseList vs. SyncList (Large Random)

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0.7537446750034354x improvement in add throughput

3.2617096018735365x improvement in contains throughput

0.9670546105640107x improvement in remove throughput

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ReentrantReadWriteLock 可以讓read operation的次數多很多，達到3倍以上。

(3) RWCoarseList vs. SyncList (Small Random)

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1.1236133122028527x improvement in add throughput

3.2494758909853254x improvement in contains throughput

0.9303955933900853x improvement in remove throughput

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(4) CoarseList vs. SyncList (Large Repeating)

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0.7279840555104452x improvement in add throughput

0.7392840160315196x improvement in contains throughput

1.5410764872521248x improvement in remove throughput

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(5) RWCoarseList vs. SyncList (Large Repeating)

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0.7709759000297531x improvement in add throughput

3.6224951519069166x improvement in contains throughput

0.8615888615888616x improvement in remove throughput

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(6) RWCoarseList vs. SyncList (Small Repeating)

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0.6785995899700362x improvement in add throughput

3.8316566063044935x improvement in contains throughput

1.2968299711815563x improvement in remove throughput

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