B.2.128.1 Prefetch Latency Ratio

The prefetch latency is the hardware delay between the execution of a prefetch instruction and the time the data being prefetched is available in the cache.

The factor must be a positive number of the form n.n.

The compiler assumes a prefetch latency value when determining how far apart to place a prefetch instruction and the load or store instruction that uses the prefetched data. The assumed latency between a prefetch and a load may not be the same as the assumed latency between a prefetch and a store.

The compiler tunes the prefetch mechanism for optimal performance across a wide range of machines and applications. This tuning may not always be optimal. For memory-intensive applications, especially applications intended to run on large multiprocessors, you may be able to obtain better performance by increasing the prefetch latency values. To increase the values, use a factor that is greater than 1 (one). A value between .5 and 2.0 will most likely pro vide the maximum performance.

For applications with data sets that reside entirely within the external cache, you may be able to obtain better performance by decreasing the prefetch latency values. To decrease the values, use a factor that is less than one.

To use the latx:factor suboption, start with a factor value near 1.0 and run performance tests against the application. Then increase or decrease the factor, as appropriate, and run the performance tests again. Continue adjusting the factor and running the performance tests until you achieve optimum performance. When you increase or decrease the factor in small steps, you will see no performance difference for a few steps, then a sudden difference, then it will level off again.