CPC(3CPC) CPU Performance Counters Library Functions CPC(3CPC)


NAME


cpc - hardware performance counters

DESCRIPTION


Modern microprocessors contain hardware performance counters that allow
the measurement of many different hardware events related to CPU
behavior, including instruction and data cache misses as well as various
internal states of the processor. The counters can be configured to count
user events, system events, or both. Data from the performance counters
can be used to analyze and tune the behavior of software on a particular
type of processor.


Most processors are able to generate an interrupt on counter overflow,
allowing the counters to be used for various forms of profiling.


This manual page describes a set of APIs that allow Solaris applications
to use these counters. Applications can measure their own behavior, the
behavior of other applications, or the behavior of the whole system.

Shared Counters or Private Counters


There are two principal models for using these performance counters. Some
users of these statistics want to observe system-wide behavior. Other
users want to view the performance counters as part of the register set
exported by each LWP. On a machine performing more than one activity,
these two models are in conflict because the counters represent a
critical hardware resource that cannot simultaneously be both shared and
private.

Configuration Interfaces


The following configuration interfaces are provided:

cpc_open(3CPC)
Check the version the application was compiled with
against the version of the library.


cpc_cciname(3CPC)
Return a printable string to describe the
performance counters of the processor.


cpc_npic(3CPC)
Return the number of performance counters on the
processor.


cpc_cpuref(3CPC)
Return a reference to documentation that should be
consulted to understand how to use and interpret
data from the performance counters.


Performance Counter Access


Performance counters can be present in hardware but not acccessible
because either some of the necessary system software components are not
available or not installed, or the counters might be in use by other
processes. The cpc_open(3CPC) function determines the accessibility of
the counters and must be invoked before any attempt to program the
counters.

Finding Events


Each different type of processor has its own set of events available for
measurement. The cpc_walk_events_all(3CPC) and cpc_walk_events_pic(3CPC)
functions allow an application to determine the names of events supported
by the underlying processor. A collection of generic, platform
independent event names are defined by generic_events(3CPC). Each
generic event maps to an underlying hardware event specific to the
underlying processor and any optional attributes. The
cpc_walk_generic_events_all(3CPC) and cpc_walk_generic_events_pic(3CPC)
functions allow an application to determine the generic events supported
on the underlying platform.

Using Attributes


Some processors have advanced performance counter capabilities that are
configured with attributes. The cpc_walk_attrs(3CPC) function can be used
to determine the names of attributes supported by the underlying
processor. The documentation referenced by cpc_cpuref(3CPC) should be
consulted to understand the meaning of a processor's performance counter
attributes.

Performance Counter Context


Each processor on the system possesses its own set of performance counter
registers. For a single process, it is often desirable to maintain the
illusion that the counters are an intrinsic part of that process
(whichever processors it runs on), since this allows the events to be
directly attributed to the process without having to make passive all
other activity on the system.


To achieve this behavior, the library associates performance counter
context with each LWP in the process. The context consists of a small
amount of kernel memory to hold the counter values when the LWP is not
running, and some simple kernel functions to save and restore those
counter values from and to the hardware registers when the LWP performs a
normal context switch. A process can only observe and manipulate its own
copy of the performance counter control and data registers.

Performance Counters In Other Processes


Though applications can be modified to instrument themselves as
demonstrated above, it is frequently useful to be able to examine the
behavior of an existing application without changing the source code. A
separate library, libpctx, provides a simple set of interfaces that use
the facilities of proc(4) to control a target process, and together with
functions in libcpc, allow truss-like tools to be constructed to measure
the performance counters in other applications. An example of one such
application is cputrack(1).


The functions in libpctx are independent of those in libcpc. These
functions manage a process using an event-loop paradigm -- that is, the
execution of certain system calls by the controlled process cause the
library to stop the controlled process and execute callback functions in
the context of the controlling process. These handlers can perform
various operations on the target process using APIs in libpctx and libcpc
that consume pctx_t handles.

SEE ALSO


cputrack(1), cpustat(1M), cpc_bind_curlwp(3CPC), cpc_buf_create(3CPC),
cpc_enable(3CPC), cpc_npic(3CPC), cpc_open(3CPC), cpc_set_create(3CPC),
cpc_seterrhndlr(3CPC), generic_events(3CPC), libcpc(3LIB),
pctx_capture(3CPC), pctx_set_events(3CPC), proc(4)


October 8, 2008 CPC(3CPC)