Modeling critical sections in Amdahl's law and its implications for multicore design

This paper presents a fundamental law for parallel performance: it shows that parallel performance is not only limited by sequential code (as suggested by Amdahl’s law) but is also fundamentally limited by synchronization through critical sections. Extending Amdahl’s software model to include critical sections, we derive the surprising result that the impact of critical sections on parallel performance can be modeled as a completely sequential part and a completely parallel part. The sequential part is determined by the probability for entering a critical section and the contention probability (i.e., multiple threads wanting to enter the same critical section). This fundamental result reveals at least three important insights for multicore design. (i) Asymmetric multicore processors deliver less performance benefits relative to symmetric processors than suggested by Amdahl’s law, and in some cases even worse performance. (ii) Amdahl’s law suggests many tiny cores for optimum ...

• Type: Conference
• Year: 2010
• Where: ISCA
• Authors: Stijn Eyerman, Lieven Eeckhout
• Keywords: Amdahl’s Law, Critical Sections, Hardware, ISCA 2010, Parallel Performance