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    Wednesday, March 15, 2017

Malthusian Locks

By: Dave Dice | Senior Research Scientist
Malthusian Locks appears in EuroSys 2017. An extended version is in arxiv.
Applications running in modern multithreaded environments are sometimes overthreaded. The excess threads do not improve performance, and in fact may act to degrade performance via scalability collapse, which can manifest even when there are fewer ready threads than available cores. Often, such software also has highly contended locks. We leverage the existence of such locks by modifying the lock admission policy so as to intentionally limit the number of distinct threads circulating over the lock in a given period. Specifically, if there are more threads circulating than are necessary to keep the lock saturated (continuously held), our approach will selectively cull and passivate some of those excess threads. We borrow the concept of swapping from the field of memory management and impose concurrency restriction (CR) if a lock suffers from contention. The resultant admission order is unfair over the short term but we explicitly provide long-term fairness by periodically shifting threads between the set of passivated threads and those actively circulating. Our approach is palliative, but is often effective at avoiding or reducing scalability collapse, and in the worst case does no harm. Specifically, throughput is either unaffected or improved, and unfairness is bounded, relative to common test-and-set locks which allow unbounded bypass and starvation. By reducing competition for shared resources, such as pipelines, processors and caches, concurrency restriction may also reduce overall resource consumption and improve the overall load carrying capacity of a system.

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