Java Random Slowdowns on Mac OS cont'd

Question

I asked this question a few weeks ago, but I'm still having the problem and I have some new hints. The original question is here:

Java Random Slowdowns on Mac OS

Basically, I have a java application that splits a job into independent pieces and runs them in separate threads. The threads have no synchronization or shared memory items. The only resources they do share are data files on the hard disk, with each thread having an open file channel.

Most of the time it runs very fast, but occasionally it will run very slow for no apparent reason. If I attach a CPU profiler to it, then it will start running quickly again. If I take a CPU snapshot, it says its spending most of its time in "self time" in a function that doesn't do anything except check a few (unshared unsynchronized) booleans. I don't know how this could be accurate because 1, it makes no sense, and 2, attaching the profiler seems to knock the threads out of whatever mode they're in and fix the problem. Also, regardless of whether it runs fast or slow, it always finishes and gives the same output, and it never dips in total cpu usage (in this case ~1500%), implying that the threads aren't getting blocked.

I have tried different garbage collectors, different sizings the parts of the memory space, writing data output to non-raid drives, and putting all data output in threads separate the main worker threads.

Does anyone have any idea what kind of problem this could be? Could it be the operating system (OS X 10.6.2) ? I have not been able to duplicate it on a windows machine, but I don't have one with a similar hardware configuration.

Answer 1

Actually this is an interesting problem, im curious to know whats the problem.

• First, in your previous question, you are saying you split the job between "multiple" processors. Are they physically multiple, like in multiple machines? or a multi core CPU?

• Second, im not sure if Snow Leopard has something to do with it, but we know that SL introduced few new features in term of multi-processor machines. So there might be some problem with the VM on the new OS. Try to use another Java version, i know SL uses Java 6 by default. Try to use Java 5.

• Third, did you try to make the Thread pool a little smaller, you are talking about 100 threads running at same time. Try to make them 20 or 40 for example. See if it makes difference.

• Finally, i would be interested in seeing how you implemented the multi-threading solution. Small parts of the code will be good

Answer 2

It's probably a bit late to reply, but I could observe similar slowdowns using Random in Threads, related to a volatile variable used within java.util.Random - see How can assigning a variable result in a serious performance drop while the execution order is (nearly) untouched? for details. If the answer I got is correct (and it sounds pretty reasonable to me), the slowdown might be related to the in-memory-addresses of the volatile variables used within Random (Have a look at the answer of user 'irreputable' to my question, which explains the problem much better than I do here).

In case you're creating the Random-instances within the run-method of your Threads, you could simply try to turn them into object-variables and initialize them within the constructor of your Thread: This would most likely ensure that the volatile fields of your Random instances will end up in 'different areas' in RAM, which do not have to get synchronized between the processor cores.

Answer 3

How do you know it's running slow? How do you know that it runs quicker when CPU profiler is active? If you do the entire run under the profiler does it ever run slow? If you restrict the number of threads to one does it ever run slow?