Provided by: libpcp4-dev_7.0.2-1_amd64 
NAME
pmSetMode - set collection time and mode parameters for the current PMAPI context
C SYNOPSIS
#include <pcp/pmapi.h>
int pmSetMode(int mode, const struct timespec *when, const struct timespec *delta);
cc ... -lpcp
DESCRIPTION
pmSetMode are used to define the collection time and/or mode for accessing performance metrics and meta‐
data in the current Performance Metrics Application Programming Interface (PMAPI) context. This mode af‐
fects the semantics of subsequent calls to the following PMAPI routines: pmFetch(3), pmFetchArchive(3),
pmLookupDesc(3), pmGetInDom(3), pmLookupInDom(3), pmLookupLabels(3) and pmNameInDom(3).
Intended mainly for retrospective analysis of performance metrics from a PCP archive, the options de‐
scribed below allow an application to implement seeking to an arbitrary time within the archive, play‐
back, fast forward, reverse, etc. by alternating calls to pmSetMode and pmFetch(3).
If mode is PM_MODE_LIVE then all information is returned from the active pool of performance metrics as
of the time that the PMAPI call is made, and the other two parameters to pmSetMode (when and delta) are
ignored. PM_MODE_LIVE is the default mode when a new PMAPI context is created with type PM_CONTEXT_HOST
(i.e. fetching metrics from pmcd (1)) or PM_CONTEXT_LOCAL, and this mode is rarely used in calls to pm‐
SetMode
The other values of mode are used with PCP archives where the associated PMAPI context must be of type
PM_CONTEXT_ARCHIVE (when it was created with pmNewContext(3)) and the when parameter defines the current
time within the archive. All requests for metric values and metadata (metric descriptions and instance
identifiers from the instance domains) will be processed to reflect the state in the archive as of the
current time.
When selecting a value for when, the time at the start of the archive can be established from the
ll_start field in the structure returned from a call to pmGetArchiveLabel(3) or the start field in the
structure returned from a call to pmGetArchiveLabel(3), and the time at the end of the archive can be es‐
tablished by calling pmGetArchiveEnd(3)
As a special case, if when is NULL then the mode and delta arguments are used as described below, but the
current time in the archive is not altered.
TIME-DRIVEN ARCHIVE MODE
If mode is PM_MODE_INTERP then as metric values are retrieved from the archive with pmFetch(3) the cur‐
rent time is returned as the timestamp in the pmResult structure and the current time moves on; if delta
is positive, the current time moves forwards, else the current time moves backwards. The adjustment to
the current time is applied even if the pmFetch(3) fails to return values for any metrics or returns an
error, e.g. PM_ERR_EOL because the current time is outside the range defined by the records in the
archive.
When metric values are being requested via pmFetch(3) the current time may not exactly match the times at
which values have been recorded in the archive, so the returned metric values are computed from the ob‐
served metric values in the archive (usually at times close to the current time).
For metrics with semantics of PM_SEM_COUNTER, the computed value is based on linear interpolation between
the last observation before the current time and the first observation after the current time.
For metrics with semantics of PM_SEM_INSTANT or PM_SEM_DISCRETE, the computed value is based on values in
the neighbourhood of the current time.
The algorithms used in these computations depend on the semantics of the metrics and the time series of
observed values in the archive; a fuller explanation may be found in the white paper Explaining Value In‐
terpolation with PCP Archives found at https://pcp.io/papers/archive-interpolation.pdf.
RECORD-DRIVEN ARCHIVE MODES
If mode is PM_MODE_FORW or PM_MODE_BACK then when metric values are being requested via pmFetch(3) the
archive will be scanned in a forwards (PM_MODE_FORW) or backwards (PM_MODE_BACK) direction in time, until
an archive record is encountered with values for at least one of the requested metrics. This archive
record is used to provide as many of the requested metrics as possible and these are returned with the
timestamp if the record in the archive, which becomes the new current time.
Note that any metrics requested via pmFetch(3) that do not have a value in the archive record at the new
current time will return no values, and so this mode is most useful for archives where all of the metrics
of interest have been logged regularly at the same time in the archive. Otherwise, each pmFetch(3) will
contain only the subset of the requested metrics and any associated instances found in the qualifying
archive record.
The delta parameter is ignored, because the current time is driven by the timestamp in the matching
archive record. So there is no concept of stepping through the archive in regular time with this mode,
although if the requested metrics appear at regular intervals in the archive the current time may advance
by regular intervals, but this is serendipitous.
If no qualifying metrics can be found in the requested direction of searching before the end or start of
the archive is encountered, then pmFetch(3) returns the special error indicator, PM_ERR_EOL.
RECOMMENDATIONS
When processing PCP archives, PM_MODE_INTERP is preferred because:
• This maximizes the information that will be returned in each pmFetch(3)
• This returns values at regular intervals of the current time, independent of the logging frequency for
requested metrics in the archive.
• This works with any PCP archive, as opposed to the record-driven modes which may work acceptably for
archives with regular logging of all requested metrics, but may fail to report complete or useful re‐
sults for other archives.
• This mode provides the closest semantic match to PM_MODE_LIVE and leads to the least user surprise when
moving between real-time monitoring and retrospective analysis.
EXAMPLES
To replay interpolated metrics from an archive at 10 second intervals, the following code fragment could
be used:
struct timeval mytime;
int mydelta = 10 * 1000; /* msec */
pmLogLabel label;
pmResult result;
pmNewContext(PM_CONTEXT_ARCHIVE, "myarchive");
pmGetArchiveLabel(&label);
mytime = label.ll_start;
pmSetMode(PM_MODE_INTERP, &mytime, mydelta)
while (pmFetch(numpmid, pmidlist, &result) != PM_ERR_EOL) {
/*
* process interpolated metric values as of
* result->timestamp
*/
. . .
pmFreeResult(result);
}
The following code fragment may be used to dump values for selected metrics in an archive in reverse tem‐
poral sequence.
struct timespec mytime;
pmResult result;
pmNewContext(PM_CONTEXT_ARCHIVE, "myarchive");
pmGetArchiveEnd(&mytime);
pmSetMode(PM_MODE_BACK, &mytime, NULL);
while (pmFetch(npmid, pmidlist, &result) != PM_ERR_EOL) {
/*
* process logged metric values as of result->timestamp
*/
. . .
pmFreeResult(result);
}
COMPATIBILITY
Prior to PCP 7.0 the when argument was a struct timeval and the delta argument was an int (in units of
milliseconds). To support PMAPI transition, the old interface and semantics can be used if applications
are recompiled with -DPMAPI_VERSION=2.
For a time in PCP 6.x there was a routine with the same semantics as the current pmSetMode called pmSet‐
ModeHighRes although this is now deprecated and compile-time support for pmSetModeHighRes will be removed
in a future release.
DIAGNOSTICS
PM_ERR_MODE
The mode parameter is invalid
SEE ALSO
pmcd(1), PMAPI(3), pmFetch(3), pmFetchArchive(3), pmGetArchiveEnd(3), pmGetArchiveLabel(3),
pmGetInDom(3), pmLookupDesc(3), pmLookupInDom(3), pmLookupLabels(3), pmNameInDom(3) and pmNewContext(3).
Performance Co-Pilot PCP PMSETMODE(3)