Provided by: libpcp3-dev_4.3.1-1_amd64 bug

NAME

       pmLookupDesc - obtain a description for a performance metric

C SYNOPSIS

       #include <pcp/pmapi.h>

       int pmLookupDesc(pmID pmid, pmDesc *desc);

       cc ... -lpcp

DESCRIPTION

       Given a Performance Metrics Identifier (PMID) as pmid, fill in the given pmDesc structure,
       pointed to by the  parameter  desc,  from  the  current  Performance  Metrics  Application
       Programming Interface (PMAPI) context.

       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.

SEE ALSO

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

DIAGNOSTICS

       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