Provided by: libpcp3-dev_4.0.1-1_amd64 
NAME
pmFetch - get performance metric values
C SYNOPSIS
#include <pcp/pmapi.h>
int pmFetch(int numpmid, pmID *pmidlist, pmResult **result);
cc ... -lpcp
DESCRIPTION
Given a list of Performance Metric Identifiers (PMID)s, e.g. as constructed by pmLookupName(3), via
pmidlist and numpmid, fetch the values for these performance metrics.
The call to pmFetch is executed in the context of a source of metrics, instance profile and collection
time, previously established by calls to the appropriate context and profile functions, namely some of
pmNewContext(3), pmDupContext(3), pmUseContext(3), pmAddProfile(3), pmDelProfile(3) and pmSetMode(3).
The principal result from pmFetch is returned in the argument result as a tree, using the following
component data structures;
typedef struct {
unsigned int vtype : 8; /* value type (same as pmDesc.type) */
unsigned int vlen : 24; /* bytes for vtype/vlen + vbuf */
char vbuf[1]; /* one or more values */
} pmValueBlock;
typedef struct {
int inst; /* instance identifier */
union {
pmValueBlock *pval; /* pointer to value-block */
int lval; /* integer value insitu */
} value;
} pmValue;
typedef struct {
pmID pmid; /* metric identifier */
int numval; /* number of values or error code */
int valfmt; /* value style, insitu or ptr */
pmValue vlist[1]; /* set of instances/values */
} pmValueSet;
/* Result returned by pmFetch() */
typedef struct {
struct timeval timestamp; /* time stamped by collector */
int numpmid; /* number of PMIDs */
pmValueSet *vset[1]; /* set of value sets */
} pmResult;
To accommodate metrics with multiple value instances, the numval field indicates how many values are
returned for each requested PMID. The field valfmt in the pmValueSet structure indicates if the values
for this metric are stored insitu in the lval field, i.e. a 32-bit integer quantity (either int, unsigned
int, long or unsigned long) or if the values are held in associated pmValueBlock structures. The
pmValueBlock structure is always used for floating point values (float or double) and also accommodates
arbitrary sized binary data such as `string-valued' metrics and metrics with aggregated or complex data
types. The maximum length of a pmValueBlock buffer is PM_VAL_VLEN_MAX bytes. If the pmValueBlock format
is used, the vtype field indicates the data type of the value. This field has the same interpretation as
the type field in the pmDesc structure, see pmLookupDesc(3).
Note that the insitu value may be a signed or unsigned 32 bit integer, signed or unsigned 32 bit long
value (on 32 bit platforms), In the special cases described below, it may also be a 32 bit floating point
value. If the application needs to know the type of an insitu value, which is almost always the case, it
is necessary to fetch the descriptor for the metric and interpret the type field, as described in detail
in pmLookupDesc(3). When the pmResult is received from a PCP1.x pmcd, insitu values may also be 32 bit
floating point values (of type PM_TYPE_FLOAT). In all cases, it is good practice to use pmLookupDesc(3)
to fetch the descriptor for the metric and interpret the type field therein. Note also that the PMAPI(3)
will automatically translate from the PCP2.0 format to the PCP1.x format when a PCP1.x client requests 32
bit floating point values from a PCP2.0 pmcd, but the reverse translation does not occur (because the
PCP2.0 pmcd cannot automatically distinguish between arbitrary 32 bit floating point values and 32 bit
integers).
If one value (i.e. associated with a particular instance) for a requested metric is `unavailable' (at the
requested time), then there is no associated pmValue structure in the result. If there are no available
values for a metric, then numval will be zero and the associated pmValue[] instance will be empty (valfmt
is undefined in these circumstances, however pmid will be correctly set to the PMID of the metric with no
values).
As an extension of this protocol, if the Performance Metrics Collection System (PMCS) is able to provide
a reason why no values are available for a particular metric, this is encoded as a standard error code in
the corresponding numval. Since the error codes are all negative, values for a requested metric are
`unavailable' if numval is less than, or equal to, zero. A performance metric's value may be
`unavailable' for a number of reasons; the following list is illustrative but not exhaustive: of the
software for the associated Performance Metric Domain
+ Collection is not currently activated in the software for the associated Performance Metric Domain
+ The associated PMID is not known
+ The current system configuration does not include the associated hardware component and/or the
associated software module, e.g. a disk is not installed, or off-line, or Oracle is not installed
+ The metric is one for which an instance profile is required, and none was provided (there are a
small number of metrics in this category, typically ones with very large, and/or very dynamic
instance domains, and/or expensive metric instantiation methods).
+ If the current context involves fetching metrics from an archive log, values may be unavailable in
the region around a <mark> record (see pmlogextract(1)) that indicate a temporal discontinuity in
the time-series of metric values.
In general, we may not be able to differentiate between the various cases, and if differentiation is not
possible, numval will simply be zero.
The argument definition and the result specifications have been constructed to ensure that for each PMID
in the requested pmidlist there is exactly one pmValueSet in the result, and further the PMIDs appear in
exactly the same sequence in both pmidlist and result. This makes the number and order of entries in
result completely deterministic, and greatly simplifies the application programming logic after the call
to pmFetch.
The result structure returned by pmFetch is dynamically allocated using a combination of malloc(3) calls
and specialized allocation strategies, and should be released when no longer required by calling
pmFreeResult(3) - under no circumstances should free(3) be called directly to release this space.
As common error conditions are encoded in the result data structure, we'd expect only cataclysmic events
to cause an error value to be returned. One example would be if the metrics source context was a remote
host, and that host or the PMCS on that host became unreachable. Otherwise the value returned by the
pmFetch function will be non-negative.
If the current context involves fetching metrics from a Performance Metrics Collector Daemon (PMCD), then
the return value may be used to encode out-of-band changes in the state of the PMCD and the associated
Performance Metrics Daemon Agents (PMDAs), as a bit-wise ``or'' of the following values:
PMCD_RESTART_AGENT An attempt has been made to restart at least one failed PMDA.
PMCD_ADD_AGENT At least one PMDA has been started.
PMCD_DROP_AGENT PMCD has noticed the termination of at least one PMDA.
PMCD_AGENT_CHANGE A convenience macro for any of the three PMDA changes.
PMCD_LABEL_CHANGE PMCD has been informed of changes to global (context) labels, or new metrics have
appeared which have associated labels.
PMCD_NAMES_CHANGE PMCD has been informed that the namespace has been modified, such that new metrics
have appeared or existing metrics have been removed.
The default is to return zero to indicate no change in state, however the pmResult returned by pmFetch
has the same interpretation independent of the return value being zero or greater than zero.
SEE ALSO
pmcd(1), pmAddProfile(3), PMAPI(3), pmDelProfile(3), pmDupContext(3), pmExtractValue(3),
pmFetchArchive(3), pmFreeResult(3), pmGetInDom(3), pmLookupDesc(3), pmLookupLabels(3), pmLookupName(3),
pmNewContext(3), pmSetMode(3), pmUseContext(3) and pmWhichContext(3).
Note that pmFetch is the most primitive method of fetching metric values from the PMCS. See the
pmFetchGroup(3) API for a higher level method that insulates the user from the intricacies of looking up
metric names and metadata, setting up instance profiles, pmResult traversal, conversions, and scaling.
DIAGNOSTICS
As mentioned above, pmFetch returns error codes insitu in the argument result. If no result is returned,
e.g. due to IPC failure using the current PMAPI context, or end of file on an archive log, then pmFetch
will return a negative error code which may be examined using pmErrStr(3).
PM_ERR_EOL
When fetching records from an archive log, pmFetch returns this error code to indicate the end of
the log has been passed (or the start of the log has been passed, if the direction of traversal is
backwards in time). If the ``mode'' for the current PMAPI context (see pmSetMode(3)) is
PM_MODE_INTERP then the time origin is advanced, even when this error code is returned. In this
way applications that position the time outside the range defined by the records in the archive,
and then commence to pmFetch will eventually see valid results once the time origin moves inside
the temporal span of the archive.
ENVIRONMENT
Many of the performance metrics exported from PCP agents have the semantics of counter meaning they are
expected to be monotonically increasing. Under some circumstances, one value of these metrics may be
smaller than the previously fetched value. This can happen when a counter of finite precision overflows,
or when the PCP agent has been reset or restarted, or when the PCP agent is exporting values from some
underlying instrumentation that is subject to some asynchronous discontinuity.
The environment variable PCP_COUNTER_WRAP may be set to indicate that all such cases of a decreasing
``counter'' should be treated as a counter overflow, and hence the values are assumed to have wrapped
once in the interval between consecutive samples. This ``wrapping'' behavior was the default in earlier
PCP versions, but by default has been disabled in PCP version 1.3 and later.