pmlookupdesc(3) — Linux manual page


PMLOOKUPDESC(3)         Library Functions Manual         PMLOOKUPDESC(3)

NAME         top

       pmLookupDesc, pmLookupDescs - obtain descriptions for performance

C SYNOPSIS         top

       #include <pcp/pmapi.h>

       int pmLookupDesc(pmID pmid, pmDesc *desc);
       int pmLookupDescs(int numpmid, pmID *pmids, pmDesc *descs);

       cc ... -lpcp

DESCRIPTION         top

       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_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
       while the running total of system calls is Count, memory
       allocation is Space and average service time is
       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         top

       These routines return a negative error code to indicate failure.

              The requested PMID is not known to the PMCS

              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

       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         top

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

COLOPHON         top

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       ⟨⟩ on 2024-06-14.
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Performance Co-Pilot               PCP                   PMLOOKUPDESC(3)

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