HP XC System 3.x Software - Manual (Page 116)

Multi-Issue Architectures
In multi-issue architectures (those that can execute more than one instruction per cycle), the
interrupt handler associates only one instruction in a bundle with an event. The other instructions
in the bundle have no associated events. This can skew the attribution of events to instructions.
Calls to exec()
If a process uses the exec() system call or its variants, HPCPI can attribute events to the wrong
image and it is possible to get samples for unexecuted instructions. Samples gathered prior to
an exec() call might be attributed to the image loaded by the exec() call. In most applications,
a process typically calls exec() soon after being created, so HPCPI collects only a few samples
prior to the exec() call and only a few samples might be attributed to the wrong image.
Unknown Locations
HPCPI is sometimes unable to map an instruction pointer value to a known code region. This is
often caused by a call to a routine outside the current object module using a trampoline, or code
generated by the compiler or linker automatically that does not exist within any routines in the
source object module. Runtime compilers, such as Java™ Just-in-Time (JIT) compilers, also
generate executable code that is unknown to HPCPI. In these cases, the HPCPI analysis tools
map these instructions to the routine unknown_rou.
Mandated Duty Groups
Some events cannot be monitored by the PMU at the same time, so HPCPI creates additional
duty groups as required. This restriction is based on requirements from the processor
manufacturers. For example, Itanium processors require separate groups for LOADS_RETIRED
and STORES_RETIRED.
Active Fraction Changes
The number of event groups can change while hpcpid is running if the xperf utility starts. If
this occurs, an event can have multiple sampling interval and/or active fraction values during
the reporting period. HPCPI calculates one effective sampling interval and one effective active
fraction value for each event for the entire time period. HPCPI maintains event counts and other
counts that are unaffected by changing values of the sampling interval and active fraction. It
uses these counts to derive effective values for the reported sampling interval and active fraction.
116 Product Specifications