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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
pmLookupDesc routine fills in the given pmDesc structure, pointed
to by the parameter desc, from the current Performance Metrics
Application 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 metrics increases, as it avoids round trip latency from
multiple individual 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 reason, 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
formats) 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
value 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 interpretation 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 structured 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
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
-1
Space.Time
while the running total of system calls is Count, memory
allocation 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
provided to manipulate values in conjunction with the pmUnits
structure that defines the dimension and scale of the values for
a particular 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
applications 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
encountered, the return value is the number of metric
descriptors 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
protocol 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.
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 2024-06-14.
(At that time, the date of the most recent commit that was found
in the repository was 2024-06-14.) 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 2024-06-26 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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