[concurrency-interest] Performance of ReentrantReadWriteLock

Brian S O'Neill bronee at gmail.com
Thu Nov 19 00:09:17 EST 2009

Lock implementations have a quick check step to see if there's no 
contention. Since you have only one thread in both tests, there is no 
contention at all. The "synchronized" monitor and the read write lock 
both need to do the same work for this quick check. The read write lock 
is not designed to be /necessarily/ faster at lock acquisition, but it 
is designed to allow multiple reads without the lock being released. For 
applications which hold the lock only briefly, use of "synchronized" 
will do fine.

Norman Elton wrote:
> I've done some very basic testing of a ReentrantReadWriteLock vs.
> synchronized. I fired up five threads, each of which is accessing a
> shared Map<Integer,Integer>. At first, I did 99% read operations, then
> 100% read operations. Initial testing, with no locking or
> synchronization, proved about 8M operations per second per thread.
> Super.
> Then I tested synchronizing the get() and put() methods. Performance
> dropped to about 700K operations per second. Synchronization obviously
> has a large overhead.
> Strange thing is, the ReentrantReadWriteLock performed just about as
> well. Even in a 100% read environment, where I would think threads
> would never have to block for one another.
> Am I missing something here? Shouldn't I be seeing significantly
> better numbers for the ReentrantReadWriteLock? Presumably, it's
> allowing multiple threads to hit the hash at the same time. I would
> expect numbers somewhere between the synchronizing hash and the
> completely untouched HashMap.
> Thoughts? Thanks!
> Norman
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