Sorting Benchmarks

Our six sorting benchmarks are defined as follows, in which MAX is for integers and approximately for doubles:

1. Uniform [U], a uniformly distributed random input, obtained by calling the C library random number generator . This function, which returns integers in the range 0 to , is initialized by each processor with the value . For the double data type, we ``normalize'' these values by first assigning the integer returned by random() a randomly chosen sign bit and then scaling the result by .
2. Gaussian [G], a Gaussian distributed random input, approximated by adding four calls to and then dividing the result by four. For the double type, we first normalize the values returned by in the manner described for [U].
3. Zero [Z], a zero entropy input, created by setting every value to a constant such as zero.
4. Bucket Sorted [B], an input that is sorted into p buckets, obtained by setting the first elements at each processor to be random numbers between 0 and , the second elements at each processor to be random numbers between and , and so forth.
5. g-Group [g-G], an input created by first dividing the processors into groups of consecutive processors of size g, where g can be any integer which partitions p evenly. If we index these groups in consecutive order, then for group j we set the first elements to be random numbers between and , the second elements at each processor to be random numbers between and , and so forth.
6. Staggered [S], created as follows: if the processor index i is , then we set all elements at that processor to be random numbers between and , and so forth. Otherwise, we set all elements to be random numbers between and , and so forth.

We selected these six benchmarks for a variety of reasons. Previous researchers have used the Uniform, Gaussian, and Zero benchmarks, and so we too included them for purposes of comparison. But benchmarks should be designed to illicit the worst case behavior from an algorithm, and in this sense the Uniform benchmark is not appropriate. For example, for , one would expect that the optimal choice of the splitters in the Uniform benchmark would be those which partition the range of possible values into equal intervals. Thus, algorithms which try to guess the splitters might perform misleadingly well on such an input. In this respect, the Gaussian benchmark is more telling. But we also wanted to find benchmarks which would evaluate the cost of irregular communication. Thus, we wanted to include benchmarks for which an algorithm which uses a single phase of routing would find contention difficult or even impossible to avoid. A naive approach to rearranging the data would perform poorly on the Bucket Sorted benchmark. Here, every processor would try to route data to the same processor at the same time, resulting in poor utilization of communication bandwidth. This problem might be avoided by an algorithm in which at each processor the elements are first grouped by destination and then routed according to the specifications of a sequence of p destination permutations. Perhaps the most straightforward way to do this is by iterating over the possible communication strides. But such a strategy would perform poorly with the g-Group benchmark, for a suitably chosen value of g. In this case, using stride iteration, those processors which belong to a particular group all route data to the same subset of g destination processors. This subset of destinations is selected so that, when the g processors route to this subset, they choose the processors in exactly the same order, producing contention and possibly stalling. Alternatively, one can synchronize the processors after each permutation, but this in turn will reduce the communication bandwidth by a factor of . In the worst case scenario, each processor needs to send data to a single processor a unique stride away. This is the case of the Staggered benchmark, and the result is a reduction of the communication bandwidth by a factor of p. Of course, one can correctly object that both the g-Group benchmark and the Staggered benchmark have been tailored to thwart a routing scheme which iterates over the possible strides, and that another sequences of permutations might be found which performs better. This is possible, but at the same time we are unaware of any single phase deterministic algorithm which could avoid an equivalent challenge.