Event-based analysis of a L2 prefetch related parallel nonscaling on intel dual core processor

Performance degradation is a common problem in parallel computing as when a workload is parallelised the speedup gained is notably lower than the factor predicted by Amdahl´s law. This work examines such a case where image pixels are summed up along the rows in parallel by two threads, but no speedup against the sequential summation is gained on images whose sizes exceed the capacity of the L2 cache of the processor. Counts collected by Intel VTune^™Performance Analyser on relevant performance events show that this nonscaling problem is not caused by those well-understood pitfalls, such as cache contention, bus overloading and unbalanced workload, but that over the parallel summation the L2 prefetchers of the processor are less effective in bringing in data before they are needed. Consequently, a considerably more number of cache lines are brought into the L2 cache by demand requests originated from the L1 data caches, and for such accesses the parallel computation pay the penalty.