HP Caliper User's Guide

What to Look for in Using HP Caliper
A useful approach for tuning performance is to start with easy-to-perform global
measurements to identify likely areas for tuning. Then you can use more specific
measurements on areas of concern.
For example, you might start by making global measurements on a variety of events,
such as:
• CPU cycles
• Instruction execution counts
• Data cache misses
• Instruction cache misses
• Branch mispredictions
The results of the global measurements will quickly identify which factors most affect
your program's performance.
The next step is to make sampled measurements on the top performance factors you
have identified.
You can vary the sample rates to find the approximate location of the problem or
problems. Usually, this is sufficient to locate trouble spots.
You can also focus on hot spots. A hot spot is an instruction or set of instructions that
has a higher execution count than most other instructions in a program. For example,
code that is inside a loop will likely be executed more times than straight-line code.
Usually the “hotness” is measured with CPU cycles, but it could also be measured with
metrics such as cache misses.
When tuning performance, you first focus on hot spots because a small change in a hot
spot, multiplied by its higher execution frequency, produces a larger improvement
than making the same change in an instruction that is rarely executed.
Here are some useful things to look for in a report:
• If the percent of cycles lost due to stalls (in the Metrics Summed for Entire Run)
is high—greater than 50 percent—the program's performance could probably be
improved.
• Although the values in the IP Samples column of the Function Details do not
represent actual execution counts, you can still use the values as a relative ranking
of how often particular lines are executed. The IP Samples values will vary slightly
with each run, but they can help you identify which parts of your code have the
most effect on your application's performance.
Syntax for caliper Command
The basic command-line syntax for making measurements with HP Caliper is:
caliper measurement [caliper_options] program [program_arguments]
where:
What to Look for in Using HP Caliper 37