Re: multithreaded cache?

Robert Klemme <>
Fri, 25 May 2012 17:36:08 +0200
On 25.05.2012 08:22, Kevin McMurtrie wrote:

Ha, this is an interview question that I use.

What you need is row level locking for the cache load.

But not all the time. See

Step 1)
Use synchronized operations to map your key to a value; creating an
uninitialized value in the map if needed. Use whatever tech you want.
A synchronized block on a HashMap is simplest and performs the fastest
on 1 or 2 core systems. A ConcurrentHashMap sometimes performs better
with 4+ core systems.

In my experience a CHM performs better even on a 1 or 2 core system.

Step 2)
Synchronize on the value. Initialize it if needed.

Step 1 blocks all cache access for only for a very short moment to make
sure that a key always has a value. Step 2 blocks access independently
for each cache value to make sure that it is loaded. It will perform
well for continuous use by several CPU cores. Google has some high
concurrency Maps that aren't too bad either.

Actually once the value is in the cache you do not need any step 2
synchronization any more.

In the 16 core range you'll find that any kind of exclusive lock causes
stalls where threads suspend while holding locks, causing a backlog that
reinforces itself. A concurrency expert can fix that using complex
Compare-And-Swap designs.

Basically this is what CHM does with putIfAbsent() internally.

Kind regards


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