False Sharing and its Effect on Shared Memory Performance
William J. Bolosky Michael L. Scott
Microsoft Research Laboratory Computer Science Department
One Microsoft Way, 9S/1049 University of Rochester
Redmond, WA 98052-6399 Rochester, NY 14627-0226
False sharing occurs when processors in a shared-memory parallel system make
references to different data objects within the same coherence block (cache
line or page), thereby inducing "unnecessary" coherence operations. False
sharing is widely believed to be a serious problem for parallel program
performance, but a precise definition and quantification of the problem has
proven to be elusive. We explain why. In the process, we present a variety
of possible definitions for false sharing, and discuss the merits and
drawbacks of each. Our discussion is based on experience gained during a
four-year study of multiprocessor memory architecture and its effect on the
behavior of applications in a sixteen-program suite.
Using trace-based simulation, we present experimental evidence to support
the claim that false sharing is a serious problem. Unfortunately, we find
that the various computationally tractable approaches to quantifying the
problem are either heuristic in nature, or fail to agree with intuition.
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