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.