MPX: Software for Multiplexing Hardware Performance Counters in Multithreaded Programs

Hardware performance counters are CPU registers that count data loads and stores, cache misses, and other events. Counter data can help programmers understand software performance. Although CPUs typically have multiple coun-ters, each can monitor only one type of event at a time, and some counters can monitor only certain events. Therefore, some CPUs cannot concurrently monitor interesting combi-nations of events. Software multiplexing partly overcomes this limitation by using time sharing to monitor multiple events on one counter. However, counter multiplexing is harder to implement for multithreaded programs than for single-threaded ones because of certain difficulties in man-aging the length of the time slices. This paper describes a software library called MPX that overcomes these difficulties. MPX allows applications to gather hardware counter data concurrently for any combi-nation of countable events. MPX data are typically within a few percent of counts recorded without multiplexing.