Saturday, May 29, 2010

SPU sorting

As you might know from the public papers, there are only 256 Kb of local memory on SPU, but DMA requests are very fast... quickSort isn't an appropriate algorithm for SPU architecture due to branching and generation of large number of spatially non-coherent memory requests. After several hours of attempts to keep SPU quickSort performance on an acceptable level and writing software implementation of memory cache for SPU, the resulting performance is slightly better than PPU version (on small arrays):

elements: 1012 (16Kb)
quickSort PPU Time: 0.000 FPS: 3717.4719
quickSort SPU Time: 0.000 FPS: 5376.3438

elements: 2024 (32Kb)
quickSort PPU Time: 0.001 FPS: 1923.0769
quickSort SPU Time: 0.000 FPS: 2652.5200

elements: 4048 (64Kb)
quickSort PPU Time: 0.001 FPS: 819.6721
quickSort SPU Time: 0.001 FPS: 908.2652

elements: 8096 (128Kb)
quickSort PPU Time: 0.003 FPS: 398.2477
quickSort SPU Time: 0.002 FPS: 407.3320

elements: 16192 (256Kb)
quickSort PPU Time: 0.005 FPS: 187.7229
quickSort SPU Time: 0.006 FPS: 180.3752

elements: 32384 (512Kb)
quickSort PPU Time: 0.012 FPS: 86.3185
quickSort SPU Time: 0.013 FPS: 78.7030

elements: 64768 (1Mb)
quickSort PPU Time: 0.027 FPS: 37.6322
quickSort SPU Time: 0.029 FPS: 34.2044

elements: 129536 (2Mb)
quickSort PPU Time: 0.056 FPS: 17.8352
quickSort SPU Time: 0.063 FPS: 15.8253

elements: 259072 (4Mb)
quickSort PPU Time: 0.124 FPS: 8.0358
quickSort SPU Time: 0.139 FPS: 7.2073

It was very difficult to sleep after this poor results... I was trying to implement radixSort on the second day in the morning... SPU instruction set fits such algorithms very well. Performance of radixSort on local SPU memory appeared to be very good especially with eliminated branching instructions. Moreover performance of DMA list operations on SPU (surprise!) is great and the resulted version of radixSort demonstrates awesome speedup:

elements: 1012 (16Kb)
radixSort PPU Time: 0.000 FPS: 4081.6326
radixSort SPU Time: 0.000 FPS: 9615.3848

elements: 2024 (32Kb)
radixSort PPU Time: 0.000 FPS: 2617.8010
radixSort SPU Time: 0.000 FPS: 4032.2581

elements: 4048 (64Kb)
radixSort PPU Time: 0.001 FPS: 1333.3334
radixSort SPU Time: 0.000 FPS: 2237.1365

elements: 8096 (128Kb)
radixSort PPU Time: 0.001 FPS: 673.4007
radixSort SPU Time: 0.001 FPS: 1168.2242

elements: 16192 (256Kb)
radixSort PPU Time: 0.003 FPS: 288.8504
radixSort SPU Time: 0.002 FPS: 597.0150

elements: 32384 (512Kb)
radixSort PPU Time: 0.008 FPS: 124.1311
radixSort SPU Time: 0.003 FPS: 298.3294

elements: 64768 (1Mb)
radixSort PPU Time: 0.022 FPS: 45.3700
radixSort SPU Time: 0.007 FPS: 149.8352

elements: 129536 (2Mb)
radixSort PPU Time: 0.049 FPS: 20.5351
radixSort SPU Time: 0.013 FPS: 75.0413

elements: 259072 (4Mb)
radixSort PPU Time: 0.101 FPS: 9.9149
radixSort SPU Time: 0.027 FPS: 37.5587

1 comment:

  1. Hi,
    I recently had a bit of a play with sorting on PPU and SPU myself and wrote up my results here:

    This article contains a link to more detailed documentation plus a subset of the code used. In the end I was performing a sort on 65k elements in around 1ms using a parallel merge sort variant.

    Hope this helps