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POWhat mechanisms other than mutexs or garbage collection can slow my multi-threaded java program?
 

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  1. POWhat mechanisms other than mutexs or garbage collection can slow my multi-threaded java program?
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    <p><strong>Problem</strong></p> <p>I have a piece of java code (JDK 1.6.0._22 if relevant) that implements a stateless, side effect free function with no mutexes. It does however use a lot of memory (I don't know if that is relevant).</p> <p>In the past I have visited Sun Laboratories and gathered the standard "performance vs number of threads" curve. As this function has no mutexs, it has a nice graph although the garbage collection kicked in as the number of threads increased. After some garbage collection tuning I was able to make this curve almost flat.</p> <p>I am now doing the same experiment on Intel hardware. The hardware has 4 CPUs each with 8 cores, and hyperthreading. This gives 64 availableProcessors(). Unfortunately the curve of "performance vs number of threads" scales nicely for 1, 2, 3 threads, and caps at 3 threads. After 3 threads I can put as many threads as I want to the task, and the performance gets no better</p> <p><strong>Attempts to fix the Problem</strong></p> <p>My first thought was that I had been stupid and introduced some synchronised code somewhere. Normally to resolve this issue I run JConsole or JVisualVM, and look at the thread stacktraces. If I have 64 threads running at the speed of 3, I would expect 61 of them to be sitting waiting to enter a mutex. I didn't find this. Instead I found all the threads running: just very slowly.</p> <p>A second thought was that perhaps the timing framework was introducing problems. I replaced my function with a dummy function that just counts to a billion using an AtomicLong. This scaled beautifully with number of threads: I was able to count to a billion 10,000 times 64 times quicker with with 64 threads than with 1 thread.</p> <p>I thought (desperation kicking in) perhaps garbage collection is taking a really really long time, so I tweaked the garbage collection parameters. While this improved my latency variation, it had no effect on throughput: I still have 64 threads running at the speed I expect 3 to run at.</p> <p>I have downloaded the intel tool VTunes, but my skill with it is weak: it is a complex tool and I don't understand it yet. I have the instruction book on order: a fun Christmas present to myself, but that is a little too late to help my current problem</p> <p><strong>Question</strong></p> <ol> <li>What tools (mental or software) could I use to improve my understanding of what is going on? </li> <li>What mechanisms other than mutexs or garbage collection could be slowing my code down?</li> </ol>
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    1. USPeter Lawrey
    1. USStave Escura
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      1. POWhat mechanisms other than mutexs or garbage collection can slow my multi-threaded java program?
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    1. COAn obvious guess is that if one function call uses much memory, then four calls use four times as much, hitting a limit where this becomes a problem.
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      1. POWhat mechanisms other than mutexs or garbage collection can slow my multi-threaded java program?
      1. USMarko Topolnik
    2. COThanks for the answer Marko. I have put a sysout for memory available, and it stands at 800M free fairly consistently. I don't seem to have a memory leak. I wondered if I might be locking "getting memory" but I have never seen this in the past, or read anything about it
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      1. USStave Escura
    3. COThe JVM observing available heap is not at all a guarantee that it isn't hitting a performance threshold, like hitting the swap file. CPU-to-memory bandwidth also comes into play.
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