Manual Pages  — HWPMC

The driver supports multi-processor systems.

PMCs are allocated using the PMC_OP_PMCALLOCATE request. A successful PMC_OP_PMCALLOCATE request will return a handle to the requesting process. Subsequent operations on the allocated PMC use this handle to denote the specific PMC. A process that has successfully allocated a PMC is termed an "owner process".

PMCs may be allocated with process or system scope.

Process-scope
	The PMC is active only when a thread belonging to a process it is attached to is scheduled on a CPU.
System-scope	The PMC operates independently of processes and measures hardware events for the system as a whole.

PMCs may be allocated for counting or for sampling:

Counting
	In counting modes, the PMCs count hardware events. These counts are retrievable using the PMC_OP_PMCREAD system call on all architectures. Some architectures offer faster methods of reading these counts.
Sampling
	In sampling modes, the PMCs are configured to sample the CPU instruction pointer (and optionally to capture the call chain leading up to the sampled instruction pointer) after a configurable number of hardware events have been observed. Instruction pointer samples and call chain records are usually directed to a log file for subsequent analysis.

Scope and operational mode are orthogonal; a PMC may thus be configured to operate in one of the following four modes:

Process-scope, counting
	These PMCs count hardware events whenever a thread in their attached process is scheduled on a CPU. These PMCs normally count from zero, but the initial count may be set using the PMC_OP_SETCOUNT operation. Applications can read the value of the PMC anytime using the PMC_OP_PMCRW operation.
Process-scope, sampling
	These PMCs sample the target processes instruction pointer after they have seen the configured number of hardware events. The PMCs only count events when a thread belonging to their attached process is active. The desired frequency of sampling is set using the PMC_OP_SETCOUNT operation prior to starting the PMC. Log files are configured using the PMC_OP_CONFIGURELOG operation.
System-scope, counting
	These PMCs count hardware events seen by them independent of the processes that are executing. The current count on these PMCs can be read using the PMC_OP_PMCRW request. These PMCs normally count from zero, but the initial count may be set using the PMC_OP_SETCOUNT operation.
System-scope, sampling
	These PMCs will periodically sample the instruction pointer of the CPU they are allocated on, and will write the sample to a log for further processing. The desired frequency of sampling is set using the PMC_OP_SETCOUNT operation prior to starting the PMC. Log files are configured using the PMC_OP_CONFIGURELOG operation. System-wide statistical sampling can only be enabled by a process with super-user privileges.

Processes are allowed to allocate as many PMCs as the hardware and current operating conditions permit. Processes may mix allocations of system-wide and process-private PMCs. Multiple processes may be using PMCs simultaneously.

Allocated PMCs are started using the PMC_OP_PMCSTART operation, and stopped using the PMC_OP_PMCSTOP operation. Stopping and starting a PMC is permitted at any time the owner process has a valid handle to the PMC.

Process-private PMCs need to be attached to a target process before they can be used. Attaching a process to a PMC is done using the PMC_OP_PMCATTACH operation. An already attached PMC may be detached from its target process using the converse PMC_OP_PMCDETACH operation. Issuing a PMC_OP_PMCSTART operation on an as yet unattached PMC will cause it to be attached to its owner process. The following rules determine whether a given process may attach a PMC to another target process: