Re: Followup to "Serious concurrency problems on fast systems"

Robert Klemme <>
Mon, 05 Jul 2010 23:36:47 +0200
On 04.07.2010 18:47, Kevin McMurtrie wrote:

In article<>,
  Robert Klemme<> wrote:

On 04.07.2010 11:28, Christian wrote:

Am 02.07.2010 16:40, schrieb Robert Klemme:

the recent thread "Serious concurrency problems on fast systems"
inspired me to put together a small demo that shows how different ways
of concurrency control affect execution. The example shares a dummy
configuration with a single long value. Multiple threads access the
shared resource read only and depending on test scenario a single thread
updates it from time to time. Concurrency control is done in these ways:

1. Plain synchronized on a single shared resource.

2. Synchronized but with redundant storage and update via observer

3. Copy on write with an immutable object and an AtomicReference.

You can download it here

You could also try the by Java provided (Reentrant)ReadWriteLock ...
it might (I am not sure there) be cheaper with lots of reads and few
writes than normal synchronization.

That's an excellent idea:

However, while it is faster than plain old synchronized on the global
resource, this confirms that centralized locking is an inferior approach
in highly concurrent systems. :-)

ALL thread interaction on a multi-CPU system is slow. Try 10 threads
writing to adjacent memory in a shared array compared to 10 threads
writing to memory in their own array. Not only is that a baseline for
all concurrency techniques, but it can hinder performance in non-obvious

In my benchmarks, a 'synchronized' block has less overhead than all of
the Java 1.5 lock mechanisms. If you must alter a set of data in a
consistent state, it seems to be the way to go.

The demo code shows that although you can get pretty fast with
synchronized there are faster alternatives. Especially the variant with
an AtomicReference (or plain volatile) is dramatically faster. It
depends on the situation what to choose. synchronize is not the one
size fits all solution to concurrency issues.

 Its downside is that a
thread can use up its CPU quanta while holding the lock and everything
else piles up for a while. Java 1.5 locks give you options for shared
read locks and fail-fast lock acquisition that may reduce blocking.




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