Re: multithreaded cache?

Robert Klemme <>
Sat, 19 May 2012 12:33:23 +0200
On 19.05.2012 00:31, Eric Sosman wrote:

On 5/18/2012 5:45 PM, Robert Klemme wrote:

On 18.05.2012 20:42, Silvio Bierman wrote:

On 05/17/2012 11:54 AM, Robert Klemme wrote:

I provide a variant of Silvio's, Eric's and Daniel's solution which
should yield higher throughput because it works without read write
locking. You can find it as gist in case the code is garbled in the
newsgroup posting:

I think you have as many locks as I suggested (being one)? My initial
implementations of something like this used a plain map with an extra
lock but later cases used the by then available ConcurrentHashMap as
well, making one lock redundant.

You didn't show it here, did you? I can's seem to find it in the thread.
Note that CHM does also do synchronization. I am not sure from your
statement what exact locking scheme you apply. There does seem to be one
difference though: I my version the second lock goes away after the
value has been computed so there is only the sync of CHM left.

It seems to me that if N threads query the same key at about
the same time, they may all miss the map and go off to perform
the slow computation. If "slow" is large compared to the cost of
a lock-release pair (and if it weren't, why cache?), the tradeoff
seems suspect.

Also, different threads may wind up using different value
instances. If the cache is purely a convenience for a value-only
object that may be all right, but it's not all right if the values
are supposed to be singletons.

Finally, there's more than a whiff of the double-checked locking
antipattern about what you're doing with the `here' flag. I'm not
absolutely sure what you've got is in fact DCL (hence, wrong), but
I'm also not absolutely sure it's DCL-free. Before using the pattern
in any important way, I'd want to check with a major-league guru,
just as "due diligence."

There is no double checked locking going on - neither in LazyCache
itself nor in the retriever instance.

Daniel has explained all the details already. I'd just add that
creation of multiple retriever objects is the price you pay for
increased concurrency due to CHM. But only one of them is ever going to
be used per key. (Documenting this is one of the tasks of the assert
Daniel questioned earlier.)

I think rw-locking is an inferior technique compared to CHM in this case
because in case of cache miss you cannot escalate a r-lock to a w-lock
(to avoid deadlocks) and hence need to release the r-lock, acquire the
w-lock, *and check again*. In other words you have two more
synchronization points with memory barriers, scheduling effects etc.

In case of cache hit you might still suffer throughput because of thread
starvation caused by all lookups for values missing from the cache might
be blocked for indefinite time when using an unfair rw-lock. Using a
fair rw-lock on the other hand is more expensive and still has the
downside of a global lock which affects the complete range of keys.

CHM improves this by partitioning value space and only lock individual
partitions for atomic operations. These partitions are completely
independent from a locking perspective. That's the main advantage of CHM.

Kind regards


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