pmdaInstance(3) — Linux manual page


PMDAINSTANCE(3)           Library Functions Manual           PMDAINSTANCE(3)

NAME         top

       pmdaInstance - return instance descriptions for a PMDA

C SYNOPSIS         top

       #include <pcp/pmapi.h>
       #include <pcp/pmda.h>

       int pmdaInstance(pmInDom indom, int inst, char *name,
               pmInResult **result, pmdaExt *pmda);

       cc ... -lpcp_pmda -lpcp

DESCRIPTION         top

       pmdaInstance uses the standard PMDA(3) data structures to return
       information concerning the instance domain indom.

       The result structure is constructed by pmdaInstance and will contain
       one or more instance names and/or identifiers as specified by the
       inst and name arguments.

       If inst has the value PM_IN_NULL and name is a null string, result
       will contain all the instances names and identifiers in the instance

       If inst is PM_IN_NULL but name is the name of an instance in the
       instance domain indom, then result will contain the instance
       identifier for instance name.  Note that if name contains no spaces,
       partial matching up to the first space in the instance name is
       performed, i.e.  ``1'' will match instance name ``1 minute''.  If
       name contains an embedded space, then no partial matching is
       performed and name should match one of the instance names exactly.

       If name is a null string but inst is an instance identifier in the
       instance domain indom, then result will contain the name for instance
       inst.  The result structure is allocated with malloc(3) and should be
       released by the caller with free(3).


       Further to the above description of name, the set of rules describing
       external instance names is provided in the pmdaCacheStore(3) manual

       Instance domains adds another dimension (set of values) to metrics.
       However, this may not suffice to describe complex multi-dimensional
       instance domain situations.  For this case the approach used by a
       number of PMDAs is to structure the external instance names using a
       delimiter (``/'' or ``::'' are most commonly used) to allow
       separation of the other dimensions.  In this situation, instance
       domain labels should be used to define names for each instance name
       component.  This allows PMAPI(3) client tools to identify and refine
       value fetches to specific dimensions of interest.

       For example, some of the Linux kernel cgroup (control group) metric
       instance domains are multi-dimensional.  The instance domain
       represents individual values across both control groups and CPUs,
       making this a two-dimensional instance domain.  The instance names
       associated with this cgroup metrics indom have been structured using
       the ``::'' delimiter to separate the two dimensions.  The instance
       domain itself has been labeled accordingly, as follows.

       $ pminfo --desc --fetch --labels cgroup.cpuacct.usage_percpu
            Data Type: 64-bit unsigned int  InDom: 3.22 0xc00016
            Semantics: counter  Units: nanosec
            inst [0 or "/::cpu0"] value 713787
            inst [1 or "/::cpu1"] value 353969
            inst [2 or "/app::cpu0"] value 407816
            inst [3 or "/app::cpu1"] value 202747
            inst [0 or "/::cpu0"] labels {"device_type":"cpu","cgroup":"/","cpu":0}
            inst [1 or "/::cpu1"] labels {"device_type":"cpu","cgroup":"/","cpu":1}
            inst [2 or "/app::cpu0"] labels {"device_type":"cpu","cgroup":"/app","cpu":0}
            inst [3 or "/app::cpu1"] labels {"device_type":"cpu","cgroup":"/app","cpu":1}

       $ pminfo --labels 3.22
       InDom: 3.22 0xc00016
            labels {"device_type":"cpu"}

       As shown above the individual instances inherit the labels from the
       instance domain, and the PMDA also applies additional per-instance
       labels describing individual cgroup and CPU names.  When this model
       has been used by the PMDA, PMAPI clients are able to restrict their
       queries to the cgroup metric instances - in the example, restricting
       to processor "cpu0" using the "cpu" label, perhaps, or to just the
       "/app" cgroup metrics using the "cgroup" label.

       Furthermore, using this labeling scheme client tools can also
       correlate related instances across different instance domains.

       $ pminfo --desc --fetch --labels kernel.percpu.cpu.irq.soft
            Data Type: 64-bit unsigned int  InDom: 60.0 0xf000000
            Semantics: counter  Units: millisec
            inst [0 or "cpu0"] value 6770
            inst [1 or "cpu1"] value 100
            inst [0 or "cpu0"] labels {"device_type":"cpu"}
            inst [1 or "cpu1"] labels {"device_type":"cpu"}

       $ pminfo --labels 60.0
       InDom: 60.0 0xf000000
            labels {"device_type":"cpu"}

       Although these two metrics have different instance domains (60.0 and
       3.22 respectively) and are sourced from different PMDAs, the
       "device_type" label identifies the common device to which these
       values relate.

DIAGNOSTICS         top

       If any errors occur during the execution of pmdaInstance, the result
       structure is deallocated.  If the instance domain indom is not
       supported by the PMDA, pmdaInstance will return PM_ERR_INDOM.

       If the inst or name does not correspond to any instances in the indom
       domain, pmdaInstance will return PM_ERR_INST.

CAVEAT         top

       The PMDA must be using PMDA_INTERFACE_2 or later, as specified in the
       call to pmdaDSO(3) or pmdaDaemon(3).  If labeling of multi-
       dimensional instance names is performed, the PMDA must use
       PMDA_INTERFACE_7 or later.

       Because of optional partial matching up to the first space in the
       instance name, the PMDA developer should ensure that if instance
       names are allowed to have spaces, the names are unique up to the
       first space.

SEE ALSO         top

       malloc(3), PMAPI(3), PMDA(3), pmdaCacheStore(3), pmdaLabel(3) and

COLOPHON         top

       This page is part of the PCP (Performance Co-Pilot) project.
       Information about the project can be found at ⟨⟩.
       If you have a bug report for this manual page, send it to  This page was obtained from the project's upstream
       Git repository ⟨⟩ on
       2020-11-01.  (At that time, the date of the most recent commit that
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       (which is not part of the original manual page), send a mail to

Performance Co-Pilot                 PCP                     PMDAINSTANCE(3)

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