Ways to Improve Performance

Profile-Based Optimization

Sharing the flow.data File Among Multiple Processes

A flow.data file can potentially be accessed by several processes at the same time. For example, this could happen when you run more than one instrumented program at the same time in the same directory, or when profiling one program while linking another with -P.

Such asynchronous access to the file could potentially corrupt the data. To prevent simultaneous access to the flow.data file in a particular directory, a lock file called flow.lock is used. Instrumented programs that need to update the flow.data file and linker processes that need to read it must first obtain access to the lock file. Only one process can hold the lock at any time. As long as the flow.data file is being actively read and written, a process will wait for the lock to become available.

A program that terminates abnormally may leave the flow.data file inactive but locked. A process that tries to access an inactive but locked flow.data file gives up after a short period of time. In such cases, you may need to remove the flow.lock file.

If an instrumented program fails to obtain the database lock, it writes the profile data to a temporary file and displays a warning message containing the name of the file. You could then use the +df option along with the +P option while optimizing, to specify the name of the temporary file instead of the flow.data file.

If the linker fails to obtain the lock, it displays an error message and terminates. In such cases, wait until all active processes that are reading or writing a profile database file in that directory have completed. If no such processes exist, remove the flow.lock file.

Forking an Instrumented Application

When instrumenting an application that creates a copy of itself with the fork system call, you must ensure that the child process calls a special function named _clear_counters(), which clears all internal profile data. If you don't do this, the child process inherits the parent's profile data, updating the data as it executes, resulting in inaccurate (exaggerated) profile data when the child terminates. The following code segment shows a valid way to call _clear_counters:

if ((pid = fork()) == 0) /* this is the child process */

{

_clear_counters();

/*

reset

profile data

for child */

. . .

/*

other

code for the

child */

}

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