C SYNOPSIS

#include <pcp/pmapi.h>

int pmLookupDesc(pmID pmid, pmDesc *desc)
int pmRequestDesc(int ctx, pmID pmid)
int pmReceiveDesc(int ctx, 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.

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 (or indeed 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 */
    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_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.

pmRequestDesc and pmReceiveDesc are used by applications which must communicate with PMCD asynchronously. These functions take explict context handle ctx which must refer to a host context (i.e. a context created by passing PM_CONTEXT_HOST to pmNewContext). pmRequestDesc sends request to PMCD and returns without waiting for the response, pmReceiveDesc reads reply from PMCD. It is the responsibility of the application to make sure the data are ready before calling pmReceiveDesc to avoid blocking.

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

PM_ERR_CTXBUSY

Context is currently in use by another asynchronous call.