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NAME | C SYNOPSIS | DESCRIPTION | MULTI-DIMENSIONAL INSTANCE NAMING | CAVEAT | DIAGNOSTICS | SEE ALSO | COLOPHON |
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PMDAINSTANCE(3) Library Functions Manual PMDAINSTANCE(3)
pmdaInstance - return instance descriptions for a PMDA
#include <pcp/pmapi.h>
#include <pcp/pmda.h>
int pmdaInstance(pmInDom indom, int inst, char *name,
pmInResult **result, pmdaExt *pmda);
cc ... -lpcp_pmda -lpcp
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 con‐
tain 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 in‐
stance 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 iden‐
tifier 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 per‐
formed 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 in‐
stance 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 de‐
scribing external instance names is provided in the
pmdaCacheStore(3) manual page.
Instance domains adds another dimension (set of values) to met‐
rics. However, this may not suffice to describe complex multi-di‐
mensional 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 in‐
stance name component. This allows PMAPI(3) client tools to iden‐
tify and refine value fetches to specific dimensions of interest.
For example, some of the Linux kernel cgroup (control group) met‐
ric 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 us‐
ing the ``::'' delimiter to separate the two dimensions. The in‐
stance domain itself has been labeled accordingly, as follows.
$ pminfo --desc --fetch --labels cgroup.cpuacct.usage_percpu
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-in‐
stance 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 ex‐
ample, restricting to processor "cpu0" using the "cpu" label, per‐
haps, or to just the "/app" cgroup metrics using the "cgroup" la‐
bel.
Furthermore, using this labeling scheme client tools can also cor‐
relate related instances across different instance domains.
$ pminfo --desc --fetch --labels kernel.percpu.cpu.irq.soft
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.
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-di‐
mensional instance names is performed, the PMDA must use PMDA_IN‐
TERFACE_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.
If any errors occur during the execution of pmdaInstance, the re‐
sult 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.
malloc(3), PMAPI(3), PMDA(3), pmdaCacheStore(3), pmdaLabel(3) and
pmGetInDom(3).
This page is part of the PCP (Performance Co-Pilot) project. In‐
formation 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
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-11.) 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 PMDAINSTANCE(3)
Pages that refer to this page: pmlogrewrite(1), pmda(3), pmdacache(3), pmdadaemon(3), pmdadso(3), pmdamain(3)
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HTML rendering created 2025-09-06 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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