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PMFETCH(3) Library Functions Manual PMFETCH(3)
pmFetch - get performance metric values
#include <pcp/pmapi.h>
int pmFetch(int numpmid, pmID *pmidlist, pmResult **result);
cc ... -lpcp
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.
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.
pmcd(1), pmAddProfile(3), PMAPI(3), pmDelProfile(3), pmDupContext(3),
pmExtractValue(3), pmFetchArchive(3), pmFreeResult(3), pmGetInDom(3),
pmLookupDesc(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.
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.
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.
This page is part of the PCP (Performance Co-Pilot) project.
Information about the project can be found at ⟨http://www.pcp.io/⟩.
If you have a bug report for this manual page, send it to
pcp@oss.sgi.com. This page was obtained from the project's upstream
Git repository ⟨git://git.pcp.io/pcp⟩ on 2017-07-05. If you discover
any rendering problems in this HTML version of the page, or you
believe there is a better or more up-to-date source for the page, or
you have corrections or improvements to the information in this
COLOPHON (which is not part of the original manual page), send a mail
to man-pages@man7.org
Performance Co-Pilot PCP PMFETCH(3)
Pages that refer to this page: pmgenmap(1), pcpintro(3), pmaddprofile(3), pmapi(3), pmcontrollog(3), pmconvscale(3), pmdafetch(3), pmdelprofile(3), pmextractvalue(3), pmfetcharchive(3), pmfetchgroup(3), pmfreeresult(3), pmgetarchiveend(3), pmgetcontexthostname(3), pmnewcontext(3), pmnewcontextzone(3), pmprintvalue(3), pmregisterderived(3), pmsetmode(3), pmsortinstances(3), pmstore(3), QmcIndom(3)