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NAME | C SYNOPSIS | DESCRIPTION | TIME-DRIVEN ARCHIVE MODE | RECORD-DRIVEN ARCHIVE MODES | RECOMMENDATIONS | EXAMPLES | COMPATIBILITY | DIAGNOSTICS | SEE ALSO | COLOPHON |
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PMSETMODE(3) Library Functions Manual PMSETMODE(3)
pmSetMode - set collection time and mode parameters for the
current PMAPI context
#include <pcp/pmapi.h>
int pmSetMode(int mode, const struct timespec *when,
const struct timespec *delta);
cc ... -lpcp
pmSetMode are used to define the collection time and/or mode for
accessing performance metrics and metadata in the current Perfor‐
mance Metrics Application Programming Interface (PMAPI) context.
This mode affects the semantics of subsequent calls to the follow‐
ing 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 described below allow an applica‐
tion to implement seeking to an arbitrary time within the archive,
playback, fast forward, reverse, etc. by alternating calls to pm‐
SetMode 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 pmSetMode
The other values of mode are used with PCP archives where the as‐
sociated 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 val‐
ues 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 struc‐
ture 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 established by calling pmGetArchiveEnd(3)
As a special case, if when is NULL then the mode and delta argu‐
ments are used as described below, but the current time in the
archive is not altered.
If mode is PM_MODE_INTERP then as metric values are retrieved from
the archive with pmFetch(3) the current 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 comput‐
ed from the observed 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 be‐
fore 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 Ex‐
plaining Value Interpolation with PCP Archives found at
https://pcp.io/papers/archive-interpolation.pdf .
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 inter‐
vals in the archive the current time may advance by regular inter‐
vals, 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 encoun‐
tered, then pmFetch(3) returns the special error indicator,
PM_ERR_EOL.
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 log‐
ging of all requested metrics, but may fail to report complete
or useful results 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.
To replay interpolated metrics from an archive at 10 second inter‐
vals, 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 select‐
ed metrics in an archive in reverse temporal 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);
}
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 pmSetModeHighRes although this is
now deprecated and compile-time support for pmSetModeHighRes will
be removed in a future release.
PM_ERR_MODE
The mode parameter is invalid
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).
This page is part of the PCP (Performance Co-Pilot) project. In‐
formation 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@groups.io. This page was obtained from the project's upstream
Git repository ⟨https://github.com/performancecopilot/pcp.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-11.) 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 PMSETMODE(3)
Pages that refer to this page: pcpintro(1), pcpintro(3), pmapi(3), pmfetch(3), pmfetcharchive(3), pmfetchgroup(3), pmgetarchiveend(3), pmnewcontext(3), pmstore(3), pmtime(3), QmcGroup(3)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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