Leveraging Cache Coherence to Detect and Repair False Sharing On-the-fly

Performance bugs due to false sharing do not man-ifest as observable correctness errors, and hence are challenging to detect and repair. Prior approaches aim to both detect and repair false sharing instances automatically but most of them suffer from one or more of the following drawbacks: (i) high performance overhead due to expensive tracking of shadow memory, (ii) reliance on imprecise hardware events, and (iii) limited applicability and portability. We present extensions to the MESI cache coherence protocol for efficiently identifying and mitigating false sharing instances. The FSDetect protocol tracks the frequency of coherence misses per cache block to identify harmful instances of falsely shared lines while incurring negligible performance overhead. The FSLite protocol extends FSDetect to transparently privatize the falsely shared lines on accesses after detection, thereby eliminating the performance problem arising from false sharing. FSLite maintains coherence by performing precise byte-level updates of privatized blocks at the LLC on termination of privatization. Our simulation results on a variety of multithreaded workloads show that FSDetect can precisely identify all known harmful instances of false sharing. FSLite, on average, improves the performance of applications suffering from false sharing by 1.39X over the unmodified baseline, at the cost of a minimal increase in the chip area. Furthermore, applications running with FSLite stress the network less and show improved energy behavior.