Optimizing CUDA code by kernel fusion: application on BLAS

Abstract

Contemporary GPUs have significantly higher arithmetic throughput than a memory throughput. Hence, many GPU kernels are memory bound and cannot exploit arithmetic power of the GPU. Examples of memory-bound kernels are BLAS-1 (vector–vector) and BLAS-2 (matrix–vector) operations. However, when kernels share data, kernel fusion can improve memory locality by placing shared data, originally passed via off-chip global memory, into a faster, but distributed on-chip memory. In this paper, we show how kernels performing map, reduce or their nested combinations can be fused automatically by our source-to-source compiler. To demonstrate the usability of the compiler, we have implemented several BLAS-1 and BLAS-2 routines and show how the performance of their sequences can be improved by fusions. Compared with similar sequences using CUBLAS, our compiler is able to generate code that is up to 2.24\(\times \) faster for the examples tested.

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Notes

  1. 1.

    The first CUDA processor, G80, has flop-to-word ratio about 24, GT200 has 27, GF110 has 33, GK110 has 63 and GM204 has 82.

  2. 2.

    For more details about CUDA, we refer to [17].

  3. 3.

    Some programming languages use map only for unary functions and introduce zipwith for n-ary functions.

  4. 4.

    Data element can be placed in registers only if their indexing can be determined at compile time [17].

  5. 5.

    This is trivially fulfilled in code generation stage, as outputs of all reductions are used outside of the fusion implementation performing the reduction, thus the global barrier is performed by finishing the kernel.

  6. 6.

    For more details about shared memory bank conflicts, we reffer to  [17].

  7. 7.

    It is naturally possible to use rectangular tiles, which decrease the reduction overhead. However, such tiles forbid efficient fusion of operations working with matrix and its transposition.

  8. 8.

    When the function f performs reduction on each row of the matrix and the reduction’s result is an input of function g processing the same row, CPU is able to hold the row in the cache and reuse it after reduction finish (thus, outer loops in f and g going over rows are fused, whereas inner loops are unfused). Considering GPU, the row needs to be partitioned among more thread blocks when it is read into on-chip memory by f, thus thread blocks need to be synchronized before the result of the reduction is available. Our compiler performs the synchronization by a new kernel invocation, thus all on-chip data are lost before the result of the reduction is available for g, so no row data can be reused. The only way to reuse row data on GPU is to use persistent threads [10], but it is not clear if it could have a positive performance impact.

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Acknowledgments

This work was supported by Ministry of Education, Youth and Sport of the Czech Republic under the Project “CERIT Scientific Cloud” (No. ED3.2.00/08.0144). The first author was supported by the Ministry of Education, Youth, and Sport Project CZ.1.07/2.3.00/30.0037—Employment of Best Young Scientists for International Cooperation Empowerment.

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Correspondence to Jiří Filipovič.

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Filipovič, J., Madzin, M., Fousek, J. et al. Optimizing CUDA code by kernel fusion: application on BLAS. J Supercomput 71, 3934–3957 (2015). https://doi.org/10.1007/s11227-015-1483-z

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Keywords

  • GPGPU
  • CUDA
  • BLAS
  • Kernel fusion
  • Code generation