HPjmeter 4.2 User's Guide

Generally, a metric is a mapping that associates numerical values with program static or dynamic

elements such as functions, variables, classes, objects, types, or threads. The numerical values

may represent various resources used by the program.

For in-depth analysis of program performance, it is useful to analyze a call graph. Call graphs

capture the “call” relationships between the methods. The nodes of the call graph represent the

program methods, while the directed arcs represent calls made from one method to another. In

a call graph, the call counts or the timing data are collected for the arcs.

Tracing

Tracing is one of two methods discussed here for collecting profile data. Java virtual machines

use tracing with reduction. Here is how it works: the profile data is collected whenever the

application makes a function call. The calling method and the called method (sometimes called

“callee”) names are recorded along with the time spent in the call. The data is accumulated (this

is “reduction”) so that consecutive calls from the same caller to the same callee increase the

recorded time value. The number of calls is also recorded.

Tracing requires frequent reading of the current time (or measuring other resources consumed

by the program), and can introduce large overhead. It produces accurate call counts and the call

graph, but the timing data can be substantially influenced by the additional overhead.

Sampling

In sampling, the program runs at its own pace, but from time to time the profiler checks the

application state more closely by temporarily interrupting the program's progress and determining

which method is executing. The sampling interval is the elapsed time between two consecutive

status checks. Sampling uses “wall clock time” as the basis for the sampling interval, but only

collects data for the CPU-scheduled threads. The methods that consume more CPU time will be

detected more frequently. With a large number of samples, the CPU times for each function are

estimated quite well.

Sampling is a complementary technique to tracing. It is characterized by relatively low overhead,

produces fairly accurate timing data (at least for long-running applications), but cannot produce

call counts. Also, the call graph is only partial. Usually a number of less significant arcs and

nodes will be missing.