in reply to Poor performances with threads

... calculate the standard deviation of the values of a small slice of the initial array ...

If it takes longer to spawn the thread, than to process that small slice, you have a net slowdown.

How long does a single (non-threaded) solution take? How many cores do you have? How many concurrent threads does that graph represent?

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Re^2: Poor performances with threads
by olafmar (Novice) on Oct 23, 2009 at 10:20 UTC
    The graph represents 1, 2, 3, 4, 8 threads and I have 8 cores.

    Actually I haven't thought about the time required to spawn the thread... since each thread has at least 55k sets of 7 values to work on, I thought the time required to spawn the thread was negligible.

    1 thread: 60 seconds
    2 threads: 40 seconds
    4 threads: 37 seconds
    8 threads: 54 seconds

    I think I will settle on 2 threads for the time being, it should be a good compromise also for smaller datasets.

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