<\/A>We\u2019ll be regularly posting answers to frequently asked questions that we\u2019ve gotten on the forum, internal email lists, etc. <\/SPAN>Here\u2019s the first \u2013 enjoy!<\/FONT><\/FONT><\/P>\n Why is the speedup not X on my X-way machine? Or, why does my parallel code run slower?<\/STRONG> <\/FONT><\/FONT><\/P>\n Less than ideal speedup can typically be attributed to two things:<\/FONT><\/FONT><\/P>\n 1.<\/FONT> <\/SPAN><\/SPAN><\/SPAN>Sequential costs<\/U>. <\/SPAN>It\u2019s often the case that parts of a particular algorithm must be executed sequentially. <\/SPAN>As the parallelizable parts get faster (with the addition of more cores), the sequential parts become more significant (Amdahl’s Law<\/A>).<\/FONT><\/FONT><\/P>\n 2.<\/FONT> <\/SPAN><\/SPAN><\/SPAN>General overheads<\/U>. <\/SPAN>For example, the costs associated with partitioning\/merging data, scheduling tasks to the runtime, synchronization to support additional features, or delegate invocations.<\/FONT><\/FONT><\/P>\n The former can sometimes be addressed by choosing a different algorithm that requires less serial execution. <\/SPAN>The latter can often be addressed by reducing communication between tasks in a parallel operation, making fine-grained parallelism more coarse-grained, etc. <\/SPAN>Also, Microsoft is constantly working to reduce the impact of both points, where possible.<\/FONT><\/FONT><\/P><\/p>\n
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