How do I select an instrumentation scheme?

Instrumentation schemes determine the order in which methods are instrumented and the likely number of instrumented methods. By instrumenting a limited subset of the application code instead of all its methods, you can reduce the profiling overhead dramatically. Choosing one or more root methods and instrumenting their call subgraphs is the recommended technique if you profile a long-running application that repeatedly performs the same or similar actions, e.g. a server-side application. However, if you want to profile application startup or a short-running command-line application, it may be more efficient to instrument most or all of the application code.

You can choose from the following three instrumentation schemes:

  • Lazy instrumentation
  • Eager instrumentation
  • Total instrumentation

Lazy instrumentation scheme.

This is the best scheme for profiling long-running applications.

When this scheme is used, the IDE instruments methods in the following way. First, the root method is instrumented. Next, whenever any instrumented method m() is executed for the very first time, its code is scanned and all methods that m() may call are instrumented in turn. In this way, we ensure that the number of methods that are instrumented is close to the number of methods that will be actually invoked by the target application during its execution lifetime. By minimizing the number of methods that are instrumented but not called from the root method, we reduce the instrumentation time itself. We also avoid the overhead that may otherwise be incurred if an instrumented method is not actually called from our root, but is still called from some other part of the program.

Nevertheless, the number of instrumented methods is usually greater than the number of methods actually invoked. The primary reason for this is that in an average Java application, many methods are virtual (that is, not static). Therefore, if there is a call like x.m() in the source code, and variable x has type X, the IDE has to instrument not only the X.m() method, but also all implementations of method m() in subclasses of X. This is because it is generally not possible to find out in advance what the actual class of x is going to be at run time (in fact it may change many times), and thus which particular implementation of m() is going to be called.

The lazy instrumentation scheme is best for profiling long-running applications, but it may not work very well if you need to profile the application during the period of time when it loads a lot of classes, or just when instrumentation is in progress. This is due to the fact that in the HotSpot JVM used by the IDE, many application methods are usually compiled into machine code to improve performance. If a method gets instrumented later, the JVM initially switches back to interpretation of its bytecodes (later it can compile this method again). A temporary switch to interpretation may happen even to some methods that are not instrumented themselves, but call instrumented methods. For this reason, during the time when the IDE is discovering the call graph, and for some time afterwards, the application may run considerably slower than normally, and the CPU profiling results obtained during this period are not representative of its normal execution. It is recommended to run the application for some time after initiating the call graph instrumentation (for example, if it's a server-side application, you can make it process a few hundred or thousand requests) and then discard the already accumulated profiling results by invoking Profile -> Reset Collected Results. Profiling results collected afterwards will be much more realistic.

Eager instrumentation scheme

Eager instrumentation is a compromise between the other two schemes. Unlike the lazy scheme, eager scheme locates all potentially reachable methods (that is, methods that can be called by the root method directly or transitively) in a class as soon as this class is loaded. Eager analysis for potential reachability may lead to many more methods instrumented than called (sometimes 10 times plus), so it is generally not recommended to use this scheme for long-running applications. However, it usually results in a much smaller number of repeated method decompilations and recompilations, and thus may be useful - for example, when profiling an action that happens during application startup, when it appears that total instrumentation results in too much overhead.

Total instrumentation scheme.

This scheme is used in the Analyze Performance -> Entire application profiling task.

With this scheme selected, the IDE works like most other profilers, i.e. instruments all of the target application's methods immediately when a class containing these methods is loaded. This scheme may result in a huge overhead for long-running applications, but it usually gives more accurate results and may even impose a smaller overhead if used for profiling application startup or for profiling relatively small, short-running applications such as command-line utilities. This is because it does not suffer from the repeated method decompilation and recompilation problem that occurs when using the lazy instrumentation scheme.

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