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NOTE: CentOS Enterprise Linux 5 is built from the Red Hat Enterprise Linux source code. Other than logo and name changes CentOS Enterprise Linux 5 is compatible with the equivalent Red Hat version. This document applies equally to both Red Hat and CentOS Enterprise Linux 5.

38.2. Configuring OProfile

Before OProfile can be run, it must be configured. At a minimum, selecting to monitor the kernel (or selecting not to monitor the kernel) is required. The following sections describe how to use the opcontrol utility to configure OProfile. As the opcontrol commands are executed, the setup options are saved to the /root/.oprofile/daemonrc file.

38.2.1. Specifying the Kernel

First, configure whether OProfile should monitor the kernel. This is the only configuration option that is required before starting OProfile. All others are optional.

To monitor the kernel, execute the following command as root:

opcontrol --setup --vmlinux=/usr/lib/debug/lib/modules/`uname -r`/vmlinux

Note

The debuginfo package must be installed (which contains the uncompressed kernel) in order to monitor the kernel.

To configure OProfile not to monitor the kernel, execute the following command as root:

opcontrol --setup --no-vmlinux

This command also loads the oprofile kernel module, if it is not already loaded, and creates the /dev/oprofile/ directory, if it does not already exist. Refer to Section 38.6, “Understanding /dev/oprofile/” for details about this directory.

Note

Even if OProfile is configured not to profile the kernel, the SMP kernel still must be running so that the oprofile module can be loaded from it.

Setting whether samples should be collected within the kernel only changes what data is collected, not how or where the collected data is stored. To generate different sample files for the kernel and application libraries, refer to Section 38.2.3, “Separating Kernel and User-space Profiles”.

38.2.2. Setting Events to Monitor

Most processors contain counters, which are used by OProfile to monitor specific events. As shown in Table 38.2, “OProfile Processors and Counters”, the number of counters available depends on the processor.

Processor cpu_type Number of Counters
Pentium Pro i386/ppro 2
Pentium II i386/pii 2
Pentium III i386/piii 2
Pentium 4 (non-hyper-threaded) i386/p4 8
Pentium 4 (hyper-threaded) i386/p4-ht 4
Athlon i386/athlon 4
AMD64 x86-64/hammer 4
Itanium ia64/itanium 4
Itanium 2 ia64/itanium2 4
TIMER_INT timer 1
IBM eServer iSeries and pSeries timer 1
  ppc64/power4 8
  ppc64/power5 6
  ppc64/970 8
IBM eServer S/390 and S/390x timer 1
IBM eServer zSeries timer 1

Table 38.2. OProfile Processors and Counters

Use Table 38.2, “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. timer is used as the processor type if the processor does not have supported performance monitoring hardware.

If timer is 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.

If timer is 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 38.3, “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
ppc64/power4 CYCLES Processor Cycles
ppc64/power5 CYCLES Processor Cycles
ppc64/970 CYCLES Processor Cycles

Table 38.3. Default Events

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):

ophelp

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 38.8, “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 opcontrol:

opcontrol --event=<event-name>:<sample-rate>

Replace <event-name> with the exact name of the event from ophelp, and replace <sample-rate> with the number of events between samples.

38.2.2.1. 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 cpu_type is not 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:

opcontrol --event=<event-name>:<sample-rate>

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.

Caution

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.

38.2.2.2. Unit Masks

Some user performance monitoring events may also require unit masks to further define the event.

Unit masks for each event are listed with the ophelp command. The values for each unit mask are listed in hexadecimal format. To specify more than one unit mask, the hexadecimal values must be combined using a bitwise or operation.

opcontrol --event=<event-name>:<sample-rate>:<unit-mask>

38.2.3. Separating Kernel and User-space Profiles

By default, kernel mode and user mode information is gathered for each event. To configure OProfile to ignore events in kernel mode for a specific counter, execute the following command:

opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:0

Execute the following command to start profiling kernel mode for the counter again:

opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:1

To configure OProfile to ignore events in user mode for a specific counter, execute the following command:

opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:<kernel>:0

Execute the following command to start profiling user mode for the counter again:

opcontrol --event=<event-name>:<sample-rate>:<unit-mask>:<kernel>:1

When the OProfile daemon writes the profile data to sample files, it can separate the kernel and library profile data into separate sample files. To configure how the daemon writes to sample files, execute the following command as root:

opcontrol --separate=<choice>

<choice> can be one of the following:

  • none — do not separate the profiles (default)

  • library — generate per-application profiles for libraries

  • kernel — generate per-application profiles for the kernel and kernel modules

  • all — generate per-application profiles for libraries and per-application profiles for the kernel and kernel modules

If --separate=library is used, the sample file name includes the name of the executable as well as the name of the library.

Note

These configuration changes will take effect when oprofile is restarted.


 
 
  Published under the terms of the GNU General Public License Design by Interspire