PMDA(3) introduction to the Performance Metrics Domain Agent support library


#include <pcp/pmapi.h>
#include <pcp/impl.h>
#include <pcp/pmda.h>

cc ... -lpcp_pmda -lpcp


To assist in the development of Performance Metric Domain Agents (PMDAs) for the Performance Co-Pilot (PCP), a procedural interface is provided that extends the Performance Metrics Application Programming Interface ( PMAPI(3)) library. These procedures are designed to enable a programmer to quickly build a PMDA which can then be tested and refined. However, this also implies that a PMDA has a particular structure which may not be suitable for all PMDA implementations.

Once you are familiar with the PCP and PMDA frameworks, you can quickly implement a new PMDA with only a few data structures and functions. This is covered in far greater detail in the Performance Co-Pilot Programmer's Guide.

A PMDA is responsible for a set of performance metrics, in the sense that it must respond to requests from pmcd(1) for information about performance metrics, instance domains, and instantiated values.

This man page contains sections of the simple PMDA which is located at $PCP_PMDAS_DIR/simple.


Two approaches may be used for connecting a PMDA to a pmcd(1) process. A Dynamic Shared Object (DSO) can be attached by pmcd(1) using dlopen(3) when the pmcd(1) process is started. A procedural interface referenced through a shared data structure is used to handle requests from pmcd(1) to the PMDA .

The preferred approach is for a separate process (daemon) to communicate with pmcd(1) using the Performance Data Units (PDU) Inter-Process Communication (IPC) protocol.

All PMDAs are launched and controlled by the pmcd(1) process on the local host. The requests from the clients are received by pmcd(1) and forwarded to the appropriate PMDAs. Responses, when required, are returned through pmcd(1) to the clients. The requests (PDUs) that may be sent to a PMDA from pmcd(1) are PDU_FETCH, PDU_PROFILE, PDU_INSTANCE_REQ, PDU_DESC_REQ, PDU_TEXT_REQ and PDU_RESULT. If the PMDA implements any dynamic metrics it may also receive PDU_PMNS_CHILD, PDU_PMNS_IDS, PDU_PMNS_NAMES and PDU_PMNS_TRAVERSE PDUs.


To allow a consistent framework, pmdaMain(3) can be used by a daemon PMDA to handle the communication protocol using the same callbacks as a DSO PMDA. The structure pmdaInterface is used to convey the common procedural interface and state information that is used by pmcd(1) and a PMDA. This state information includes tables describing the supported metrics and instance domains.

As most of the procedural interface is identical for all PMDAs, they are provided as part of this support library (pmdaProfile(3), pmdaFetch(3), pmdaInstance(3), pmdaDesc(3), pmdaText(3) and pmdaStore(3)). However, these routines require access to the pmdaInterface state information so it must be correctly initialized using pmdaConnect(3), pmdaDaemon(3), pmdaOpenLog(3), pmdaDSO(3), pmdaGetOpt(3) and pmdaInit(3).


Three structures are declared in /usr/include/pcp/pmda.h which provide a framework for declaring the metrics and instances supported by the PMDA.

Every instance requires a unique integer identifier and a unique name, as defined by the structure pmdaInstid:

 * Instance description: index and name
typedef struct {
    int         i_inst;        /* internal instance identifier */
    char        *i_name;       /* external instance identifier */
} pmdaInstid;

An instance domain requires its own unique identification (pmInDom), the number of instances the domain represents, and a pointer to an array of instance descriptions. This is defined in the structure pmdaIndom:

 * Instance domain description: unique instance id, 
 * number of instances in this domain, and the list of 
 * instances (not null terminated).
typedef struct {
    pmInDom     it_indom;       /* indom, filled in */
    int         it_numinst;     /* number of instances */
    pmdaInstid  *it_set;        /* instance identifiers */
} pmdaIndom;

The simple PMDA has one instance domain for simple.color with three instances (red, green and blue), and a second instance domain for with instances which can be specified at run-time. These instance domains are defined as:

static pmdaInstid _color[] = {
    { 0, "red" }, { 1, "green" }, { 2, "blue" }
static pmdaInstid *_timenow = NULL;
static pmdaIndom indomtab[] = {
#define COLOR_INDOM     0
    { COLOR_INDOM, 3, _color },
#define NOW_INDOM 1
    { NOW_INDOM, 0, NULL },

The preprocessor macros COLOR_INDOM and NOW_INDOM are used in the metric description table to identify the instance domains of individual metrics. These correspond to the serial value in the instance domain pmInDom structure (the domain field is set by pmdaInit(3) at run-time). The serial value must be unique for each instance domain within the PMDA.

The indom table shown above which is usually passed to pmdaInit(3) does not need to be created if one wants to write one's own Fetch and Instance functions. See pmdaInit(3) for more details.


Every PMDA has its own unique namespace using the format defined in pmns(5). In summary, the namespace matches the names of the metrics to the unique identifier. The simple PMDA defines five metrics: simple.numfetch, simple.color, simple.time.user, simple.time.sys and The namespace for these metrics is defined in $PCP_PMDAS_DIR/simple/pmns and is installed as:

simple {
    numfetch    253:0:0
    color       253:0:1
    now         253:2:4
simple.time {
    user        253:1:2
    sys         253:1:3

The domain number of 253 is obtained from $PCP_VAR_DIR/pmns/stdpmid. New PMDAs should specify a unique domain number in this file, and obtain the number during installation. This allows the domain number to change by modifying only the file $PCP_VAR_DIR/pmns/stdpmid.

The simple.time and metrics are defined in separate clusters to the other metrics which allows a PMDA to support more than 1024 metrics, as well as grouping similar metrics together. Therefore, the item numbers for a new cluster may be identical to the item numbers in other clusters. The simple PMDA continues to increment the item numbers to permit direct mapping (see pmdaInit(3)).

The namespace file should be installed and removed with the agent using pmnsadd(1) and pmnsdel(1). See the later sections on INSTALLATION and REMOVAL.

A simple ASCII namespace can be constructed by creating a file similar to $PCP_PMDAS_DIR/simple/root:

 * fake "root" for validating the local PMNS subtree
#include "$PCP_VAR_DIR/pmns/stdpmid"
root { simple }
#include "pmns"

and can be referred to with the -n option in most PCP tools.


Each metric requires a description (pmDesc), which contains its Performance Metric Identifier (PMID), data type specification, instance domain, semantics and units (see pmLookupDesc(3)). A handle is also provided for application specific information in the pmdaMetric structure:

 * Metric description: handle for extending description, 
 * and the description.
typedef struct {
    void*       m_user;         /* for users external use */
    pmDesc      m_desc;         /* metric description */
} pmdaMetric;

The simple PMDA defines the metrics as:

static pmdaMetric metrictab[] = {
/* numfetch */
    { (void *)0, 
        { 0,0,0,0,0,0} }, },
/* color */
    { (void *)0, 
        { 0,0,0,0,0,0} }, },
/* time.user */
    { (void*)0,
        { 0, 1, 0, 0, PM_TIME_SEC, 0 } }, },
/* time.sys */
    { (void*)0,
        { 0, 1, 0, 0, PM_TIME_SEC, 0 } }, },
/* now */
    { NULL,
        { 0,0,0,0,0,0 } }, },

The macro PMDA_PMID (defined in /usr/include/pcp/pmda.h) is used to specify each metric's cluster and unit number in the __pmID_int structure defined in /usr/include/pcp/impl.h. As with instance domains, the domain field is set by pmdaInit(3) at run-time, however, the default domain is assumed to be defined by the PMDA in the macro MYDOMAIN.

The metric table shown above which is usually passed to pmdaInit(3) does not need to be created if one wants to write one's own Fetch and Descriptor functions. See pmdaInit(3) for more details.


A PMDA that is run as a DSO is opened by pmcd(1) with dlopen(3). pmcd(1) will call the PMDA's initialization function that is specified in $PCP_PMCDCONF_PATH. This function is passed a pointer to a pmdaInterface structure which must be completed. Any callbacks which are not the default PMDA support library callbacks must be specified in the pmdaInterface structure.

The simple PMDA uses its own store and fetch callback. simple_fetch() calls pmdaFetch(3) which requires a callback to be set with pmdaSetFetchCallBack(3) as can be seen in $PCP_PMDAS_DIR/simple/simple.c.

The flag _isDSO is used to determine if the PMDA is a daemon or a DSO so that the correct initialization routine, pmdaDaemon(3) or pmdaDSO(3), is called.


A PMDA that is run as a daemon is forked and executed by pmcd(1). Therefore, unlike a DSO PMDA, the starting point for a daemon PMDA is main(). The agent should parse the command line arguments, create a log file and initialize some data structures that pmcd(1) would initialize for a DSO agent.

The pmdaInterface structure must be completely defined by the daemon PMDA. The function pmdaDaemon(3) can be called at the start of main() to set most of these fields. Command line parsing can be simplified by using pmdaGetOpt(3), which is similar to getopt(2), but extracts a common set of options into the pmdaInterface structure. stderr can be mapped to a log file using pmdaOpenLog(3) to simplify debugging and error messages. The connection to pmcd(1) can be made with pmdaConnect(3) and the loop which handles the incoming PDUs, pmdaMain(3), should be the last function called. This can be seen in $PCP_PMDAS_DIR/simple/simple.c.

The simple_init() routine is common to an agent that can be run as both a Daemon and DSO PMDA.


Each PMDA must be able to provide pmcd(1) with the help text for each metric. Most PMDAs use specially created files with indexes to support efficient retrieval of the help text. Tools are provided with PCP to create the help text files of appropriate format. See newhelp(1).


A series of shell procedures are defined in $PCP_SHARE_DIR/lib/ which greatly simplify the installation and removal of a PMDA.

The Install scripts for most PMDAs should only need to specify the name of the PMDA in iam, call pmdaSetup which sets up some default variables, checks permissions (you have to be ``root'' to install or remove a PMDA), checks that you're in the right directory (somewhere that ends with /pmdas/$iam), optionally generate the Performance Metrics Name Space (PMNS) and PMDA domain number files for Perl or Python PMDAs, checks the PMDA domain number is valid, etc., specify the communication protocols, and finally call pmdaInstall to do all the work of updating the PMNS, updating the pmcd(1) control file, notifying or restarting pmcd(1),

Beyond pmdaSetup and pmdaInstall, another optional helper routines is pmdaChooseConfigFile that may be used to interactively select or create a PMDA-specific configuration file, e.g. pmdalogger(1).

The Remove scripts are even simpler as setting up the communication protocols are not required, so set the name of the PMDA in iam, then call pmdaSetup followed by a call to pmdaRemove.

Further information is contained in the $PCP_SHARE_DIR/lib/ file and the following section.


As outlined below there are a number of variables that can be set in a PMDA's Install script to influence the behaviour of the installation procedures. These would typically need to be set before the call to pmdaInstall, but in some instances (like $iam and the cases specifically noted below), before the call to pmdaSetup.

The following variables control the communication options between the PMDA and pmcd(1). At least one of $daemon_opt, $dso_opt, $perl_opt or $python_opt must be set to define the supported mode(s) of communication. If more than one of these is set the user will be prompted to make a selection when the Install script is run.

We are willing to install the PMDA as a daemon.
Default: true
We are willing to install the PMDA as a DSO, so pmcd(1) will use the dynamic linking loader to attach the PMDA's DSO at run-time and communication from pmcd(1) to the PMDA and back uses procedure calls, not an IPC channel.
Default: false
For a DSO PMDA, this is the name of the PMDA's initialization routine.
Default: ${iam}_init
For a DSO PMDA, this is the full pathanme of the PMDA's DSO file.
Default: $PCP_PMDAS_DIR/$iam/pmda_$iam.$dso_suffix
If true then the PMDA needs to run with the user-id of ``root'' or something other than the user-id of ``pcp''. This means pmcd(1) needs to be restarted and run initially as ``root'' until all the PMDAs are started at which time pmcd(1) reverts to the user ``pcp''. Otherwise, the new PMDA will be run as the user ``pcp'' after pmcd(1) is sent a SIGHUP signal.
Default: true
For a daemon PMDA, is the default IPC channel via a pipe(2)?
Default: Platform-specific, so true for most, but false for Windows
We are willing to install the PMDA as a Perl script and pmcd(1) will use the perl(1) interpreter to run the PMDA.
Default: false
Full pathname to the directory where the PMDA's installation files (executable, script, PMNS source, help text source, etc) are to be found.
Default: output from pwd(1)

If set, must be done before the call to pmdaSetup.

For a daemon PMDA, this is the name of the PMDA's executable binary relative to the $pmda_dir directory.
Default: pmda$iam
We are willing to install the PMDA as a Python script and pmcd(1) will use the python(1) interpreter to run the PMDA.
Default: false
There are multiple versions of the interface protocol between a PMDA and the libpcp_pmda library. If the PMDA is built to be installed as a daemon or a DSO, it will included a call to pmdaDaemon(3) or pmdaDSO(3), and the interface argument there will be one of the PMDA_INTERFACE macros from <pcp/pmda.h>. The value that is used there should match the value for pmda_interface. The value is only used in the installation procedure to determine the format of help text to be generated by newhelp(1) and in this context all values of pmda_interface greater than 1 are equivalent (the format changed between PMDA_INTERFACE_1 and PMDA_INTERFACE_2).
Default: 3
For a daemon PMDA using a socket(2) as the IPC channel the default Internet port number or service name (if known).
Default: ""
For a daemon PMDA, is the default IPC channel via a socket(2)?
Default: Platform-specific, so false for most, but true for Windows

The following variables control the PMNS options.

Most PMDAs do not have duplicate names for the same PMID in their PMNS. But if this is not the case, pmns_dupok should be set to true.
Default: false
Each PMDA will add one or more non-leaf nodes to the top of the PMNS. The most common case is that all of the metrics for a PMDA will be placed below the node named $iam. If this is not the case, and especially when the PMDA adds more than one non-leaf node at the top of the PMNS, pmns_name needs to be set to the list of node names (separated by white space), e.g. for pmdaproc(1) pmns_name is set to "proc cgroup hotproc".
Default: $iam

It is most important that if pmns_name is set to a non-default value in the Install script then it must also be set to the same value in the Remove script.

The name of the PMDA's PMNS source file. By default, the name is interpreted as a relative pathname from the $pmda_dir directory.
Default: pmns

The following variables provide assorted additional options associated with the installation of a PMDA.

Additional command line args for the PMDA. These will be appended to the PMDA's control line in $PCP_PMCDCONF_PATH.
Default: ""
Delay (in seconds) after finishing the PMDA installation (or removal) before checking the availability of metrics from the PMDA. May need to be increased if the PMDA has a lengthy startup procedure.
Default: 0.3
Delay (in seconds) after notifying pmcd(1) with a signal. Required to allow pmcd(1) to complete processing before proceeding to the next step of the installation (or removal).
Default: 1
Determines the directory in which a PMDA's configuration file will be stored. Used by pmdaChooseConfigFile so should be set before calling that procedure.
Default: $PCP_VAR_DIR/config/$iam
Preferred configuration file for the PMDA. Used by pmdaChooseConfigFile so should be set before calling that procedure.
Default: ""
Default configuration file for the PMDA. Used by pmdaChooseConfigFile so should be set before calling that procedure.
Default: ""
Standard suffix for a DSO. Should not need to be changed under normal circumstances.
Default: Platform-specific, so 'so' for Linux, 'dylib' for Mac OS X, 'dll' for Windows, etc.

If set, must be done before the call to pmdaSetup.

The name of the help text source file that should be used as input to pmnewhelp(1). By default, the name is interpreted as a relative pathname from the $pmda_dir directory.
Default: help
Full pathname of the Python script for a Python PMDA.
Default: $pmda_dir/pmda$iam.python or $pmda_dir/pmda$

The shell procedures in $PCP_SHARE_DIR/lib/ manipulate a number of temporary files using the variable $__tmp as the prefix for the name of the temporary files. $__tmp is a directory that is created, used and removed internally within the procedures of $PCP_SHARE_DIR/lib/ but can also be used as the prefix for temporary files needed by a PMDA's Install or Remove scripts. When used in this way, $__tmp should be followed by a ``.'' and then a suffix, e.g. $__tmp.myfoo. The Install and Remove scripts should not use other temporary file name prefixes nor use sh(1) trap statements to clean up temporary files as this is all done within $PCP_SHARE_DIR/lib/


Any PMDA which uses this library can set PMAPI(3) debug control variable pmDebug (with -D on the command line) to DBG_TRACE_LIBPMDA to enable the display of debugging information which may be useful during development (see pmdbg(1)).

The status field of the pmdaInterface structure should be zero after pmdaDaemon, pmdaDSO, pmdaGetOpt, pmdaConnect and pmdaInit are called. A value less than zero indicates that initialization has failed.

Some error messages that are common to most functions in this library are:

PMDA interface version interface not supported
Most of the functions require that the comm.version field of the pmdaInterface structure be set to PMDA_INTERFACE_2 or later. PMDA_INTERFACE_2 or PMDA_INTERFACE_3 implies that the version.two fields are correctly initialized, while PMDA_INTERFACE_4 implies that the version.four fields are correctly initialized (see pmdaDaemon(3) and pmdaDSO(3)).


Failing to complete any of the data structures or calling any of the library routines out of order may cause unexpected behavior in the PMDA.

Due to changes to the PMAPI(3) and PMDA(3) API in the PCP 2.0 release, as described in the product release notes, PMDAs built using PCP 2.0 must specify PMDA_INTERFACE_2 or later and link with and Pre-existing Daemon PMDAs specifying PMDA_PROTOCOL_1 will continue to function using the backwards compatible and libraries and may be recompiled using the headers installed in /usr/include/pcp1.x/ without any modification. These backwards compatible headers and libraries are contained in the pcp.sw.compat subsystem.


Header file for the PMDA support library.
Dynamic library containing PMDA support library routines.
The source of the trivial PMDA.
The source of the simple PMDA.
The source of the txmon PMDA.
Configuration file for pmcd(1).
Location of namespace descriptions for every PMDA.
The unique domain identifiers for each PMDA.
Shell procedures for installing and removing a PMDA.


Environment variables with the prefix PCP_ are used to parameterize the file and directory names used by PCP. On each installation, the file /etc/pcp.conf contains the local values for these variables. The $PCP_CONF variable may be used to specify an alternative configuration file, as described in pcp.conf(5). Values for these variables may be obtained programmatically using the pmGetConfig(3) function.