path: root/man/man3/pmda.3

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.TH PMDA 3 "PCP" "Performance Co-Pilot"
.SH NAME
\f3PMDA\f1 \- introduction to the Performance Metrics Domain Agent support library
.SH "C SYNOPSIS"
.ft 3
#include <pcp/pmapi.h>
.br
#include <pcp/impl.h>
.br
#include <pcp/pmda.h>
.sp
cc ... \-lpcp_pmda \-lpcp
.ft 1
.SH DESCRIPTION
.de CW
.ie t \f(CW\\$1\f1\\$2
.el \fI\\$1\f1\\$2
..
To assist in the development of Performance Metric Domain Agents 
.RB ( PMDA s)
for the Performance Co-Pilot (PCP),
a procedural interface is provided that extends the Performance Metrics
Application Programming Interface (
.BR PMAPI (3)) 
library.  These procedures are designed to enable a programmer to quickly 
build a 
.B PMDA
which can then be tested and refined.  However, this also 
implies that a 
.B PMDA
has a particular structure which may not be suitable for 
all applications.
.PP
Once you are familiar with the PCP and
.B PMDA
frameworks, you can quickly implement a new 
.B PMDA
with only a few data structures and functions.  This is covered in far greater
detail in the
.IR "Performance Co-Pilot Programmer's Guide" .
.PP
A
.B PMDA
is responsible for a set of performance metrics, in the sense that it must
respond to requests from
.BR pmcd(1)
for information about performance metrics, instance domains, and instantiated
values.  The
.BR pmcd (1)
process generates requests on behalf of performance tools that make requests
using 
.BR PMAPI (3)
routines.
.PP
This man page contains sections of the 
.B simple PMDA
which is located at
.IR $PCP_PMDAS_DIR/simple .
.SH COMMUNICATING WITH PMCD
Two approaches may be used for connecting a
.B PMDA 
to a 
.BR pmcd (1)
process.  A Dynamic Shared Object (DSO) can be attached by
.BR pmcd (1)
using
.BR dlopen (3)
when the
.BR pmcd (1)
process is started.  A procedural interface referenced through a shared data
structure is used to handle requests from
.BR pmcd (1)
to the
.BR PMDA .
.PP
The preferred approach is for a separate process (daemon) to communicate with 
.BR pmcd (1)
using the Performance Data Units (PDU) Inter-Process Communication (IPC)
protocol.
.PP
All
.BR PMDA s
are launched and controlled by the
.BR pmcd (1)
process on the local host.  The requests from the clients are received by
.BR pmcd (1)
and forwarded to the appropriate
.BR PMDA s.
Responses, when required, are returned through 
.BR pmcd (1)
to the clients.  The requests (PDUs) that may be sent to a
.B PMDA
from
.BR pmcd (1)
are
.BR PDU_FETCH ,
.BR PDU_PROFILE ,
.BR PDU_INSTANCE_REQ ,
.BR PDU_DESC_REQ ,
.BR PDU_TEXT_REQ 
and
.BR PDU_RESULT .
.SH DEFAULT CALLBACKS FOR HANDLING PDUs
To allow a consistent framework,
.BR pmdaMain (3)
can be used by a daemon
.B PMDA
to handle the communication protocol using the same callbacks as a DSO
.BR PMDA .
The structure
.B pmdaInterface
is used to convey the common procedural interface and state information that is
used by 
.BR pmcd (1)
and a
.BR PMDA .
This state information includes tables describing the supported metrics and
instance domains.
.PP
As most of the
procedural interface is identical for all 
.BR PMDA s,
they are provided as part of
this support library
.RB ( pmdaProfile (3),
.BR pmdaFetch (3),
.BR pmdaInstance (3),
.BR pmdaDesc (3),
.BR pmdaText (3)
and
.BR pmdaStore (3)). 
However, these routines require access to the 
.B pmdaInterface
state information so it must be correctly initialized using 
.BR pmdaConnect (3),
.BR pmdaDaemon (3),
.BR pmdaOpenLog (3),
.BR pmdaDSO (3),
.BR pmdaGetOpt (3)
and
.BR pmdaInit (3).
.SH INSTANCES AND INSTANCE DOMAINS
Three structures are declared in
.I /usr/include/pcp/pmda.h
which provide a framework for declaring the metrics and instances supported by
the 
.BR PMDA .
.PP
Every instance requires a unique integer identifier and a unique name, as defined by
the structure
.BR pmdaInstid :
.PP
.nf
.ft CW
.in +0.5i
/*
 * Instance description: index and name
 */

typedef struct {
    int         i_inst;        /* internal instance identifier */
    char        *i_name;       /* external instance identifier */
} pmdaInstid;
.in
.fi
.PP
An instance domain requires its own unique identification
.RB ( pmInDom ),
the number of instances the domain represents, and a pointer to an array of
instance descriptions.  This is defined in the structure 
.BR pmdaIndom :
.PP
.nf
.ft CW
.in +0.5i
/*
 * 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;
.in
.fi
.ft 1
.PP
The
.B simple PMDA 
has one instance domain for 
.IR simple . color
with three instances
.RI ( red ,
.I green 
and
.IR blue ),
and a second instance domain for
.IR simple . now
with instances which can be specified at run-time.
These instance domains are defined as:
.PP
.nf
.ft CW
.in +0.5i
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 },
};
.in
.fi
.PP
The preprocessor macros
.B COLOR_INDOM
and
.B NOW_INDOM
are used in the metric description table to identify the instance domains of
individual metrics.  These correspond to the
.I serial
value in the instance domain 
.B pmInDom
structure (the 
.I domain
field is set by
.BR pmdaInit (3)
at run-time).  The serial value must be unique for each instance domain
within the
.BR PMDA . 
.PP
The indom table shown above which is usually passed to 
.BR pmdaInit (3)
does not need to be created 
if one wants to write one's own Fetch and Instance functions.
See 
.BR pmdaInit (3)
for more details.
.SH NAMESPACE
Every 
.B PMDA
has its own unique 
.B namespace 
using the format defined in 
.BR pmns (5). 
In summary, the namespace matches the names of the metrics to the unique
identifier.  The
.B simple PMDA
defines five metrics:
.IR simple . numfetch ,
.IR simple . color ,
.IR simple . time . user,
.IR simple . time . sys
and
.IR simple . now .
The namespace for these metrics is defined in
.I $PCP_PMDAS_DIR/simple/pmns
and is installed as:
.PP
.nf
.ft CW
.in +0.5in
simple {
    numfetch    253:0:0
    color       253:0:1
    time
    now         253:2:4
}

simple.time {
    user        253:1:2
    sys         253:1:3
}
.in
.fi
.PP
The domain number of
.I 253
is obtained from
.IR $PCP_VAR_DIR/pmns/stdpmid .
New 
.BR PMDA s
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
.IR $PCP_VAR_DIR/pmns/stdpmid .
.PP
The
.I simple.time
and
.I simple.now
metrics are defined in separate clusters to the other metrics which allows a
.B 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 
.B simple PMDA 
continues to increment the item numbers to permit direct mapping (see
.BR pmdaInit (3)).
.PP
The namespace file should be installed and removed with the agent using 
.BR pmnsadd (1)
and
.BR pmnsdel (1).
See the later sections on INSTALLATION and REMOVAL.
.PP
A simple ASCII namespace can be constructed by creating a file similar to
.IR $PCP_PMDAS_DIR/simple/root :
.PP
.nf
.ft CW
.in +0.5i
/*
 * fake "root" for validating the local PMNS subtree
 */

#include "$PCP_VAR_DIR/pmns/stdpmid"

root { simple }

#include "pmns"

.in
.fi
.PP
and can be referred to with the
.B \-n
option in most PCP tools.
.SH METRIC DESCRIPTIONS
Each metric requires a description
.RB ( pmDesc ), 
which contains its PMID, data type specification, instance domain, semantics 
and units (see 
.BR pmLookupDesc (3)).
A handle is also provided for application specific information in the
.B pmdaMetric
structure:
.PP
.nf
.ft CW
.in +0.5i
/*
 * Metric description: handle for extending description, 
 * and the description.
 */

typedef struct {
    void*       m_user;         /* for users external use */
    pmDesc      m_desc;         /* metric description */
} pmdaMetric;
.in
.fi
.PP
The 
.B simple PMDA
defines the metrics as:
.PP
.nf
.ft CW
.in +0.5i
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 } }, },
};
.in
.fi
.PP
The macro
.B PMDA_PMID
(defined in
.IR /usr/include/pcp/pmda.h )
is used to specify each metric's
.I cluster
and
.I unit 
number in the 
.B __pmID_int
structure defined in
.IR /usr/include/pcp/impl.h .
As with instance domains, the
.I domain
field is set by
.BR pmdaInit (3)
at run-time, however, the default domain is assumed to be defined by the
.B PMDA 
in the macro
.BR MYDOMAIN .
.PP
The metric table shown above which is usually passed to 
.BR pmdaInit (3)
does not need to be created 
if one wants to write one's own Fetch and Descriptor functions.
See 
.BR pmdaInit (3)
for more details.
.SH DSO PMDA
A 
.B PMDA
that is run as a DSO is opened by 
.BR pmcd (1)
with
.BR dlopen (3).
.B pmcd
will call the 
.BR PMDA "'s"
initialization function that is specified in
.IR $PCP_PMCDCONF_PATH. 
This function is passed a pointer to a
.B pmdaInterface 
structure which must be completed.  Any callbacks which are
.I not
the default 
.B PMDA
support library callbacks must be specified in the
.B pmdaInterface
structure. 
.PP
The
.B simple PMDA 
uses its own store and fetch callback.  
.BR simple_fetch ()
calls 
.BR pmdaFetch (3)
which requires a callback to be set with
.BR pmdaSetFetchCallBack (3)
as can be seen in
.IR $PCP_PMDAS_DIR/simple/simple.c .
.PP
The flag
.B _isDSO
is used to determine if the 
.B PMDA
is a daemon or a DSO so that the correct initialization
routine,
.BR pmdaDaemon (3)
or
.BR pmdaDSO (3),
is called.
.SH DAEMON PMDA
A 
.B PMDA
that is run as a daemon is forked and executed by 
.BR pmcd (1).
Therefore, unlike a DSO 
.BR PMDA ,
the starting point for a daemon 
.B PMDA 
is
.BR main ().
The agent should parse the command line arguments, create
a log file and initialize some data structures that 
.B pmcd
would initialize for a DSO agent.
.PP
The
.B pmdaInterface
structure must be completely defined by the daemon
.BR PMDA .
The function
.BR pmdaDaemon (3)
can be called at the start of 
.BR main ()
to set most of these fields.  Command line parsing can be simplified by using
.BR pmdaGetOpt (3),
which is similar to 
.BR getopt (2),
but extracts a common set of options into the 
.B pmdaInterface
structure.
.I stderr 
can be mapped to a log file using 
.BR pmdaOpenLog (3)
to simplify debugging and error messages.  The connection to 
.B pmcd
can be made with
.BR pmdaConnect (3)
and the loop which handles the incoming PDUs,
.BR pmdaMain (3),
should be the last function called.  This can be seen in
.IR $PCP_PMDAS_DIR/simple/simple.c .
.PP
The
.BR simple_init ()
routine is common to an agent that can be run as both a Daemon and DSO
.BR PMDA .
.SH HELP TEXT
Each 
.B PMDA
must be able to provide 
.B pmcd
with the help text for each metric.  Most 
.BR PMDA s
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 
.BR newhelp (1).
.SH INSTALLATION AND REMOVAL
A series of shell procedures are defined in 
.I $PCP_SHARE_DIR/lib/pmdaproc.sh 
which greatly simplify the installation and removal of a
.BR PMDA .
The
.I Install
scripts for most
.BR PMDAs
should only need to specify the name of the
.B PMDA
in
.BR iam ,
call
.B _setup
which check licenses and whether the 
.B PMDA
has been previously installed,
specify the communication protocols,
and finally call
.BR _install .
The 
.I Remove
scripts are even simpler as the communication protocols are not required.
Further information is contained in the
.I $PCP_SHARE_DIR/lib/pmdaproc.sh
file.
.SH DIAGNOSTICS
Any 
.B PMDA
which uses this library can set
.BR PMAPI (3)
debug control variable
.B pmDebug
(with \-D on the command line) to 
.B DBG_TRACE_LIBPMDA
to enable the display of debugging information which may be useful during
development
(see
.BR pmdbg (1)).
.PP
The
.I status 
field of the 
.B pmdaInterface
structure should be zero after 
.BR pmdaDaemon ,
.BR pmdaDSO ,
.BR pmdaGetOpt ,
.BR pmdaConnect
and
.B pmdaInit
are called.  A value less than zero indicates that initialization has failed.
.PP
Some error messages that are common to most functions in this library are:
.TP 15
.BI "PMDA interface version " interface " not supported"
Most of the functions require that the
.I comm.version
field of the
.B pmdaInterface
structure be set to
.B PMDA_INTERFACE_2
or later.
.B PMDA_INTERFACE_2
or
.B PMDA_INTERFACE_3
implies that the 
.I version.two
fields are correctly initialized,
while
.B PMDA_INTERFACE_4
implies that the 
.I version.four
fields are correctly initialized
(see
.BR pmdaDaemon (3)
and
.BR pmdaDSO (3)).
.SH CAVEAT
Failing to complete any of the data structures or calling any of the library
routines out of order may cause unexpected behavior in the
.BR PMDA .
.PP
Due to changes to the
.BR PMAPI (3)
and
.BR PMDA (3)
API in the PCP 2.0 release, as described in the product release notes, 
.BR PMDA s
built using PCP 2.0 must specify 
.B PMDA_INTERFACE_2
or later and link with
.I libpcp_pmda.so.2
and
.IR libpcp.so.2 .
Pre-existing Daemon PMDAs specifying 
.B PMDA_PROTOCOL_1
will continue to function using the backwards compatible 
.I libpcp_pmda.so.1
and
.I libpcp.so.1
libraries and may be recompiled using the headers installed in
.I "/usr/include/pcp1.x/"
without any modification.  These backwards compatible headers and libraries
are contained in the
.I pcp.sw.compat
subsystem.
.SH FILES
.TP 10
.I /usr/include/pcp/pmda.h
Header file for the 
.B PMDA
support library.
.TP
.I /usr/lib/libpcp_pmda.so
Dynamic library containing 
.B PMDA 
support library routines.
.TP
.I $PCP_PMDAS_DIR/trivial
The source of the
.BR "trivial PMDA" .
.TP
.I $PCP_PMDAS_DIR/simple
The source of the
.BR "simple PMDA" .
.TP
.I $PCP_PMDAS_DIR/txmon
The source of the
.BR "txmon PMDA" .
.TP
.I $PCP_PMCDCONF_PATH
Configuration file for
.BR pmcd (1).
.TP
.I $PCP_VAR_DIR/pmns
Location of namespace descriptions for every
.BR PMDA .
.TP
.I $PCP_VAR_DIR/pmns/stdpmid
The unique domain identifiers for each
.BR PMDA .
.TP
.I $PCP_SHARE_DIR/lib/pmdaproc.sh
Shell procedures for installing and removing a
.BR PMDA .
.SH "PCP ENVIRONMENT"
Environment variables with the prefix
.B PCP_
are used to parameterize the file and directory names
used by PCP.
On each installation, the file
.I /etc/pcp.conf
contains the local values for these variables.
The
.B $PCP_CONF
variable may be used to specify an alternative
configuration file,
as described in
.BR pcp.conf (5).
Values for these variables may be obtained programmatically
using the
.IR pmGetConfig (3)
function.
.SH SEE ALSO
.BR dbpmda (1),
.BR newhelp (1),
.BR pmcd (1),
.BR pmnsadd (1),
.BR pmnsdel (1),
.BR PMAPI (3),
.BR pmdaConnect (3),
.BR pmdaDaemon (3),
.BR pmdaDesc (3),
.BR pmdaDSO (3),
.BR pmdaFetch (3),
.BR pmdaGetOpt (3),
.BR pmdaInit (3),
.BR pmdaInstance (3),
.BR pmdaMain (3),
.BR pmdaOpenLog (3),
.BR pmdaProfile (3),
.BR pmdaStore (3),
.BR pmdaText (3),
.BR pmLookupDesc (3)
and
.BR pmns (5).
.PP
For a complete description of the
.I pcp_pmda
library and the PMDA development process, refer to the Insight book
.IR "Performance Co-Pilot Programmer's Guide" .