NAME

pmdammv - memory mapped values performance metrics domain agent (PMDA)

SYNOPSIS

$PCP_PMDAS_DIR/mmv/pmdammv [-d domain] [-l logfile] [-U username]

DESCRIPTION

pmdammv is a Performance Metrics Domain Agent (PMDA) which exports application level performance metrics using memory mapped files. It offers an extremely low overhead instrumentation facility that is well-suited to long running, mission critical applications where it is desirable to have performance metrics and availability information permanently enabled.

The mmv PMDA exports instrumentation that has been added to an application using the MMV APIs (refer to mmv_stats_init(3) and mmv(5) for further details). These APIs can be called from several languages, including C, C++, Perl, Python, Java (via the separate ``Parfait'' class library) and GoLang (via the separate ``Speed'' library).

A brief description of the pmdammv command line options follows:

-d: It is absolutely crucial that the performance metrics domain number specified here is unique and consistent. That is, domain should be different for every PMDA on the one host, and the same domain number should be used for the same PMDA on all hosts.
-l: Location of the log file. By default, a log file named mmv.log is written in the current directory of pmcd(1) when pmdammv is started, i.e. $PCP_LOG_DIR/pmcd. If the log file cannot be created or is not writable, output is written to the standard error instead.
-U: User account under which to run the agent. The default is the unprivileged "pcp" account in current versions of PCP, but in older versions the superuser account ("root") was used by default.

INSTALLATION

Firstly, a security model for interprocess communication between instrumented applications and pmdammv (running as a shared library inside pmcd) must be chosen and established.

This communication occurs through memory mapped files in a location known to both pmdammv and the instrumented applications. That location is $PCP_TMP_DIR/mmv, although that is often hidden behind application APIs such as ``Parfait'' and mmv_stats_init(3). The permissions set on this directory indicate the security model in use.

The more secure method is to use an existing unprivileged group which is shared by instrumented applications, pmcd and pmdammv such as the "pcp" group.



# . /etc/pcp.env
# mkdir -m 775 $PCP_TMP_DIR/mmv
# chgrp pcp $PCP_TMP_DIR/mmv

Since this method requires instrumented applications to use this non-default group (see newgrp(1) and setgid(2)) a common alternative involves the use of a world-writeable $PCP_TMP_DIR/mmv directory with the sticky-bit set (similar to /tmp and /var/tmp, for example).

This allows any application, running under any user account, to communicate with the PMDA (which runs under the "pcp" account by default). This may not be desirable for all environments and one should consider the security implications of any directory setup like this (similar classes of issues exist as those that affect the system temporary file directories).

pmdammv is enabled by default on all modern PCP installations, and thus the names, help text and values for the mmv performance metrics are available without the usual PMDA ./Install process.

If the $PCP_TMP_DIR/mmv directory does not exist, then the ./Install script can be used to create a world-writeable sticky-bit-set directory for communication. This is for backwards compatibility, as well as ease of use - to use this mechanism, do the following as root:



# rmdir $PCP_TMP_DIR/mmv
# cd $PCP_PMDAS_DIR/mmv
# ./Install

This installation process will not overwrite any existing $PCP_TMP_DIR/mmv directory.

If you want to undo the installation, do the following as root:



# cd $PCP_PMDAS_DIR/mmv
# ./Remove

pmdammv is launched by pmcd and should never be executed directly. The Install and Remove scripts notify pmcd when the agent is installed or removed.

FILES

$PCP_PMCDCONF_PATH: command line options used to launch pmdammv
$PCP_TMP_DIR/mmv: directory housing memory mapped value files
$PCP_PMDAS_DIR/mmv/help: default help text file for the mmv metrics
$PCP_PMDAS_DIR/mmv/Install: installation script for the pmdammv agent
$PCP_PMDAS_DIR/mmv/Remove: undo installation script for the pmdammv agent
$PCP_LOG_DIR/pmcd/mmv.log: default log file for error messages and other information from pmdammv

PCP ENVIRONMENT

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).