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Thread: For loops inside kernels

  1. #1
    Junior Member
    Join Date
    Nov 2012

    For loops inside kernels

    I'm using the book "openCL in action" to learn how to program openCL. In this book the author claims that for-loops inside kernel-functions is a bad idea because comparison statements are time consuming on gpus which I understand considering general gpu architectures. However in his matrix-examples and in other matrix-examples from other sources, for loops are used quite extensively inside kernels. Isn't this sort of against the whole idea behind using openCL? If I need many for-loops why dispatch kernels at all instead of just writing normal c/c++ code?

    Could someone please shed some light on these issues for me? It would be of great help before I start implementing my own algorithms.

  2. #2
    Senior Member
    Join Date
    Dec 2011

    Re: For loops inside kernels

    For loops in kernels are fine if all work items in the workgroup are looping the same number of times. If each work item takes a different number of loops then you have divergence and that's what can really slow things down.

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