PMDA(3) Library Functions Manual PMDA(3)
PMDA - 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
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 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
$PCP_SHARE_DIR/lib/pmdaproc.sh which greatly simplify the
installation and removal of a PMDA.
The Install scripts for most PMDAs should only need to specify the
name of the PMDA in iam, call pmdaSetup which sets up some default
variables, checks permissions (you have to be ``root'' to install or
remove a PMDA), checks that you're in the right directory (somewhere
that ends with /pmdas/$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.
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
pmda_interface 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
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.
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
libpcp_pmda.so.2 and libpcp.so.2. Pre-existing Daemon PMDAs
specifying PMDA_PROTOCOL_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.
/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),
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
pcp@oss.sgi.com. This page was obtained from the project's upstream
Git repository ⟨git://git.pcp.io/pcp⟩ on 2017-07-05. 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 man-pages@man7.org
Performance Co-Pilot PCP PMDA(3)
Pages that refer to this page: dbpmda(1), genpmda(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), pmdamain(3), pmdaname(3), pmdaopenlog(3), pmdapmid(3), pmdaprofile(3), pmdarootconnect(3), pmdastore(3), pmdatext(3), pmdatrace(3), pmjsonget(3)