PMSETMODE(3)

NAME
pmSetMode, pmSetModeHighRes - set collection time and mode parameters for the current PMAPI context

C SYNOPSIS
#include <pcp/pmapi.h>

int pmSetMode(int mode, const struct timeval *when, int delta);
int pmSetModeHighRes(int mode, const struct timespec *when, const struct timespec *delta);

cc ... -lpcp

DESCRIPTION
pmSetMode and pmSetModeHighRes are used to define the collection time and/or mode for accessing performance metrics and metadata in the current Performance Metrics Application Programming Interface (PMAPI) context. This mode affects the semantics of subsequent calls to the following PMAPI routines: pmFetch(3), pmFetchHighRes(3), pmFetchArchive(3), pmLookupDesc(3), pmGetInDom(3), pmLookupInDom(3), pmLookupLabels(3) and pmNameInDom(3).

Intended mainly for retrospective analyis of performance metrics from a PCP archive, the options described below allow an application to implement seeking to an arbitrary time within the archive, playback, fast forward, reverse, etc. by alternating calls to pmSetMode and pmFetch(3) or pmFetchHighRes(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 or pmSetModeHighRes.

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 field in the structure returned from a call to pmGetArchiveLabel(3) or the field in the structure returned from a call to pmGetHighResArchiveLabel(3), and the time at the end of the archive can be established by calling pmGetArchiveEnd(3) or pmGetHighResArchiveEnd(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) or pmFetchHighRes(3) the current time is returned as the timestamp in the 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) or pmFetchHighRes(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.

By default the delta parameter of pmSetMode is interpreted as milliseconds (but see the LARGE DELTA VALUES section below).

In the pmSetModeHighRes variant of this interface, the delta parameter is a so this can directly represent any interval.

When metric values are being requested via pmFetch(3) or pmFetchHighRes(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 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 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 Interpolation 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) or pmFetchHighRes(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) or pmFetchHighRes(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) or pmFetchHighRes(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) or pmFetchHighRes(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) or pmFetchHighRes(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 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.

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 temporal sequence.

struct timeval mytime;
pmResult result;

pmNewContext(PM_CONTEXT_ARCHIVE, "myarchive");
pmGetArchiveEnd(&mytime);
pmSetMode(PM_MODE_BACK, &mytime, 0);

while (pmFetch(npmid, pmidlist, &result) != PM_ERR_EOL) {
    /*
     * process logged metric values as of result->timestamp
     */
    . . .
    pmFreeResult(result);
}

LARGE DELTA VALUES
The simplest mechanism to set large values for delta is to use the pmSetModeHighRes interface. However, the pmSetMode interface supports the XTB (eXtended Time Base) mechanism to allow for values in units other than milliseconds.

Because the delta parameter to pmSetMode is an int and treated as milliseconds by default there is a limit on the maximum absolute value of delta that can be specified with this default interpretation, namely about 24 days if a signed int has 31 bits of precision. To accommodate longer values of delta the high-order bits of the mode parameter is also used to optionally set the units of time for the delta parameter. To specify the units of time use the PM_XTB_SET macro with one of the values PM_TIME_NSEC, PM_TIME_MSEC, PM_TIME_SEC, etc. to set the mode as follows:

PM_MODE_INTERP | PM_XTB_SET(PM_TIME_XXXX)

The following code shows how this could be done if the desired delta is initially encoded in interval (a struct timeval).

struct timeval interval;
int mode;

mode = ...

if (abs(interval.tv_sec / (3600*24)) <= 24) {
    /* default encoding of milliseconds is fine */
    mode = PM_MODE_INTERP;
    delta = interval.tv_sec * 1000 + (interval.tv_usec + 500)/ 1000;
}
else {
    /* encode delta in units of seconds */
    mode = PM_MODE_INTERP | PM_XTB_SET(PM_TIME_SEC);
    delta = interval.tv_sec + (interval.tv_usec + 500000)/ 1000000;
}

For millisecond encoding of delta, using PM_XTB_SET(PM_TIME_MSEC) is functionally equivalent to not using PM_XTB_SET at all.

DIAGNOSTICS

PM_ERR_MODE
    The mode parameter is invalid

SEE ALSO
pmcd(1), PMAPI(3), pmFetch(3), pmFetchArchive(3), pmFetchHighRes(3), pmGetArchiveEnd(3), pmGetArchiveLabel(3), pmGetHighResArchiveEnd(3), pmGetHighResArchiveLabel(3), pmGetInDom(3), pmLookupDesc(3), pmLookupInDom(3), pmLookupLabels(3), pmNameInDom(3) and pmNewContext(3).