1PMLOOKUPDESC(3) Library Functions Manual PMLOOKUPDESC(3)
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6 pmLookupDesc, pmLookupDescs - obtain descriptions for performance met‐
7 rics
8
10 #include <pcp/pmapi.h>
11 int pmLookupDesc(pmID pmid, pmDesc *desc);
13 int pmLookupDescs(int numpmid, pmID *pmids, pmDesc *descs);
14 cc ... -lpcp
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18 Given a Performance Metrics Identifier (PMID) as pmid, the pmLookupDesc
19 routine fills in the given pmDesc structure, pointed to by the parame‐
20 ter desc, from the current Performance Metrics Application Programming
21 Interface (PMAPI) context.
22 The pmLookupDescs variant provides equivalent functionality for numpmid
24 metrics at once, with the pmids array providing the metric identifiers
25 to lookup. It is more efficient as the number of metrics increases, as
26 it avoids round trip latency from multiple individual requests. Note
27 that the error protocol guarantees there is a 1:1 relationship between
28 the elements of descs and pmids, hence both lists contain exactly nump‐
29 mid elements. For this reason, the caller is expected to have pre-al‐
30 located a suitably sized array for descs.
31 The pmDesc structure provides all of the information required to de‐
33 scribe and manipulate a performance metric via the PMAPI, and has the
34 following declaration.
35 /* Performance Metric Descriptor */
37 typedef struct {
38 pmID pmid; /* unique identifier */
39 int type; /* base data type (see below) */
40 pmInDom indom; /* instance domain */
41 int sem; /* semantics of value (see below) *
42 pmUnits units; /* dimension and units (see below) */
43 } pmDesc;
44 /* pmDesc.type -- data type of metric values */
46 #define PM_TYPE_NOSUPPORT -1 /* not impl. in this version */
47 #define PM_TYPE_32 0 /* 32-bit signed integer */
48 #define PM_TYPE_U32 1 /* 32-bit unsigned integer */
49 #define PM_TYPE_64 2 /* 64-bit signed integer */
50 #define PM_TYPE_U64 3 /* 64-bit unsigned integer */
51 #define PM_TYPE_FLOAT 4 /* 32-bit floating point */
52 #define PM_TYPE_DOUBLE 5 /* 64-bit floating point */
53 #define PM_TYPE_STRING 6 /* array of char */
54 #define PM_TYPE_AGGREGATE 7 /* arbitrary binary data */
55 #define PM_TYPE_AGGREGATE_STATIC 8 /* static pointer to aggregate */
56 #define PM_TYPE_EVENT 9 /* packed pmEventArray */
57 #define PM_TYPE_UNKNOWN 255 /* used in pmValueBlock, not pmDesc */
58 /* pmDesc.sem -- semantics/interpretation of metric values */
61 #define PM_SEM_COUNTER 1 /* cumulative ctr (monotonic incr) */
62 #define PM_SEM_INSTANT 3 /* instant. value continuous domain */
63 #define PM_SEM_DISCRETE 4 /* instant. value discrete domain */
64 The type field in the pmDesc describes various encodings (or formats)
66 for a metric's value.
67 If a value is counted in the underlying base instrumentation with less
69 than 32 bits of integer precision, it is the responsibility of the Per‐
70 formance Metrics Domain Agent (PMDA) to promote the value to a 32-bit
71 integer before it is exported into the Performance Metrics Collection
72 Subsystem (PMCS); i.e. applications above the PMAPI never have to deal
73 with 8-bit and 16-bit counters.
74 If the value of a performance metric is of type PM_TYPE_AGGREGATE,
76 PM_TYPE_AGGREGATE_STATIC, PM_TYPE_EVENT or PM_TYPE_STRING, the inter‐
77 pretation of the value is unknown to the PMCS. In these cases, the ap‐
78 plication using the value, and the PMDA providing the value must have
79 some common understanding about how the value is structured and inter‐
80 preted.
81 Each value for a performance metric is assumed to be drawn from a set
83 of values that can be described in terms of their dimensionality and
84 scale by a compact encoding as follows. The dimensionality is defined
85 by a power, or index, in each of 3 orthogonal dimensions, namely Space,
86 Time and Count (or Events, which are dimensionless). For example I/O
87 throughput might be represented as
88 -1
89 Space.Time
90 while the running total of system calls is Count, memory allocation is
91 Space and average service time is
92 -1
93 Time.Count
94 In each dimension there are a number of common scale values that may be
95 used to better encode ranges that might otherwise exhaust the precision
96 of a 32-bit value. This information is encoded in the pmUnits struc‐
97 ture which is embedded in the pmDesc structure.
98 /*
100 * Encoding for the units (dimensions Time and Space) and scale
101 * for Performance Metric Values
102 *
103 * For example, a pmUnits struct of
104 * { 1, -1, 0, PM_SPACE_MBYTE, PM_TIME_SEC, 0 }
105 * represents Mbytes/sec, while
106 * { 0, 1, -1, 0, PM_TIME_HOUR, 6 }
107 * represents hours/million-events
108 */
109 typedef struct {
110 int dimSpace:4; /* space dimension */
111 int dimTime:4; /* time dimension */
112 int dimCount:4; /* event dimension */
113 unsigned int scaleSpace:4; /* one of PM_SPACE_* below */
114 unsigned int scaleTime:4; /* one of PM_TIME_* below */
115 int scaleCount:4; /* one of PM_COUNT_* below */
116 } pmUnits; /* dimensional units and scale of value */
117 /* pmUnits.scaleSpace */
119 #define PM_SPACE_BYTE 0 /* bytes */
120 #define PM_SPACE_KBYTE 1 /* Kilobytes (1024) */
121 #define PM_SPACE_MBYTE 2 /* Megabytes (1024^2) */
122 #define PM_SPACE_GBYTE 3 /* Gigabytes (1024^3) */
123 #define PM_SPACE_TBYTE 4 /* Terabytes (1024^4) */
124 /* pmUnits.scaleTime */
125 #define PM_TIME_NSEC 0 /* nanoseconds */
126 #define PM_TIME_USEC 1 /* microseconds */
127 #define PM_TIME_MSEC 2 /* milliseconds */
128 #define PM_TIME_SEC 3 /* seconds */
129 #define PM_TIME_MIN 4 /* minutes */
130 #define PM_TIME_HOUR 5 /* hours */
131 /*
132 * pmUnits.scaleCount (e.g. count events, syscalls, interrupts,
133 * etc.) these are simply powers of 10, and not enumerated here,
134 * e.g. 6 for 10^6, or -3 for 10^-3
135 */
136 #define PM_COUNT_ONE 0 /* 1 */
137 Special routines (e.g. pmExtractValue(3), pmConvScale(3)) are provided
139 to manipulate values in conjunction with the pmUnits structure that de‐
140 fines the dimension and scale of the values for a particular perfor‐
141 mance metric.
142 Below the PMAPI, the information required to complete the pmDesc struc‐
144 ture, is fetched from the PMDAs, and in this way the format and scale
145 of performance metrics may change dynamically, as the PMDAs and their
146 underlying instrumentation evolve with time. In particular, when some
147 metrics suddenly become 64-bits long, or change their units from Mbytes
148 to Gbytes, well-written applications using the services provided by the
149 PMAPI will continue to function correctly.
150
152 These routines return a negative error code to indicate failure.
153 PM_ERR_PMID
155 The requested PMID is not known to the PMCS
156 PM_ERR_NOAGENT
158 The PMDA responsible for providing the metric is currently not
159 available
160 pmLookupDesc returns zero to indicate success.
162 The result from pmLookupDescs depends on the presence of any lookup
164 failures, their severity and the number of metrics being looked up.
165 1. If there are no lookup failures, the return value will be numpmid.
167 2. If a fatal error is encountered, the return value will be less than
169 0. For example PM_ERR_IPC.
170 3. If numpmid is greater than one and non-fatal error(s) are encoun‐
172 tered, the return value is the number of metric descriptors that
173 have successfully been looked up (greater than or equal to zero and
174 less than or equal to numpmid).
175 4. If numpmid is one and a non-fatal error is encountered, the return
177 value is the error code (less than zero).
178 When errors are encountered, any metrics that cannot be looked up re‐
180 sult in the corresponding descriptor element of descs having its pmid
181 field set to PM_ID_NULL. The slightly convoluted error protocol allows
182 bulk lookups, then probing for more error details in the case of a spe‐
183 cific failure.
184
186 PMAPI(3), pmAtomStr(3), pmConvScale(3), pmExtractValue(3), pmGetCon‐
187 fig(3), pmTypeStr(3), pmUnitsStr(3), pcp.conf(5) and pcp.env(5).
188Performance Co-Pilot PCP PMLOOKUPDESC(3)