PMAPI(3) Library Functions Manual PMAPI(3)
PMAPI - introduction to the Performance Metrics Application Program‐
ming 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 archive logs 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
archive logs, this would be the time when the metrics were written
into the archive log.
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 archive logs.
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), pmGetConfig(3), pcp.conf(5) and
pcp.env(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
pcp@groups.io. This page was obtained from the project's upstream
Git repository ⟨https://github.com/performancecopilot/pcp.git⟩ on
2018-02-02. (At that time, the date of the most recent commit that
was found in the repository was 2018-02-02.) 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
man-pages@man7.org
Performance Co-Pilot PCP PMAPI(3)
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