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.
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.
| FreeBSD Version | Latest pmc-rs Release |
|---|---|
| FreeBSD 11 | v0.1.1 |
| FreeBSD 12.3+ | v0.2.0 |
| FreeBSD 13 | v0.2.0 |
Versions prior to FreeBSD 11 are untested, but may work.
FreeBSD 12.0 linked C++ into the libpmc stack causing compatibility issues with Rust (see this issue). Fortunately it was fixed in FreeBSD 12.3 and 13.0.
- Support sampling PMCs.
- Read counters with the
RDPMCinstruction to avoid context switching.