NAME

       pmLookupDesc - obtain a description for a performance met­
       ric


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_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.

       tion  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  Sub­
       system  (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  (or  indeed PM_TYPE_STRING), the inter­
       pretation of the value is unknown to the PMCS.   In  these
       cases,  the application using the value, and the PMDA pro­
       viding 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  val­
       ues  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 */
                int scaleSpace:4;  /* one of PM_SPACE_* below */
                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_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  instrumen­
       tation 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 ser­
       vices provided by the PMAPI will continue to function cor­
       rectly.


SEE ALSO

       PMAPI(3), pmAtomStr(3), pmConvScale(3), pmExtractValue(3),
       pmGetConfig(3), pmTypesStr(3), pmUnitsStr(3),  pcp.conf(4)
       and pcp.env(4).


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


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