PMLOOKUPDESC(3)

NAME
pmLookupDesc, pmLookupDescs - obtain descriptions for performance metrics

C SYNOPSIS
#include <pcp/pmapi.h>

int pmLookupDesc(pmID pmid, pmDesc *desc);
int pmLookupDescs(int numpmid, pmID *pmids, pmDesc *descs);

cc ... -lpcp

DESCRIPTION
Given a Performance Metrics Identifier (PMID) as pmid, the pmLookupDesc routine fills in the given 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 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 field in the 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
while the running total of system calls is memory allocation is and average service time is
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 structure which is embedded in the 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 structure that defines the dimension and scale of the values for a particular performance metric.

Below the PMAPI, the information required to complete the 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).