PMLOOKUPDESC(3) | Library Functions Manual | PMLOOKUPDESC(3) |

# NAME

**pmLookupDesc**, **pmLookupDescs** - obtain descriptions
for performance metrics

# C SYNOPSIS

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

int pmLookupDesc(pmIDpmid, pmDesc *desc); int pmLookupDescs(intnumpmid, pmID *pmids, pmDesc *descs);

cc ... -lpcp

# DESCRIPTION

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.

# DIAGNOSTICS

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.

# SEE ALSO

PMAPI(3), pmAtomStr(3), pmConvScale(3), pmExtractValue(3), pmGetConfig(3), pmTypeStr(3), pmUnitsStr(3), pcp.conf(5) and pcp.env(5).

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