Provided by: libpcp3-dev_6.3.0-1_amd64 bug

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 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).