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NAME | C SYNOPSIS | DESCRIPTION | DIAGNOSTICS | SEE ALSO | COLOPHON |
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PMLOOKUPDESC(3) Library Functions Manual PMLOOKUPDESC(3)
pmLookupDesc, pmLookupDescs - obtain descriptions for performance
metrics
#include <pcp/pmapi.h>
int pmLookupDesc(pmID pmid, pmDesc *desc);
int pmLookupDescs(int numpmid, pmID *pmids, pmDesc *descs);
cc ... -lpcp
Given a Performance Metrics Identifier (PMID) as pmid, the pm‐
LookupDesc routine fills in the given pmDesc structure, pointed to
by the parameter desc, from the current Performance Metrics Appli‐
cation Programming Interface (PMAPI) context.
The pmLookupDescs variant provides equivalent functionality for
numpmid metrics at once, with the pmids array providing the metric
identifiers to lookup. It is more efficient as the number of met‐
rics increases, as it avoids round trip latency from multiple in‐
dividual requests. Note that the error protocol guarantees there
is a 1:1 relationship between the elements of descs and pmids,
hence both lists contain exactly numpmid elements. For this rea‐
son, the caller is expected to have preallocated a suitably sized
array for descs.
The pmDesc structure provides all of the information required to
describe and manipulate a performance metric via the PMAPI, and
has the following declaration.
/* Performance Metric Descriptor */
typedef struct {
pmID pmid; /* unique identifier */
int type; /* base data type (see below) */
pmInDom indom; /* instance domain */
int sem; /* semantics of value (see below) *
pmUnits units; /* dimension and units (see below) */
} pmDesc;
/* pmDesc.type -- data type of metric values */
#define PM_TYPE_NOSUPPORT -1 /* not impl. in this version */
#define PM_TYPE_32 0 /* 32-bit signed integer */
#define PM_TYPE_U32 1 /* 32-bit unsigned integer */
#define PM_TYPE_64 2 /* 64-bit signed integer */
#define PM_TYPE_U64 3 /* 64-bit unsigned integer */
#define PM_TYPE_FLOAT 4 /* 32-bit floating point */
#define PM_TYPE_DOUBLE 5 /* 64-bit floating point */
#define PM_TYPE_STRING 6 /* array of char */
#define PM_TYPE_AGGREGATE 7 /* arbitrary binary data */
#define PM_TYPE_AGGREGATE_STATIC 8 /* static pointer to aggregate */
#define PM_TYPE_EVENT 9 /* packed pmEventArray */
#define PM_TYPE_UNKNOWN 255 /* used in pmValueBlock, not pmDesc */
/* pmDesc.sem -- semantics/interpretation of metric values */
#define PM_SEM_COUNTER 1 /* cumulative ctr (monotonic incr) */
#define PM_SEM_INSTANT 3 /* instant. value continuous domain */
#define PM_SEM_DISCRETE 4 /* instant. value discrete domain */
The type field in the pmDesc describes various encodings (or for‐
mats) for a metric's value.
If a value is counted in the underlying base instrumentation with
less than 32 bits of integer precision, it is the responsibility
of the Performance Metrics Domain Agent (PMDA) to promote the val‐
ue to a 32-bit integer before it is exported into the Performance
Metrics Collection Subsystem (PMCS); i.e. applications above the
PMAPI never have to deal with 8-bit and 16-bit counters.
If the value of a performance metric is of type PM_TYPE_AGGREGATE,
PM_TYPE_AGGREGATE_STATIC, PM_TYPE_EVENT or PM_TYPE_STRING, the in‐
terpretation of the value is unknown to the PMCS. In these cases,
the application using the value, and the PMDA providing the value
must have some common understanding about how the value is struc‐
tured and interpreted.
Each value for a performance metric is assumed to be drawn from a
set of values that can be described in terms of their dimensional‐
ity and scale by a compact encoding as follows. The dimensionali‐
ty is defined by a power, or index, in each of 3 orthogonal dimen‐
sions, namely Space, Time and Count (or Events, which are dimen‐
sionless). For example I/O throughput might be represented as
-1
Space.Time
while the running total of system calls is Count, memory alloca‐
tion is Space and average service time is
-1
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.
/*
* Encoding for the units (dimensions Time and Space) and scale
* for Performance Metric Values
*
* For example, a pmUnits struct of
* { 1, -1, 0, PM_SPACE_MBYTE, PM_TIME_SEC, 0 }
* represents Mbytes/sec, while
* { 0, 1, -1, 0, PM_TIME_HOUR, 6 }
* represents hours/million-events
*/
typedef struct {
int dimSpace:4; /* space dimension */
int dimTime:4; /* time dimension */
int dimCount:4; /* event dimension */
unsigned int scaleSpace:4; /* one of PM_SPACE_* below */
unsigned int scaleTime:4; /* one of PM_TIME_* below */
int scaleCount:4; /* one of PM_COUNT_* below */
} pmUnits; /* dimensional units and scale of value */
/* pmUnits.scaleSpace */
#define PM_SPACE_BYTE 0 /* bytes */
#define PM_SPACE_KBYTE 1 /* Kilobytes (1024) */
#define PM_SPACE_MBYTE 2 /* Megabytes (1024^2) */
#define PM_SPACE_GBYTE 3 /* Gigabytes (1024^3) */
#define PM_SPACE_TBYTE 4 /* Terabytes (1024^4) */
/* pmUnits.scaleTime */
#define PM_TIME_NSEC 0 /* nanoseconds */
#define PM_TIME_USEC 1 /* microseconds */
#define PM_TIME_MSEC 2 /* milliseconds */
#define PM_TIME_SEC 3 /* seconds */
#define PM_TIME_MIN 4 /* minutes */
#define PM_TIME_HOUR 5 /* hours */
/*
* pmUnits.scaleCount (e.g. count events, syscalls, interrupts,
* etc.) these are simply powers of 10, and not enumerated here,
* e.g. 6 for 10^6, or -3 for 10^-3
*/
#define PM_COUNT_ONE 0 /* 1 */
Special routines (e.g. pmExtractValue(3), pmConvScale(3)) are pro‐
vided to manipulate values in conjunction with the pmUnits struc‐
ture that defines the dimension and scale of the values for a par‐
ticular performance metric.
Below the PMAPI, the information required to complete the pmDesc
structure, is fetched from the PMDAs, and in this way the format
and scale of performance metrics may change dynamically, as the
PMDAs and their underlying instrumentation evolve with time. In
particular, when some metrics suddenly become 64-bits long, or
change their units from Mbytes to Gbytes, well-written applica‐
tions using the services provided by the PMAPI will continue to
function correctly.
These routines return a negative error code to indicate failure.
PM_ERR_PMID
The requested PMID is not known to the PMCS
PM_ERR_NOAGENT
The PMDA responsible for providing the metric is currently
not available
pmLookupDesc returns zero to indicate success.
The result from pmLookupDescs depends on the presence of any
lookup failures, their severity and the number of metrics being
looked up.
1. If there are no lookup failures, the return value will be
numpmid.
2. If a fatal error is encountered, the return value will be less
than 0. For example PM_ERR_IPC.
3. If numpmid is greater than one and non-fatal error(s) are en‐
countered, the return value is the number of metric descrip‐
tors that have successfully been looked up (greater than or
equal to zero and less than or equal to numpmid).
4. If numpmid is one and a non-fatal error is encountered, the
return value is the error code (less than zero).
When errors are encountered, any metrics that cannot be looked up
result in the corresponding descriptor element of descs having its
pmid field set to PM_ID_NULL. The slightly convoluted error pro‐
tocol allows bulk lookups, then probing for more error details in
the case of a specific failure.
PMAPI(3), pmAtomStr(3), pmConvScale(3), pmExtractValue(3),
pmGetConfig(3), pmTypeStr(3), pmUnitsStr(3), pcp.conf(5) and
pcp.env(5).
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
pcp@groups.io. 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 man-pages@man7.org
Performance Co-Pilot PCP PMLOOKUPDESC(3)
Pages that refer to this page: pmdaopenmetrics(1), pmgenmap(1), pmie(1), pmlogcheck(1), pmlogdump(1), pmlogrewrite(1), pmseries(1), sheet2pcp(1), pmaddprofile(3), pmapi(3), pmatomstr(3), pmconvscale(3), pmda(3), pmdadesc(3), pmdainit(3), pmdelprofile(3), pmextractvalue(3), pmfetch(3), pmgetderivedcontrol(3), pmgetindom(3), pmgetindomarchive(3), pmiaddmetric(3), pmidstr(3), pmindomstr(3), pmiunits(3), pmlookupindom(3), pmlookupindomarchive(3), pmlookupindomtext(3), pmlookuplabels(3), pmlookuptext(3), pmnameindom(3), pmnameindomarchive(3), pmparseunitsstr(3), pmprintdesc(3), pmprintvalue(3), pmregisterderived(3), pmsemstr(3), pmsetmode(3), pmtypestr(3), pmunitsstr(3), pmwebapi(3), QmcDesc(3), LOGARCHIVE(5)
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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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