Relaxing the problem-size bound for out-of-core columnsort

Previous implementations of out-of-core columnsort limit the problem size to N ≤(M/P)³/2, where N is the number of records to sort, P is the number of processors, and M is the total number of records that the entire system can hold in its memory. We implemented two variations to out-of-core columnsort that relax this restriction. Subblock columnsort is based on an algorithmic modification of the underlying columnsort algorithm, and it improves the problem-size bound to N ≤(M/P)^5/3 / 4^2/3 but at the cost of additional disk I/O. M-columnsort changes the notion of the column size in columnsort, improving the maximum problem size to N ≤M³ / 2 but at the cost of additional computation and communication. Experimental results on a Beowulf cluster show that both subblock columnsort and M-columnsort run well but that M-columnsort is faster. A further advantage of M-columnsort is that it handles a wider range of problem sizes than subblock columnsort.