Kernel profiling

Kernel profiling is a mechanism that allows you to determine where the operating system is spending its time during operation. It consists of commands that control the profiling process and commands that generate reports. (See profiler(1M) for a description of these commands.) The system profiler samples the program counter on every clock interrupt and increments the counter corresponding to the kernel function shown by that value of the program counter.

When enabled, the system profiler initializes the sampling mechanism with the addresses of kernel functions. During initialization, any dynamically loadable kernel modules that are present in memory are locked in place. Likewise, any modules that are later loaded are locked in place if profiling is enabled. Therefore it's important to disable profiling after your measurements are complete; if you don't, the dynamically loaded modules remain resident in memory, decreasing the amount of memory available for other purposes and making your system run slower than usual.

Here's a brief description of the profiling commands:

• prfld initializes (loads) the profiler with kernel function addresses. This command is no longer required, but is still available for compatibility, or when you have booted off a kernel other than /stand/unix.

• prfstat turns the sampling mechanism on or off. If no arguments are specified, it reports whether profiling is loaded, enabled, or disabled. If profiling has not been initialized, you can initialize it by entering prfstat on. When profiling is disabled (via prfstat off), the profiler is de-initialized and dynamically loadable kernel modules are no longer locked in memory.

• The prfdc and prfsnap commands collect profiling data and save them in a file. If profiling isn't initialized, prfdc and prfsnap initialize it. prfsnap collects one set of the profiling counters and then exits. prfdc collects multiple sets of the profiling counters over a predetermined interval of time. Because the amount of time spent in each function is determined by the difference between two sets of samples, at least two samples of the counters must be taken to print the profiling results. Multiple sets of samples are printed as the difference between each subsequent set. Hence prfsnap can be used repeatedly to collect samples in the same file.

• The prfpr command prints the profiling results from the data collected by either prfdc or prfsnap.

To operate the system profiler, run either prfdc or prfsnap to collect samples of the profiling counters, and run prfpr to print the profiling results. When profiling is complete, run prfstat off to disable the profiler and unlock the dynamically loadable kernel modules.

For example:

   /usr/sbin/prfsnap temp
     <some time passes, with or without system activity>
   prfsnap temp
   prfpr temp
   prfstat off

If you want the profiler to begin automatically when you boot the system to multiuser state, you can add the following lines to the /etc/init.d/perf file:

   /usr/sbin/prfstat on

   profiling enabled
   

   xxx kernel text addresses loaded