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 domain.

       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 page.

       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"

       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.

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.

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.

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 2023-12-22.
       (At that time, the date of the most recent commit that was found
       in the repository was 2023-12-16.)  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

Performance Co-Pilot               PCP                   PMDAINSTANCE(3)

Pages that refer to this page: pmlogrewrite(1)pmda(3)pmdacache(3)pmdadaemon(3)pmdadso(3)pmdamain(3)