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HTML rendering created 2024-06-26 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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PMDA(3) Library Functions Manual PMDA(3)
PMDA - introduction to the Performance Metrics Domain Agent
support library
#include <pcp/pmapi.h>
#include <pcp/pmda.h>
... assorted routines ...
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 simple.now 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 simple.now. 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
time
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 simple.now 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,
{ PMDA_PMID(0,0), PM_TYPE_U32, PM_INDOM_NULL, PM_SEM_INSTANT,
{ 0,0,0,0,0,0} }, },
/* color */
{ (void *)0,
{ PMDA_PMID(0,1), PM_TYPE_32, COLOR_INDOM, PM_SEM_INSTANT,
{ 0,0,0,0,0,0} }, },
/* time.user */
{ (void*)0,
{ PMDA_PMID(1,2), PM_TYPE_DOUBLE, PM_INDOM_NULL, PM_SEM_COUNTER,
{ 0, 1, 0, 0, PM_TIME_SEC, 0 } }, },
/* time.sys */
{ (void*)0,
{ PMDA_PMID(1,3), PM_TYPE_DOUBLE, PM_INDOM_NULL, PM_SEM_COUNTER,
{ 0, 1, 0, 0, PM_TIME_SEC, 0 } }, },
/* now */
{ NULL,
{ PMDA_PMID(2,4), PM_TYPE_U32, NOW_INDOM, PM_SEM_INSTANT,
{ 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 item fields of the
associated pmID. 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
$PCP_SHARE_DIR/lib/pmdaproc.sh which greatly simplify the
installation and removal of a PMDA.
The Install scripts for most PMDAs should only need to specify
the name of the PMDA in iam, call pmdaSetup which sets up some
default variables, checks permissions (you have to be ``root'' to
install or remove a PMDA), checks that you're in the right
directory (somewhere that ends with /pmdas/$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/pmdaproc.sh file and the following section.
Optionally, a PMDA may provide an Upgrade script alongside
Install and Remove. If present this script will be used by the
pmcd startup process to ensure corrections to an installation
have been made before starting the PMDA. Examples of such
corrections include: updates to pmcd.conf when a PMDA script or
binary has been renamed, when the PMDA supports a new format of
its configuration file, or if there is some latent problem from
an earlier install (e.g. some PMDAs may need to introduce use of
the notready keyword in pmcd.conf, as described below).
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.
daemon_opt
We are willing to install the PMDA as a daemon.
Default: true
dso_opt
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
dso_entry
For a DSO PMDA, this is the name of the PMDA's
initialization routine.
Default: ${iam}_init
dso_name
For a DSO PMDA, this is the full pathanme of the PMDA's
DSO file.
Default: $PCP_PMDAS_DIR/$iam/pmda_$iam.$dso_suffix
pipe_opt
For a daemon PMDA, is the default IPC channel via a
pipe(2)?
Default: Platform-specific, so true for most, but false
for Windows
perl_opt
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
pmda_dir
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.
pmda_name
For a daemon PMDA, this is the name of the PMDA's
executable binary relative to the $pmda_dir directory.
Default: pmda$iam
python_opt
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
ipc_prot
For a daemon PMDA, this can be set to either binary or
text. The default is binary and text is rarely used. In
addition, an optional IPC parameter notready can be used
to signify that the PMDA will start up in the notready
state, e.g. ipc_prot="binary notready". Note that the
quotes are required. The IPC parameters for a PMDA appear
in pmcd.conf in the IPC Params column. For further
details, see pmcd(1) but basically pmcd will not issue any
requests to a PMDA that has started in the notready state
until the PMDA sends a PM_ERR_PMDAREADY PDU. This allows
PMDAs with long startup times to initialize correctly
without timing out. For details, see pmdaSendError(3) and
pmcd(1). When a PMDA is in the notready state, any client
requests sent to pmcd for the PMDA domain will return with
the PM_ERR_PMDANOTREADY error.
socket_inet_def
For a daemon PMDA using a socket(2) as the IPC channel the
default Internet port number or service name (if known).
Default: ""
socket_opt
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.
pmns_dupok
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
pmns_name
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.
pmns_source
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.
args Additional command line args for the PMDA. These will be
appended to the PMDA's control line in $PCP_PMCDCONF_PATH.
Default: ""
check_delay
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
signal_delay
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
configdir
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
configfile
Preferred configuration file for the PMDA. Used by
pmdaChooseConfigFile so should be set before calling that
procedure.
Default: ""
default_configfile
Default configuration file for the PMDA. Used by
pmdaChooseConfigFile so should be set before calling that
procedure.
Default: ""
dso_suffix
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.
help_source
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
python_name
Full pathname of the Python script for a Python PMDA.
Default: $pmda_dir/pmda$iam.python or
$pmda_dir/pmda$iam.py
The shell procedures in $PCP_SHARE_DIR/lib/pmdaproc.sh 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/pmdaproc.sh 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/pmdaproc.sh.
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 libpcp_pmda.so.2 and libpcp.so.2. Pre-existing Daemon PMDAs
specifying PMDA_INTERFACE_1 will continue to function using the
backwards compatible libpcp_pmda.so.1 and libpcp.so.1 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.
Any PMDA which uses this library can set PMAPI(3) debugging
control option libpmda (with -Dlibpmda on the command line or via
3pmSetDebug(3)) to 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)).
/usr/include/pcp/pmda.h
Header file for the PMDA support library.
/usr/lib/libpcp_pmda.so
Dynamic library containing PMDA support library routines.
$PCP_PMDAS_DIR/trivial
The source of the trivial PMDA.
$PCP_PMDAS_DIR/simple
The source of the simple PMDA.
$PCP_PMDAS_DIR/txmon
The source of the txmon PMDA.
$PCP_PMCDCONF_PATH
Configuration file for pmcd(1).
$PCP_VAR_DIR/pmns
Location of namespace descriptions for every PMDA.
$PCP_VAR_DIR/pmns/stdpmid
The unique domain identifiers for each PMDA.
$PCP_SHARE_DIR/lib/pmdaproc.sh
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.
dbpmda(1), newhelp(1), pmcd(1), pmnsadd(1), pmnsdel(1), PMAPI(3),
PMWEBAPI(3), pmdaConnect(3), pmdaDaemon(3), pmdaDesc(3),
pmdaDSO(3), pmdaFetch(3), pmdaGetOpt(3), pmdaInit(3),
pmdaInstance(3), pmdaMain(3), pmdaOpenLog(3), pmdaProfile(3),
pmdaStore(3), pmdaText(3), pmLookupDesc(3) and PMNS(5).
For a complete description of the pcp_pmda library and the PMDA
development process, refer to the Insight book Performance Co-
Pilot Programmer's Guide.
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 protected]. This page was obtained from the
project's upstream Git repository
⟨https://github.com/performancecopilot/pcp.git⟩ on 2024-06-14.
(At that time, the date of the most recent commit that was found
in the repository was 2024-06-14.) 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
[email protected]
Performance Co-Pilot PCP PMDA(3)
Pages that refer to this page: dbpmda(1), genpmda(1), indomcachectl(1), pcpcompat(1), pmcd(1), pmdaroot(1), pmdasimple(1), pmdatrivial(1), pmdatxmon(1), pmdaweblog(1), pmapi(3), pmda(3), pmdaattribute(3), pmdacache(3), pmdachildren(3), pmdaconnect(3), pmdadaemon(3), pmdadesc(3), pmdadso(3), pmdaeventarray(3), pmdaeventclient(3), pmdaeventqueue(3), pmdafetch(3), pmdagetoptions(3), pmdahelp(3), pmdainit(3), pmdainstance(3), pmdainterfacemoved(3), pmdalabel(3), pmdamain(3), pmdaname(3), pmdaopenlog(3), pmdapmid(3), pmdaprofile(3), pmdarootconnect(3), pmdasenderror(3), pmdastore(3), pmdatext(3), pmdatrace(3), pmjsonget(3), mmv(5)
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