HP UX Caliper Software manual Function Details, How Instruction Cache Metrics Are Obtained

Table 27 Information in icache Measurement Reports (continued)

Column	Description
Load Module	Shared library or the main executable.
Function	Routine from your application.
File	Source file associated with a function.
Line	The column contains one of these:
Slot	• A source-code line number for rows showing statements
Col,Offset
	• An instruction slot number for rows showing instructions not on a bundle boundary
	• A source-code column number followed by an offset from the beginning address of a function
	for rows showing instructions on a bundle boundary
	Column and line numbers are preceded by “~” when they are approximate due to optimization.
>Statement	The column contains either a source statement, preceded by “>”, or a disassembled instruction.
Instruction	Statements that are out of order due to optimization are preceded by “*>”.

Function Details

A cache line is the smallest unit of data that is transferred at one time between main memory and the instruction cache. On Itanium 2 systems, cache lines are 64 bytes (12 instructions). Cache lines are the finest level of granularity available in icache measurement reports.

These reports show data associated with a cache line on the same row as the first instruction of the cache line. Each set of instructions that make up a cache line are preceded and followed by a row of dashes (“- - - -”). The cache lines shown might not be contiguous.

Non-contiguous cache lines are separated by a row of tildes (“~ ~ ~ ~”).

How Instruction Cache Metrics Are Obtained

HP Caliper obtains instruction cache metrics from the processor's performance monitoring unit (PMU).

Exact counts are obtained from the PMU's set of performance monitor configuration (PMC)/performance monitor data (PMD) register pairs. Sampled instruction cache metrics are obtained from the PMU's instruction event address register (I-EAR). Both sets of metrics focus on the L1 cache.

HP Caliper takes samples every Nth instruction cache miss, where N is defined in the icache measurement configuration file in the HP Caliper home directory config/) subdirectory. At each sample point, HP Caliper records both the cache line that resulted in an instruction cache miss and the latency, number of clock cycles, incurred by the miss. You can override the value in the measurement configuration file by using the -soption.

HP Caliper attributes samples for a given cache line to the function associated with the start address of the cache line. Because cache lines can cross function boundaries, data attributed to functions will not always be accurate. However, only cache-line data at the boundaries of the function are potentially misattributed.

More frequent sampling increases HP Caliper's perturbation of your application. In the extreme case of taking one sample for each cache miss event, the kernel will trap on every event, making the resulting data of limited, if any, value.

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