1PMLOOKUPDESC(3)            Library Functions Manual            PMLOOKUPDESC(3)
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NAME

6       pmLookupDesc - obtain a description for a performance metric
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C SYNOPSIS

9       #include <pcp/pmapi.h>
10
11       int pmLookupDesc(pmID pmid, pmDesc *desc);
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13       cc ... -lpcp
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DESCRIPTION

16       Given  a  Performance  Metrics  Identifier  (PMID) as pmid, fill in the
17       given pmDesc structure, pointed to by the parameter desc, from the cur‐
18       rent Performance Metrics Application Programming Interface (PMAPI) con‐
19       text.
20
21       The pmDesc structure provides all of the information  required  to  de‐
22       scribe  and  manipulate a performance metric via the PMAPI, and has the
23       following declaration.
24
25            /* Performance Metric Descriptor */
26            typedef struct {
27                pmID    pmid;   /* unique identifier */
28                int     type;   /* base data type (see below) */
29                pmInDom indom;  /* instance domain */
30                int     sem;    /* semantics of value (see below) *
31                pmUnits units;  /* dimension and units (see below) */
32            } pmDesc;
33
34            /* pmDesc.type -- data type of metric values */
35            #define PM_TYPE_NOSUPPORT        -1    /* not impl. in this version */
36            #define PM_TYPE_32               0    /* 32-bit signed integer */
37            #define PM_TYPE_U32              1    /* 32-bit unsigned integer */
38            #define PM_TYPE_64               2    /* 64-bit signed integer */
39            #define PM_TYPE_U64              3    /* 64-bit unsigned integer */
40            #define PM_TYPE_FLOAT            4    /* 32-bit floating point */
41            #define PM_TYPE_DOUBLE           5    /* 64-bit floating point */
42            #define PM_TYPE_STRING           6    /* array of char */
43            #define PM_TYPE_AGGREGATE        7    /* arbitrary binary data */
44            #define PM_TYPE_AGGREGATE_STATIC 8    /* static pointer to aggregate */
45            #define PM_TYPE_EVENT            9    /* packed pmEventArray */
46            #define PM_TYPE_UNKNOWN          255  /* used in pmValueBlock, not pmDesc */
47
48
49            /* pmDesc.sem -- semantics/interpretation of metric values */
50            #define PM_SEM_COUNTER  1  /* cumulative ctr (monotonic incr) */
51            #define PM_SEM_INSTANT  3  /* instant. value continuous domain */
52            #define PM_SEM_DISCRETE 4  /* instant. value discrete domain */
53
54       The type field in the pmDesc describes various encodings  (or  formats)
55       for a metric's value.
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57       If  a value is counted in the underlying base instrumentation with less
58       than 32 bits of integer precision, it is the responsibility of the Per‐
59       formance  Metrics  Domain Agent (PMDA) to promote the value to a 32-bit
60       integer before it is exported into the Performance  Metrics  Collection
61       Subsystem  (PMCS); i.e. applications above the PMAPI never have to deal
62       with 8-bit and 16-bit counters.
63
64       If the value of a performance  metric  is  of  type  PM_TYPE_AGGREGATE,
65       PM_TYPE_AGGREGATE_STATIC,  PM_TYPE_EVENT  or PM_TYPE_STRING, the inter‐
66       pretation of the value is unknown to the PMCS.  In these cases, the ap‐
67       plication  using  the value, and the PMDA providing the value must have
68       some common understanding about how the value is structured and  inter‐
69       preted.
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71       Each  value  for a performance metric is assumed to be drawn from a set
72       of values that can be described in terms of  their  dimensionality  and
73       scale  by a compact encoding as follows.  The dimensionality is defined
74       by a power, or index, in each of 3 orthogonal dimensions, namely Space,
75       Time  and  Count (or Events, which are dimensionless).  For example I/O
76       throughput might be represented as
77                    -1
78          Space.Time
79       while the running total of system calls is Count, memory allocation  is
80       Space and average service time is
81                    -1
82          Time.Count
83       In each dimension there are a number of common scale values that may be
84       used to better encode ranges that might otherwise exhaust the precision
85       of  a  32-bit value.  This information is encoded in the pmUnits struc‐
86       ture which is embedded in the pmDesc structure.
87
88            /*
89             * Encoding for the units (dimensions Time and Space) and scale
90             * for Performance Metric Values
91             *
92             * For example, a pmUnits struct of
93             *      { 1, -1, 0, PM_SPACE_MBYTE, PM_TIME_SEC, 0 }
94             * represents Mbytes/sec, while
95             *      { 0, 1, -1, 0, PM_TIME_HOUR, 6 }
96             * represents hours/million-events
97             */
98            typedef struct {
99                int dimSpace:4;             /* space dimension */
100                int dimTime:4;              /* time dimension */
101                int dimCount:4;             /* event dimension */
102                unsigned int scaleSpace:4;  /* one of PM_SPACE_* below */
103                unsigned int scaleTime:4;   /* one of PM_TIME_* below */
104                int scaleCount:4;           /* one of PM_COUNT_* below */
105            } pmUnits;                      /* dimensional units and scale of value */
106
107            /* pmUnits.scaleSpace */
108            #define PM_SPACE_BYTE   0       /* bytes */
109            #define PM_SPACE_KBYTE  1       /* Kilobytes (1024) */
110            #define PM_SPACE_MBYTE  2       /* Megabytes (1024^2) */
111            #define PM_SPACE_GBYTE  3       /* Gigabytes (1024^3) */
112            #define PM_SPACE_TBYTE  4       /* Terabytes (1024^4) */
113            /* pmUnits.scaleTime */
114            #define PM_TIME_NSEC    0       /* nanoseconds */
115            #define PM_TIME_USEC    1       /* microseconds */
116            #define PM_TIME_MSEC    2       /* milliseconds */
117            #define PM_TIME_SEC     3       /* seconds */
118            #define PM_TIME_MIN     4       /* minutes */
119            #define PM_TIME_HOUR    5       /* hours */
120            /*
121             * pmUnits.scaleCount (e.g. count events, syscalls, interrupts,
122             * etc.) these are simply powers of 10, and not enumerated here,
123             * e.g. 6 for 10^6, or -3 for 10^-3
124             */
125            #define PM_COUNT_ONE    0       /* 1 */
126
127       Special routines (e.g. pmExtractValue(3), pmConvScale(3)) are  provided
128       to manipulate values in conjunction with the pmUnits structure that de‐
129       fines the dimension and scale of the values for  a  particular  perfor‐
130       mance metric.
131
132       Below the PMAPI, the information required to complete the pmDesc struc‐
133       ture, is fetched from the PMDAs, and in this way the format  and  scale
134       of  performance  metrics may change dynamically, as the PMDAs and their
135       underlying instrumentation evolve with time.  In particular, when  some
136       metrics suddenly become 64-bits long, or change their units from Mbytes
137       to Gbytes, well-written applications using the services provided by the
138       PMAPI will continue to function correctly.
139

DIAGNOSTICS

141       PM_ERR_PMID
142              The requested PMID is not known to the PMCS
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144       PM_ERR_NOAGENT
145              The  PMDA  responsible for providing the metric is currently not
146              available
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SEE ALSO

149       PMAPI(3), pmAtomStr(3),  pmConvScale(3),  pmExtractValue(3),  pmGetCon‐
150       fig(3), pmTypeStr(3), pmUnitsStr(3), pcp.conf(5) and pcp.env(5).
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