10 Producing a Sampled Call Graph Profile Analysis

HP Caliper can produce a sampled call graph profile report (using the scgprof measurement) from any compiled program. You do not need to compile your program in any special way to use this feature.

The call graph is produced by sampling the processor's performance monitoring unit (PMU) to determine function calls. The call graph is not exact, because it does not show every function call, but it is statistically useful.

This chapter provides an overview. For details about the sampled call graph profile report, see “scgprof Measurement Report Description” (p. 224).

HP Caliper can also produce an exact call graph, using the cgprof measurement, which instruments the application code. The cgprof measurement runs on HP-UX only. For details about cgprof,

see “cgprof Report Description ” (p. 180). Also see “Differences Between scgprof and cgprof” (p. 122).

NOTE: Using the scgprof measurement on HP-UX 11.23 (v2) requires kernel patch PHKL_34020.

How Sampled Call Graph Profile Analysis Works

The HP Caliper sampled call graph profile analysis produces a gprof-like report. It includes these results:

•Total execution times for each function in the program

•Call counts for each function in the program

•Cycle information, member functions, time and call counts

•Most likely hot call paths in the program

Because the calls are discovered by sampling the PMU, it cannot show every function call performed by a process. (This is similar to how the dcache measurement works. It finds data cache misses by sampling the PMU.)

Statistically, the sampled call graph produced by scgprof is generally accurate in terms of which calls are more frequently made, and how much more frequent some calls are than others. The sampled call graph produced will be more useful for longer-lived processes than shorter-lived processes.

The --branch-sampling-specoption is used with the scgprof measurement (and used only with this measurement). It helps produce the statistical call graph. See “--branch-sampling-spec” (p. 55).

Differences Between scgprof and cgprof

These are differences between scgprof and cgprof collection runs:

•scgprof collection runs have significantly less overhead than cgprof collection runs, because the call graphs are produced using sampling instead of instrumentation.

•The --durationoption is supported for use with scgprof (but not cgprof).

•The --branch-sampling-specoption is supported for use with scgprof (but not cgprof).

•When you are using scgprof and attaching to a process, dependent libraries do not have to be mapped private with chatr before attaching with HP Caliper.

•An scgprof report differs slightly from a cgprof report. In particular, there are two Sampling Specification sections in the summary area: IP Sampling and Branch Sampling. See “Sampled Call Graph Text Report Example” (p. 124).

122 Producing a Sampled Call Graph Profile Analysis