pmc-rs

pmc-rs provides a safe abstraction for interacting with libpmc/hwpmc's Performance Monitor Counters on FreeBSD.

PMCs are part of the CPU hardware and are typically used to profile CPU micro-architecture events such as L1/L2/L3 cache hits & misses, instructions processed per CPU tick, TLB lookups, branch mispredictions, etc for a particular application or algorithm. Using PMCs an algorithm can be tuned for performance by minimising CPU stalls, optimising CPU cache usage, and ensuring the CPU is always doing useful work.

The events are defined by the CPU manufacturer (here is the Intel 64 and IA-32 Architectures Developer's Manual: vol. 3B where the events can be found in section 18.2.1.2 "Pre-defined Architectural Performance Events", Table 18-1 "UMask and Event Select Encodings for Pre-Defined Architectural Performance Events").

pmc-rs makes use of the libpmc userland interace to the hwpmc kernel module on FreeBSD.

Version Compatibility

The latest release of pmc-rs generally targets the latest FreeBSD release. Due to changes in libpmc between FreeBSD versions, compatibility with older FreeBSD versions may require pinning pmc-rs to an older release.

Versions prior to FreeBSD 11 are untested, but may work.

FreeBSD 12 linked C++ into the libpmc stack causing compatibility issues with Rust (see this issue). Fortunately this is not the case with FreeBSD 13.

Future improvements

• Support sampling PMCs.
• Read counters with the RDPMC instruction to avoid context switching.