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POParallelization: What causes Java threads to block other than synchronization & I/O?
 

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  1. POParallelization: What causes Java threads to block other than synchronization & I/O?
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    <p>Short version is in the title.</p> <p>Long version: I am working on a program for scientific optimization using Java. The workload of the program can be divided into parallel and serial phases -- parallel phases meaning that highly parallelizable work is being performed. To speed up the program (it runs for hours/days) I create a number of threads equal to the number of CPU cores on the machine I'm using -- typically 4 or 8 -- and divide the work between them. I then start these threads and join() them before proceeding to a serial phase.</p> <p>So far so good. What's bothering me is that the CPU utilization and speedup of the parallel phases is nowhere near the "theoretical maximum" -- e.g. if I have 4 cores, I expect to see somewhere between 350-400% "utilization" (as reported by top) but instead it bounces around between 180 and about 310. Using only a single thread, I get 100% CPU utilization.</p> <p>The only reasons I know of for threads not to run at full speed are: -blocking due to I/O -blocking due to synchronization</p> <p>No I/O whatsoever is going on in my parallel threads, nor any synchronization -- the only data structures shared by the threads are read-only, and are either basic types or (non-concurrent) collections. So I'm looking for other explanations. One possibility would be that several threads are repeatedly blocking for garbage collection, but that would only seem to make sense in a situation with memory pressure, and I am allocating well above the required maximum heap space. </p> <p>Any suggestions would be appreciated.</p> <p><b>Update:</b> Just in case anyone is curious, after some more investigation I tweaked the code for general performance and am seeing better utilization, even though nothing I changed has to do with synchronization. However, some of the changes should have resulted in fewer new heap allocations in particular I got rid of some use of <b>iterators</b> and <b>termporary boxed numbers</b> (The CERN "Colt" library for high-performance Java computing was useful here: it provides collections like IntArrayList, DoubleArrayList etc for basic types.). So I think garbage collection was probably the culprit. </p>
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    1. COJoe, since I am curious, what you are saying is that after tweaking you see better CPU utilization. What are the numbers?
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