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 IDs (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 any of
the following reasons;
- +
- The metric is not supported in this version 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).
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(3C) calls and specialized allocation
strategies, and should be released when no longer required by calling
pmFreeResult(3) - under no circumstances should
free(3C) 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.
SEE ALSO¶
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. More user friendly interfaces to the PMCS are available or
currently under development - these higher level fetch methods insulate the
user from the intricacies of context creation, setting up instance profiles,
pmResult traversal, and splitting fetches into batches to minimize PDU
traffic or according to other optimization criteria.
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.