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NAME | C SYNOPSIS | DESCRIPTION | EXAMPLE | UNUSUAL SITUATIONS | DIAGNOSTICS | SEE ALSO | COLOPHON |
PMFETCHGROUP(3) Library Functions Manual PMFETCHGROUP(3)
pmCreateFetchGroup, pmExtendFetchGroup_item, pmExtendFetch‐
Group_indom, pmExtendFetchGroup_event, pmExtendFetchGroup_timestamp,
pmFetchGroup, pmGetFetchGroupContext, pmDestroyFetchGroup - simpli‐
fied performance metrics value fetch and conversion
#include <pcp/pmapi.h>
int pmCreateFetchGroup(pmFG *ptr, int type, const char *name);
int pmExtendFetchGroup_item(pmFG pmfg, const char *metric, const char
*instance, const char *scale, pmAtomValue *out_value, int
out_type, int out_sts);
int pmExtendFetchGroup_indom(pmFG pmfg, const char *metric, const
char *scale, int out_inst_codes[], char *out_inst_names[],
pmAtomValue out_values[], int out_type, int out_stss[],
unsigned int out_maxnum, unsigned int *out_num, int
*out_sts);
int pmExtendFetchGroup_event(pmFG pmfg, const char *metric, const
char *instance, const char *field, const char *scale, struct
timespec out_times[], pmAtomValue out_values[], int out_type,
int out_stss[], unsigned int out_maxnum, unsigned int
*out_num, int *out_sts);
int pmExtendFetchGroup_timestamp(pmFG pmfg, struct timeval
*out_value);
int pmGetFetchGroupContext(pmFG pmfg);
int pmFetchGroup(pmFG pmfg);
int pmDestroyFetchGroup(pmFG pmfg);
cc ... -lpcp
The fetchgroup functions implement a registration-based mechanism to
fetch groups of performance metrics, including automation for general
unit, rate and type conversions as well as convenient instance and
value encodings. They constitute a powerful and compact alternative
to the classic Performance Metrics Application Programming Interface
(PMAPI(3)) sequence of separate lookup, check, fetch, iterate,
extract and convert functions.
The general idea consists of two stages. In the setup stage, the
application identifies metrics of interest by name and with desired
conversions, and register a unique pmAtomValue output location where
the fetchgroup system is to later deposit the result. It is also
possible to identify a metric with an instance domain, and register a
unique vector of pmAtomValue objects for them. In the operation
stage, one simple pmFetchGroup function fetches, decodes, converts,
and stores all metrics to their destinations, where the application
can read them directly. This function may be called repeatedly, and
each time new pmAtomValue values will be stored in the same
destinations. Rate conversions between consecutive samples may be
requested.
Each fetchgroup is associated with a private PMAPI context, so it can
manipulate instance profiles and other such state without disrupting
other contexts. This private PMAPI context belongs to the
fetchgroup, is used for all of its internal operations, and will be
destroyed.
Multiple fetchgroups may be used concurrently, independently. An
opaque type pmFG is used to identify a fetchgroup, which is passed to
all related function calls.
Creating a fetchgroup
int pmCreateFetchGroup(pmFG *ptr, int type, const char *name);
This function creates a new fetchgroup, associated with a new PMAPI
context. The type and name parameters are relayed to pmNewContext(3)
for creation of the context. The fetchgroup identifier is returned
upon success through the ptr pointer. This object is later used as a
parameter to all other fetchgroup functions. The private PMAPI con‐
text may be accessed with pmGetFetchGroupContext, if required.
The normal function return code is zero, and ptr is set. This func‐
tion may fail in case of pmNewContext or memory allocation errors.
Those are indicated with a negative return code and a cleared ptr
value.
Getting the private PMAPI context
int pmGetFetchGroupContext(pmFG pmfg);
This function returns the private PMAPI context used by the given
fetchgroup. It may be safely used to adjust some configuration pa‐
rameters of the context, such as via pmSetMode(3), before fetchgroup
extension and fetching begins.
However, mutation of this context by PMAPI functions after this time
may disrupt fetchgroup functionality. For example, a pmSetMode call
could invalidate one rate-conversion time-step.
The normal function return code is the context number.
Extending a fetchgroup with a metric instance of interest
int pmExtendFetchGroup_item(pmFG pmfg, const char *metric, const char
*instance, const char *scale, pmAtomValue *out_value, int
out_type, int *out_sts);
This function registers interest in a single metric and optional in‐
stance. The metric name is given in the mandatory metric parameter,
which is checked immediately via pmLookupName(3) and other calls. If
and only if the metric has an instance domain, the specific instance
of interest may be named by the instance parameter, which is checked
immediately via pmNameInDom(3); otherwise pass NULL. If the fetch‐
group context is a set of archives, it is possible that the metric /
instance pair is not yet defined at the current time origin. There‐
fore, this function may attempt to seek to the end of the current set
of archives temporarily to retry the metric / instance lookup.
The optional scale parameter specifies desired unit/scale/rate con‐
versions for the metric value. It can take the following values:
NULL
No unit/scale conversion. If metric has PM_SEM_COUNTER seman‐
tics, perform rate conversion.
rate
Perform rate conversion regardless of semantics, and no
unit/scale conversion.
instant
Perform no rate conversion regardless of semantics, and no
unit/scale conversion.
EXPRESSION
Perform unit/scale/rate conversion as specified by the EXPRES‐
SION, which is parsed by pmParseUnitsStr(3). This may be useful
to assert a canonical scaling for the resulting metric value, in‐
dependent of PCP version or configuration. Dimensionality must
match the metric, except if rate conversion is requested, in
which case the time dimension must be one smaller than the met‐
ric's time dimension. Note that the type of rate conversion per‐
formed here matches the rate(x) function in derived metric ex‐
pressions, in that it is calculated as the naive difference be‐
tween previous and current values of a metric, divided by elapsed
time. For example, if a counter wraps around, or a non-counter
value decreases, a negative output rate may be computed.
The optional but usual out_value parameter specifies the pmAtomValue
where the converted result should later be stored. If the value is
NULL, fetching and conversions will be attempted, and possible errors
reported, but the result tossed away. The mandatory out_type parame‐
ter specifes the PM_TYPE_* requested for the output value. It need
not match the metric's native type, as the fetchgroup facility is ca‐
pable of casting between all supported types (including to and from
strings).
Any errors subsequently encountered during fetching, unit/scale/rate
conversion, or casting, will result in the assignment of a sentinel
value to the output pmAtomValue (see the ``UNUSUAL SITUATIONS'' sec‐
tion below). In addition, if the optional out_sts parameter is spec‐
ified, an appropriate PMAPI error code will be stored there.
As a review, only the pmfg, metric, and out_type parameters are
mandatory. Others may be NULL to indicate applicaton disinterest.
The normal function return code is zero. This function may fail in
case of various lookup, type- and conversion- checking errors. Those
are indicated with a negative return code.
Extending a fetchgroup with a metric instance domain of interest
int pmExtendFetchGroup_indom(pmFG pmfg, const char* metric, const
char *scale, int out_inst_codes[], char *out_inst_names[],
pmAtomValue out_values[], int out_type, int out_stss[],
unsigned int out_maxnum, unsigned int *out_num, int
*out_sts);
This function generalizes the pmExtendFetchGroup_item function by
registering interest in a whole instance domain. Therefore, the
function registers preallocated vectors for output variables (instead
of a singleton). Instances will be stored in sorted order in ele‐
ments of those vectors. The concepts are otherwise the same.
The metric name is specified by the mandatory metric parameter. Note
that it may refer to a metric without an instance domain, in which
case the single output value will appear as one unnamed instance.
The optional scale parameter specifies desired unit/scale/rate con‐
versions for the metric value, same as above.
The optional out_inst_codes parameter specifies a vector of integers,
where the raw instance number of the fetched metrics should later be
stored.
The optional out_inst_names parameter specifies a vector of strings,
where the instance names of the fetched metrics should later be
stored. If an instance does not have a corresponding name, a NULL
pointer is stored instead. The application must not modify or
free(3) strings in that vector.
The optional out_values parameter specifies a vector of pmAtomValue
objects where the converted result should later be stored. The
mandatory out_type parameter specifies the PM_TYPE_* requested for
the all output values, same as above.
The optional out_stss parameter specifies a vector of integers where
per-instance error codes should be stored.
The mandatory out_maxnum parameter specifies the number of elements
of the vectors above. In other words, it tells the fetchgroup the
maximum number of instances which are expected. The optional out_num
parameter specifies an integer where the actual number of instances
should later be stored. It will range between 0 and out_maxnum. It
is initialized to 0 by this function.
Finally, the optional out_sts parameter specifies a single location
where an integer status code for the overall fetch for this metric
should be stored. Normally, this will be zero. Other than a severe
fetch error, one may see a PM_ERR_TOOBIG here if the number of in‐
stances actually encountered was larger than out_maxnum.
Any errors subsequently encountered during fetching, unit/scale/rate
conversion, or casting, will result in the assignment of a sentinel
value to the appropriate output pmAtomValue (see the ``UNUSUAL SITUA‐
TIONS'' section below). In addition, if the optional out_stss param‐
eter was specified, a PMAPI error code will be stored in the appro‐
priate position.
As a review, only the pmfg, metric, out_type, and out_maxnum parame‐
ters are mandatory. Others may be NULL to indicate applicaton disin‐
terest.
The normal function return code is zero. This function may fail in
case of various lookup, type- and conversion- checking errors. Those
are indicated with a negative return code.
Extending a fetchgroup with an event field
int pmExtendFetchGroup_event(pmFG pmfg, const char *metric, const
char *instance, const char *field, const char *scale, struct
timespec out_times[], pmAtomValue out_values[], int out_type,
int out_stss[], unsigned int out_maxnum, unsigned int
*out_num, int *out_sts);
This function registers interest in all instances of one field of all
records of an event metric. Since event metrics may return multiple
records per fetch, and each record may have multiple fields of a giv‐
en field metric type, this function registers preallocated vectors
for output variables, similarly to pmExtendFetchGroup_indom. They
are filled in temporal/sequential order.
The metric name is specified by the mandatory metric parameter. It
must be of PM_TYPE_EVENT. If the metric has an instance domain, the
instance parameter is mandatory to identify the instance of interest.
The field to extract from event records is specified by the mandatory
field parameter, which is a metric name of normal scalar type. As is
typical for event field metrics, it should not have an instance do‐
main. The optional scale parameter specifies desired unit/scale con‐
versions on this metric value. Rate conversions are not available,
because of ambiguity about which previous value to compute rates
from.
The optional out_times parameter specifies a vector of timespec
structs, which will receive a copy of the timestamp of the event
record where each particular field was found.
The optional out_values parameter specifies a vector of pmAtomValue
objects where the converted result should later be stored. The
mandatory out_type parameter specifies the PM_TYPE_* requested for
the output values.
The optional out_stss parameter specifies a vector of integers where
per-field error codes should be stored.
The mandatory out_maxnum parameter specifies the number of elements
of the vectors above. In other words, it tells the fetchgroup the
maximum number of instances which are expected. The optional out_num
parameter specifies an integer where the the actual number of in‐
stances should later be stored. It will range between zero and
out_maxnum. It is initialized to zero by this function.
Finally, the optional out_sts parameter specifies a single location
where an integer status code for the overall fetch for this metric
should be stored. Normally, this will be zero, even if no event
field values were found (out_num would then be zero). Other than a
severe fetch error, one may see a PM_ERR_TOOBIG here if the number of
fields actually encountered was larger than out_maxnum.
Any errors subsequently encountered during fetching, unit/scale con‐
version, or casting, will result in the assignment of a sentinel val‐
ue to the appropriate output pmAtomValue (see the ``UNUSUAL SITUA‐
TIONS'' section below). In addition, if the optional out_stss param‐
eter was specified, a PMAPI error code will be stored in the appro‐
priate position.
As a review, only the pmfg, metric, field, out_type, and out_maxnum
parameters are mandatory. Others may be NULL to indicate applicaton
disinterest.
The normal function return code is zero. This function may fail in
case of various lookup, type- and conversion- checking errors. Those
are indicated with a negative return code.
Extending a fetchgroup with the fetch timestamp
int pmExtendFetchGroup_timestamp(pmFG pmfg, struct timeval
*out_value);
This function registers interest in the pmResult timestamp. If the
out_value pointer is non-NULL, at every future pmFetchGroup call, the
corresponding result timestamp will be copied there.
Fetching all metrics in a fetchgroup
int pmFetchGroup(pmFG pmfg);
This function performs one pmFetch on its private PMAPI context, in‐
cluding all the metrics that were registered via prior pmExtendFetch‐
Group_* calls. It runs all the data extraction and conversion opera‐
tions necessary to populate all the requested output variables.
The normal function return code is zero or positive, as per the un‐
derlying pmFetch function. This function may fail in case of severe
fetch errors, which are indicated with a negative return code.
In the case of per-metric availability or conversion errors, or se‐
vere fetch errors, output variables are reset to sentinel values and
individual error codes are set. PM_ERR_AGAIN signals rate-conversion
failure due to lack of a previous value.
However, temporarily absent metrics with discrete semantics are ex‐
empt from some sentinel/error processing: if a pmFetchGroup fails to
collect a result for a discrete metric (pmResult pmValueSet.num‐
val==0), then the last seen valid value (if any) is retained. This
is intended to ease the processing of sets of archives with a mixture
of once- and repeatedly-sampled metrics.
Destroying a fetchgroup
int pmDestroyFetchGroup(pmFG pmfg);
When the fetchgroup is no longer needed, it may be explicitly freed
with this function. It releases any dynamically stored state, as
well as the private PMAPI context. It clears frees any pointers such
as indom instance names or strings that may have been stored in out‐
put variables.
The following program demonstrates fetchgroup usage. Run it with
different $PCP_DISK_UNITS environment variables to see different
unit/rate conversion in effect.
#include <pcp/pmapi.h>
#include <stdio.h>
#define pcpassert(sts) \
while (sts<0) { fprintf(stderr, "%s\n", pmErrStr(sts)); exit(42); }
int main()
{
pmFG fg;
pmAtomValue v, v2;
enum { v3_maxnum = 100 };
pmAtomValue v3_values[v3_maxnum];
char *v3_names[v3_maxnum];
int v3_stss[v3_maxnum];
unsigned int v3_num;
int sts, i;
char *diskunits = getenv("PCP_DISK_UNITS");
struct timeval t;
sts = pmCreateFetchGroup(&fg, PM_CONTEXT_HOST, "local:");
pcpassert(sts);
sts = pmExtendFetchGroup_item(fg, "kernel.all.load", "1 minute",
NULL, &v, PM_TYPE_FLOAT, NULL);
pcpassert(sts);
sts = pmExtendFetchGroup_item(fg, "kernel.all.idletime", NULL,
"hour", &v2, PM_TYPE_DOUBLE, NULL);
pcpassert(sts);
sts = pmExtendFetchGroup_indom(fg, "disk.dev.total", diskunits,
NULL, v3_names,
v3_values, PM_TYPE_STRING,
v3_stss, v3_maxnum, &v3_num, NULL);
pcpassert(sts);
sts = pmExtendFetchGroup_timestamp(fg, &t);
pcpassert(sts);
for (i=0; i < 10; i++) {
unsigned int j;
char stamp[28];
sts = pmFetchGroup(fg);
pcpassert(sts);
printf("%s", pmCtime(&t.tv_sec, stamp));
printf("1-minute load: %f; idletime: %f h\n", v.f, v2.d);
for (j=0; j < v3_num; j++) {
if (v3_stss[j] == 0)
printf("disk %s i/o operations (%s): %s\n",
v3_names[j] ? v3_names[j] : "?",
diskunits ? diskunits : "-",
v3_values[j].cp);
}
sleep(1);
}
sts = pmDestroyFetchGroup(fg);
pcpassert(sts);
return 0;
}
The fetchgroup API supports only the numeric, string and event metric
types. Aggregates are rejected during pmExtendFetchGroup_*.
Any strings supplied by the fetchgroup API to the application are
"owned" by the API. The application should consider them read-only,
so it should not modify them nor free them.
Error codes are always negative integers, whether returned from
fetchgroup functions as return value, or stored in out_sts type
variables. Normal result codes are always zero.
Because of the unique ways in which extracted data is shared between
the application and a fetchgroup, the functions in this API are not
protected by the multi-threading mutexes conventional in other parts
of PMAPI. Specifically, for any given pmFG, it is not safe to
concurrently call two or more fetchgroup API functions, nor to
traverse the registered output variables while calling one of the
functions. Instead, the calling application must ensure that only
one thread at a time uses these calls and the registered output
variables. On the other hand, concurrency between different pmFG
instances is unrestricted, because they share no global data.
Any pointers passed to a successful pmFetchGroupExtent_* call must
stay valid throughout the lifetime of the fetchgroup, since future
pmFetchGroup calls may write into them.
The fetchgroup API offers several options for collecting diagnostics.
Negative integer error codes may be returned from each function for
serious conditions.
In addition, each output pmAtomValue may have a corresponding integer
variable, where pmFetchGroup can store per-metric per-instance error
codes.
As an alternative, per-metric per-instance error conditions are also
signalled by setting the corresponding pmAtomValue to a sentinel
value. If unambiguous and precise error detection is not required,
this may be sufficient. The sentinel value is negative one for all
integers (including unsigned integers - i.e. all bits are set), NaN
for floating point types, a NULL pointer for strings, and 0.0s for
the timestamp. The fetchgroup API guarantees that once an output
pmAtomValue is registered (during a successful pmExtendFetchGroup_*
call), it will be cleared to the sentinel value or to a valid
converted metric value, from the time of registration until the
pmDestroyFetchGroup call.
PMAPI(3), pmLookupName(3), pmFetch(3), pmParseUnitsStr(3),
pmUseContext(3), pmRegisterDerived(3) and pmExtractValue(3).
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
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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 PMFETCHGROUP(3)
Pages that refer to this page: pmclient(1), pmapi(3), pmextractvalue(3), pmfetch(3)