This documentation is intended for internal Performance Co-Pilot
(PCP) developer use.
These interfaces are not part of the PCP APIs that are guaranteed to
remain fixed across releases, and they may not work, or may provide
different semantics at some point in the future.
__pmControlLog may be used to enable or disable the archive logging
for particular performance metrics, as identified by the request
parameter; see pmFetch(3) for an explanation of the pmResult
The application must have previously issued a call to
__pmConnectLogger(3) to establish a control-port connection to the
pmlogger(1) instance to whom the control request is to be directed,
and fd (the result from __pmConnectLogger(3)) identifies this
Within request, only the details of the performance metrics and their
associated instances will be used, i.e. the values of the metrics,
if any, will be ignored. request would typically be constructed as
the result of an earlier call to pmFetch(3). For metrics with a
singular value (having an instance domain of PM_INDOM_NULL) the
corresponding pmValueSet should have the value one in the numval
field and PM_IN_NULL as the inst field of the single pmValue
supplied. If multiple explicit instances are to be logged, the
numval field of the pmValueSet should contain the number of instances
supplied and the inst fields of the pmValue structures should contain
specific instance identifiers (which may not have the reserved value
If the numval field within any of the pmValueSet structures in
request has a value of zero, it indicates that all available
instances of the metric should be used. Enumeration of the instance
domain is deferred until the logger fetches the metric prior to
writing it to the log, rather than being performed when the
__pmControlLog request is received. This is useful for metrics with
instance domains that change over time. It is an error to specify
numval equal to zero if the corresponding metric has a singular value
(no instance domain).
There are several sorts of logging control available, namely
mandatory or advisory, as defined by the control argument, and on,
off or maybe as defined by the state argument. These different types
of control may be used to ensure that some performance metrics can be
guaranteed to always be in the log, while others may be dynamically
enabled or disabled as determined by the level and type of system
The actual action to be performed is defined by the combination of
control and state as follows. If control is PM_LOG_MANDATORY and
state is PM_LOG_ON, then logging is enabled. If control is
PM_LOG_MANDATORY and state is PM_LOG_OFF, then logging is disabled.
If control is PM_LOG_MANDATORY and state is PM_LOG_MAYBE, then
subsequent advisory controls will be honored. If the logging state
prior to the request was mandatory (on or off), the state is changed
to advisory off. If the logging state was already advisory (either
on or off), it remains unchanged. If control is PM_LOG_ADVISORY and
the last mandatory control for the metric was PM_LOG_MAYBE, then
logging is enabled or disabled as specified by the state argument,
i.e. PM_LOG_ON or PM_LOG_OFF. When the arguments state and control
specify a request to change the logging behavior, the argument delta
defines the logging interval in milliseconds to be applied to all
metrics and instances identified in request.
The result argument status returns the current logging state for each
of the nominated performance metrics. There is a 1:1 correspondence
between the elements of request and status. For metrics in request
that have pmValueSets with numval equal to zero, the corresponding
pmValueSet in result will contain a value for each available instance
at the time of the call. Each metric value in status will have the
current logging state encoded in it. The detailed outcome of the
operation for each metric can be determined by comparing these values
to that requested via control, state and delta.
Macros defined in libpcp.h may be used to extract the state and
logging interval from the returned metric values. PMLC_GET_ON
returns true if logging is on, or false if it is off; PMLC_GET_MAND
returns true if logging is mandatory, or false if it is advisory;
PMLC_GET_INLOG returns true if the metric has been logged at least
once, or false otherwise; PMLC_GET_AVAIL returns true if the metric
was available from its source the last time it was supposed to be
logged, or false if it was unavailable; and PMLC_GET_DELTA returns
the current logging interval for the metric (in milliseconds).
PMLC_MAX_DELTA defines the greatest delta that can be returned in an
encoded metric value.
As a special case, when control is PM_LOG_ENQUIRE, state and delta
are ignored, and status returns the current logging state of the
nominated performance metrics (this variant makes no changes to the
If the value of the logging interval is 0, either for delta in a
request to change state to PM_LOG_ON, or encoded in the value
returned from PM_LOG_ENQUIRE, then this corresponds to the special
``once only'' logging of metrics that appear once in the archive log,
and are never logged again.
__pmControlLog returns zero on success.
This routine is not thread-safe as there is no serialization on the
use of the communication channel between the sending of the request
and receiving the reply. It is assumed that the caller is single-
threaded, which is true for the only current user of this routine,
The number of metrics in request is less than one.
One or more of the pmValueSets in request had numval (the
number of instances) less than one.
EINVAL An invalid combination of control and state was specified, or
delta was negative.
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
email@example.com. This page was obtained from the project's upstream
Git repository ⟨https://github.com/performancecopilot/pcp.git⟩ on
2020-08-13. (At that time, the date of the most recent commit that
was found in the repository was 2020-08-11.) 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
Performance Co-Pilot PCP PMCONTROLLOG(3)