43.2.2. Setting Events to Monitor
Most processors contain counters, which are used by OProfile to monitor specific events. As shown in Table 43.2, “OProfile Processors and Counters”, the number of counters available depends on the processor.
Table 43.2. OProfile Processors and Counters
|Processor|| ||Number of Counters|
|Pentium 4 (non-hyper-threaded)||i386/p4||8|
|Pentium 4 (hyper-threaded)||i386/p4-ht||4|
|IBM eServer iSeries and pSeries||timer||1|
|IBM eServer S/390 and S/390x||timer||1|
|IBM eServer zSeries||timer||1|
Use Table 43.2, “OProfile Processors and Counters” to verify that the correct processor type was detected and to determine the number of events that can be monitored simultaneously.
timeris used as the processor type if the processor does not have supported performance monitoring hardware.
timeris used, events cannot be set for any processor because the hardware does not have support for hardware performance counters. Instead, the timer interrupt is used for profiling.
timeris not used as the processor type, the events monitored can be changed, and counter 0 for the processor is set to a time-based event by default. If more than one counter exists on the processor, the counters other than counter 0 are not set to an event by default. The default events monitored are shown in Table 43.3, “Default Events”.
Table 43.3. Default Events
|Processor||Default Event for Counter||Description|
|Pentium Pro, Pentium II, Pentium III, Athlon, AMD64||CPU_CLK_UNHALTED||The processor's clock is not halted|
|Pentium 4 (HT and non-HT)||GLOBAL_POWER_EVENTS||The time during which the processor is not stopped|
|Itanium 2||CPU_CYCLES||CPU Cycles|
|TIMER_INT||(none)||Sample for each timer interrupt|
The number of events that can be monitored at one time is determined by the number of counters for the processor. However, it is not a one-to-one correlation; on some processors, certain events must be mapped to specific counters. To determine the number of counters available, execute the following command:
ls -d /dev/oprofile/[0-9]*
The events available vary depending on the processor type. To determine the events available for profiling, execute the following command as root (the list is specific to the system's processor type):
The events for each counter can be configured via the command line or with a graphical interface. For more information on the graphical interface, refer to Section 43.8, “Graphical Interface”. If the counter cannot be set to a specific event, an error message is displayed.
To set the event for each configurable counter via the command line, use
Replace <event-name> with the exact name of the event from
ophelp, and replace <sample-rate> with the number of events between samples.
188.8.131.52. Sampling Rate
By default, a time-based event set is selected. It creates a sample every 100,000 clock cycles per processor. If the timer interrupt is used, the timer is set to whatever the jiffy rate is and is not user-settable. If the
timer, each event can have a sampling rate set for it. The sampling rate is the number of events between each sample snapshot.
When setting the event for the counter, a sample rate can also be specified:
Replace <sample-rate> with the number of events to wait before sampling again. The smaller the count, the more frequent the samples. For events that do not happen frequently, a lower count may be needed to capture the event instances.
Be extremely careful when setting sampling rates. Sampling too frequently can overload the system, causing the system to appear as if it is frozen or causing the system to actually freeze.