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PMAPI(3) Library Functions Manual PMAPI(3)
PMAPI - introduction to the Performance Metrics Application
Programming Interface
#include <pcp/pmapi.h>
... assorted routines ...
cc ... -lpcp
Within the framework of the Performance Co-Pilot (PCP), client
applications are developed using the Performance Metrics
Application Programming Interface (PMAPI) that defines a
procedural interface with services suited to the development of
applications with a particular interest in performance metrics.
This description presents an overview of the PMAPI and the
context in which PMAPI applications are run. The PMAPI is more
fully described in the Performance Co-Pilot Programmer's Guide,
and the manual pages for the individual PMAPI routines.
For a description of the Performance Metrics Name Space (PMNS)
and associated terms and concepts, see PCPIntro(1).
Not all PMIDs need be represented in the PMNS of every
application. For example, an application which monitors disk
traffic will likely use a name space which references only the
PMIDs for I/O statistics.
Applications which use the PMAPI may have independent versions of
a PMNS, constructed from an initialization file when the
application starts; see pmLoadASCIINameSpace(3),
pmLoadNameSpace(3), and PMNS(5).
Internally (below the PMAPI) the implementation of the
Performance Metrics Collection System (PMCS) uses only the PMIDs,
and a PMNS provides an external mapping from a hierarchic
taxonomy of names to PMIDs that is convenient in the context of a
particular system or particular use of the PMAPI. For the
applications programmer, the routines pmLookupName(3) and
pmNameID(3) translate between names in a PMNS and PMIDs, and vice
versa. The PMNS may be traversed using pmGetChildren(3)
andpmTraversePMNS. The pmFetchGroup(3) functions combine metric
name lookup, fetch, and conversion operations.
An application using the PMAPI may manipulate several concurrent
contexts, each associated with a source of performance metrics,
e.g. pmcd(1) on some host, or a set of archives of performance
metrics as created by pmlogger(1).
Contexts are identified by a ``handle'', a small integer value
that is returned when the context is created; see pmNewContext(3)
and pmDupContext(3). Some PMAPI functions require an explicit
``handle'' to identify the correct context, but more commonly the
PMAPI function is executed in the ``current'' context. The
current context may be discovered using pmWhichContext(3) and
changed using pmUseContext(3).
If a PMAPI context has not been explicitly established (or the
previous current context has been closed using
pmDestroyContext(3)) then the current PMAPI context is undefined.
In addition to the source of the performance metrics, the context
also includes the instance profile and collection time (both
described below) which controls how much information is returned,
and when the information was collected.
When performance metric values are returned across the PMAPI to a
requesting application, there may be more than one value for a
particular metric. Multiple values, or instances, for a single
metric are typically the result of instrumentation being
implemented for each instance of a set of similar components or
services in a system, e.g. independent counts for each CPU, or
each process, or each disk, or each system call type, etc. This
multiplicity of values is not enumerated in the name space but
rather, when performance metrics are delivered across the PMAPI
by pmFetch(3), the format of the result accommodates values for
one or more instances, with an instance-value pair encoding the
metric value for a particular instance.
The instances are identified by an internal identifier assigned
by the agent responsible for instantiating the values for the
associated performance metric. Each instance identifier has a
corresponding external instance identifier name (an ASCII
string). The routines pmGetInDom(3), pmLookupInDom(3) and
pmNameInDom(3) may be used to enumerate all instance identifiers,
and to translate between internal and external instance
identifiers.
All of the instance identifiers for a particular performance
metric are collectively known as an instance domain. Multiple
performance metrics may share the same instance domain.
If only one instance is ever available for a particular
performance metric, the instance identifier in the result from
pmFetch(3) assumes the special value PM_IN_NULL and may be
ignored by the application, and only one instance-value pair
appears in the result for that metric. Under these
circumstances, the associated instance domain (as returned via
pmLookupDesc(3)) is set to PM_INDOM_NULL to indicate that values
for this metric are singular.
The difficult issue of transient performance metrics (e.g. per-
filesystem information, hot-plug replaceable hardware modules,
etc.) means that repeated requests for the same PMID may return
different numbers of values, and/or some changes in the
particular instance identifiers returned. This means
applications need to be aware that metric instantiation is
guaranteed to be valid at the time of collection only. Similar
rules apply to the transient semantics of the associated metric
values. In general however, it is expected that the bulk of the
performance metrics will have instantiation semantics that are
fixed over the execution life-time of any PMAPI client.
The PMAPI supports a wide range of format and type encodings for
the values of performance metrics, namely signed and unsigned
integers, floating point numbers, 32-bit and 64-bit encodings of
all of the above, ASCII strings (C-style, NULL byte terminated),
and arbitrary aggregates of binary data.
The type field in the pmDesc structure returned by
pmLookupDesc(3) identifies the format and type of the values for
a particular performance metric within a particular PMAPI
context.
Note that the encoding of values for a particular performance
metric may be different for different PMAPI contexts, due to
differences in the underlying implementation for different
contexts. However it is expected that the vast majority of
performance metrics will have consistent value encoding across
all versions of all implementations, and hence across all PMAPI
contexts.
The PMAPI supports routines to automate the handling of the
various value formats and types, particularly for the common case
where conversion to a canonical format is desired, see
pmExtractValue(3) and pmPrintValue(3).
Independent of how the value is encoded, the value for a
performance metric is assumed to be drawn from a set of values
that can be described in terms of their dimensionality and scale
by a compact encoding as follows. The dimensionality is defined
by a power, or index, in each of 3 orthogonal dimensions, namely
Space, Time and Count (or Events, which are dimensionless). For
example I/O throughput might be represented as Space/Time, while
the running total of system calls is Count, memory allocation is
Space and average service time is Time/Count. In each dimension
there are a number of common scale values that may be used to
better encode ranges that might otherwise exhaust the precision
of a 32-bit value. This information is encoded in the pmUnits
structure which is embedded in the pmDesc structure returned from
pmLookupDesc(3).
The routine pmConvScale(3) is provided to convert values in
conjunction with the pmUnits structures that defines the
dimensionality and scale of the values for a particular
performance metric as returned from pmFetch(3), and the desired
dimensionality and scale of the value the PMAPI client wishes to
manipulate. Alternatively, the pmFetchGroup(3) functions can
perform data format and unit conversion operations, specified by
textual descriptions of desired unit / scales.
The set of instances for performance metrics returned from a
pmFetch(3) call may be filtered or restricted using an instance
profile. There is one instance profile for each PMAPI context
the application creates, and each instance profile may include
instances from one or more instance domains.
The routines pmAddProfile(3) and pmDelProfile(3) may be used to
dynamically adjust the instance profile.
For each set of values for performance metrics returned via
pmFetch(3) there is an associated ``timestamp'' that serves to
identify when the performance metric values were collected; for
metrics being delivered from a real-time source (i.e. pmcd(1) on
some host) this would typically be not long before they were
exported across the PMAPI, and for metrics being delivered from a
set of archives, this would be the time when the metrics were
written into the archive.
There is an issue here of exactly when individual metrics may
have been collected, especially given their origin in potentially
different Performance Metric Domains, and variability in the
metric updating frequency at the lowest level of the Performance
Metric Domain. The PMCS opts for the pragmatic approach, in
which the PMAPI implementation undertakes to return all of the
metrics with values accurate as of the timestamp, to the best of
our ability. The belief is that the inaccuracy this introduces
is small, and the additional burden of accurate individual
timestamping for each returned metric value is neither warranted
nor practical (from an implementation viewpoint).
Of course, in the case of collection of metrics from multiple
hosts the PMAPI client must assume the sanity of the timestamps
is constrained by the extent to which clock synchronization
protocols are implemented across the network.
A PMAPI application may call pmSetMode(3) to vary the requested
collection time, e.g. to rescan performance metrics values from
the recent past, or to ``fast-forward'' through a set of
archives.
Across the PMAPI, all arguments and results involving a ``list of
something'' are declared to be arrays with an associated argument
or function value to identify the number of elements in the list.
This has been done to avoid both the varargs(3) approach and
sentinel-terminated lists.
Where the size of a result is known at the time of a call, it is
the caller's responsibility to allocate (and possibly free) the
storage, and the called function will assume the result argument
is of an appropriate size. Where a result is of variable size
and that size cannot be known in advance (e.g. for
pmGetChildren(3), pmGetInDom(3), pmNameInDom(3), pmNameID(3),
pmLookupLabels(3), pmLookupText(3) and pmFetch(3)) the PMAPI
implementation uses a range of dynamic allocation schemes in the
called routine, with the caller responsible for subsequently
releasing the storage when no longer required. In some cases
this simply involves calls to free(3), but in others (most
notably for the result from pmFetch(3)), special routines (e.g.
pmFreeResult(3) and pmFreeLabelSets(3)) should be used to release
the storage.
As a general rule, if the called routine returns an error status
then no allocation will have been done, and any pointer to a
variable sized result is undefined.
Where error conditions may arise, the functions that comprise the
PMAPI conform to a single, simple error notification scheme, as
follows;
+ the function returns an integer
+ values >= 0 indicate no error, and perhaps some positive
status, e.g. the number of things really processed
+ values < 0 indicate an error, with a global table of error
conditions and error messages
The PMAPI routine pmErrStr(3) translates error conditions into
error messages. By convention, the small negative values are
assumed to be negated versions of the Unix error codes as defined
in <errno.h> and the strings returned are as per strerror(3).
The larger, negative error codes are PMAPI error conditions.
One error, common to all PMAPI routines that interact with
pmcd(1) on some host is PM_ERR_IPC, which indicates the
communication link to pmcd(1) has been lost.
The original design for PCP was based around single-threaded
applications, or more strictly applications in which only one
thread was ever expected to call the PCP libraries. This
restriction has been relaxed for libpcp to allow the most common
PMAPI routines to be safely called from any thread in a multi-
threaded application.
However the following groups of functions and services in libpcp
are still restricted to being called from a single-thread, and
this is enforced by returning PM_ERR_THREAD when an attempt to
call the routines in each group from more than one thread is
detected.
1. Any use of a PM_CONTEXT_LOCAL context, as the DSO PMDAs that
are called directly from libpcp may not be thread-safe.
Most environment variables are described in PCPIntro(1). In
addition, environment variables with the prefix PCP_ are used to
parameterize the file and directory names used by PCP. On each
installation, the file /etc/pcp.conf contains the local values
for these variables. The $PCP_CONF variable may be used to
specify an alternative configuration file, as described in
pcp.conf(5). Values for these variables may be obtained
programmatically using the pmGetConfig(3) function.
PCPIntro(1), PCPIntro(3), PMDA(3), PMWEBAPI(3), pmGetConfig(3),
pcp.conf(5), pcp.env(5) and PMNS(5).
This page is part of the PCP (Performance Co-Pilot) project.
Information about the project can be found at
⟨http://www.pcp.io/⟩. If you have a bug report for this manual
page, send it to [email protected]. This page was obtained from the
project's upstream Git repository
⟨https://github.com/performancecopilot/pcp.git⟩ on 2024-06-14.
(At that time, the date of the most recent commit that was found
in the repository was 2024-06-14.) If you discover any rendering
problems in this HTML version of the page, or you believe there
is a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
Performance Co-Pilot PCP PMAPI(3)
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HTML rendering created 2024-06-26 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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