pmSetMode (3) - Linux Manuals

pmSetMode: set collection time parameters for the current PMAPI context

NAME

pmSetMode - set collection time parameters for the current PMAPI context

C SYNOPSIS

#include <pcp/pmapi.h>

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

cc ... -lpcp

DESCRIPTION

pmSetMode is 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 affects the semantics of subsequent calls to the following PMAPI routines: pmFetch(3), pmFetchArchive(3), pmLookupDesc(3), pmGetInDom(3), pmLookupInDom(3) and pmNameInDom(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 are ignored. PM_MODE_LIVE is the default mode when a new PMAPI context of type PM_CONTEXT_HOST is created.

If the mode is not PM_MODE_LIVE, then the when parameter defines a time origin, and all requests for meta-data (metric descriptions and instance identifiers from the instance domains) will be processed to reflect the state of the meta-data as of the time origin, i.e. we use the last state of this information at, or before, the time origin.

If the mode is PM_MODE_INTERP then, in the case of pmFetch(3), the underlying code will use an interpolation scheme to compute the values of the metrics from the values recorded for times in the proximity of the time origin. A mode of PM_MODE_INTERP may only be used with an archive context.

If the mode is PM_MODE_FORW then, in the case of pmFetch(3), the collection of recorded metric values will be scanned in a forwards direction in time, until values for at least one of the requested metrics is located after the time origin, and then all requested metrics stored in the set of archives at that time will be returned with the corresponding timestamp. A mode of PM_MODE_FORW may only be used with an archive context.

If the mode is PM_MODE_BACK then, the situation is the same as for PM_MODE_FORW, except a pmFetch(3) will be serviced by scanning the collection of recorded metrics in a backwards direction in time for metrics before the time origin. A mode of PM_MODE_BACK may only be used with an archive context.

If the mode is PM_MODE_FORW or PM_MODE_BACK, and no qualifying metrics can be found in the requested direction of searching before the end or start of the set of archive logs is found, then pmFetch(3) returns the special error indicator, PM_ERR_EOL.

For modes other than PM_MODE_LIVE, after each successful pmFetch(3), the time origin is reset to the timestamp returned via the pmResult structure from pmFetch(3).

The pmSetMode parameter delta defines an additional number of time units that should be used to adjust the time origin (forwards or backwards), after the new time origin from the pmResult has been determined. This automatic adjustment of the time origin only occurs when the mode is PM_MODE_INTERP, and the adjustment is applied, even if the pmFetch(3) fails because the time origin is outside the range defined by the records in a set of archive logs, i.e. returns PM_ERR_EOL.

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

Using these mode options, an application can implement replay, playback, fast forward, reverse, etc. for performance metric values held in the set of archive logs by alternating calls to pmSetMode and pmFetch(3).

As a special case, if when is NULL then the mode and delta arguments are used as described above, but the current time in the archive is not altered.

EXAMPLES

The following code fragment may be used to dump just those values recorded in an archive in correct temporal sequence, for a selected set of performance metrics; this uses the default collection time mechanisms.

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

Alternatively, to replay interpolated metrics from the log in reverse chronological order, at 10 second intervals (of recorded time), the following code fragment could be used.

struct timeval mytime;

mytime.tv_sec = 0x7fffffff; /* or use pmGetArchiveEnd(&mtime) */
pmSetMode(PM_MODE_BACK, &mytime, 0);
pmFetchArchive(&result);
mytime = result->timestamp;
pmFreeResult(result);
pmSetMode(PM_MODE_INTERP | PM_XTB_SET(PM_TIME_SEC), &mytime, -10);

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

LARGE DELTA VALUES

Because delta 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