OpenCL, TBB, OpenMP

Question

I have implemented few normal looping applications in OpenMP, TBB and OpenCL. In all these applications, OpeCL gives far better performance than others too when I am only running it on CPU with no specific optimizations done in kernels. OpenMP and TBB gives good performance too but far less than OpenCL, what could be reason for it because these both are CPU specialized frameworks and should gives at least a performance equal to OpenMP/TBB.

My second concern is that when it comes to OpenMP and TBB, OpenMP is always better in performance than TBB in my implementations in which I havent tuned it for a very good optimizations as I am not so expert. Is there a reason that OpenMP is normally better in performance than TBB? Because I think they both or even OpenCL too uses same kind of thread pooling at low level.... Any expert opinions? Thanks

Answer 1

One advantage that OpenCL has over TBB and OpenMP is that it can take better advantage of SIMD parallelism in your hardware. Some OpenCL implementations will run your code such that each work item runs in a SIMD vector lane of the machine, as well as running on separate cores. Depending on the algorithm, this could provide lots of performance benefits.

C compilers can also take some advantage of SIMD parallelism as well, using auto-vectorization, but the memory aliasing rules in C make it hard for this to work in some cases. Since OpenCL requires programmers to call out the work items and fence memory accesses explicitly, an OpenCL compiler can be more aggressive.

In the end, it depends on your code. One could find an algorithm for which any of OpenCL, OpenMP, or TBB are best.

Answer 2

OpenCL runtime for CPU and MIC provided by Intel uses TBB under the hood. It's far from just 'thread pooling at low level' since it takes advantage of sophisticated scheduling and partitioning algorithms provided by TBB for better load balance and so better utilization of CPUs.

As for TBB vs. OpenMP. Usually, it comes down to incorrect measurements. For example, TBB has no implicit barrier like in OpenMP, so a warm-up loop is not enough. You have to make sure all the threads are created and this overhead is not included into your measurements. Another example: sometimes, compilers are not able to vectorize the same code with TBB which is vectorized with OpenMP.

Answer 3

OpenCL kernels are compiled for the given hardware. The potential for vendor/hardware specific optimisations is huge.