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dbpmda - debugger for Performance Co-Pilot PMDAs


dbpmda [-ei] [-n pmnsfile] [-q timeout] [-U username]


dbpmda is an interactive interface to the interactions between a Performance Metric Domain
Agent (PMDA(3)) and the Performance Metric Collector Daemon (pmcd(1)). This allows PMDAs
to be attached, initialized and exercised to test for correctness.

dbpmda interactively prompts the user for commands, many of which emulate the Protocol
Data Units (PDUs) that may be sent by a pmcd(1) process. After running dbpmda, enter the
command help to get a list of the available commands. The example section below
illustrates a session using dbpmda to test a PMDA.

To simplify repetitive testing of a PMDA, the file .dbpmdarc in the current working
directory can contain a list of commands that will be executed by dbpmda on startup,
before the user is prompted to enter further commands interactively. While processing the
.dbpmdarc file, interactive mode and command echoing are enabled and then reset at the end
of the .dbpmdarc file (see the -i and -e command line arguments below).

If the system supports readline(3) then this will be used to read commands when input is
from a tty device, so history and command line editing are available.

dbpmda accepts the following command line arguments:

-e Echo the input to stdout. This is useful when the input is redirected from a file.

-i Emulate interactive behavior and prompt for new commands, even if standard input is
not a tty device.

-n pmnsfile
Normally dbpmda operates on the distributed Performance Metrics Name Space (PMNS),
however if the -n option is specified an alternative local PMNS is loaded from the
file pmnsfile.

-q timeout
The pmcd to agent version exchange protocol (new in PCP 2.0 - introduced to provide
backward compatibility) uses this timeout to specify how long dbpmda should wait
before assuming that no version response is coming from an agent. If this timeout
is reached, the agent is assumed to be an agent which does not understand the PCP
2.0 protocol. The default timeout interval is five seconds, but the -q option
allows an alternative timeout interval (which must be greater than zero) to be
specified. The unit of time is seconds.

-U username
User account under which to run dbpmda.

As there are no timeout constraints on a PMDA while using dbpmda (as compared to pmcd(1)),
another debugger like gdb(1) can be used on the PMDA process once it has been attached to


Below is a dbpmda session using the simple PMDA. A .dbpmdarc file is used to set the
debugging flag, open the PMDA and display the current status of the debugger:

$ cat .dbpmdarc
debug libpmda
open dso pmda_simple.so simple_init 253

When dbpmda is run, the commands in the .dbpmdarc file are executed first:

$ dbpmda
.dbpmdarc> debug libpmda
.dbpmdarc> open dso pmda_simple.so simple_init 253
[Fri Sep 19 10:19:55] dbpmda(11651) Debug: pmdaInit: PMDA simple DSO: Metric 0.0.1(1) matched to indom 253.0(0)
[Fri Sep 19 10:19:55] dbpmda(11651) Debug: pmdaInit: PMDA simple DSO: help file $PCP_PMDAS_DIR/simple/help opened
[Fri Sep 19 10:19:55] dbpmda(11651) Info: name = simple DSO
[Fri Sep 19 10:19:55] dbpmda(11651) Info: domain = 253
[Fri Sep 19 10:19:55] dbpmda(11651) Info: num metrics = 4
[Fri Sep 19 10:19:55] dbpmda(11651) Info: num indom = 1
[Fri Sep 19 10:19:55] dbpmda(11651) Info: direct map = 1
.dbpmdarc> status

Namespace: (default)
PMDA: ./pmda_simple.so
Connection: dso
DSO Interface Version: 2
PMDA PMAPI Version: 2
pmDebug: 32768 ( libpmda )
Timer: off
Getdesc: off

Dump Instance Profile state=INCLUDE, 0 profiles


To examine the metric and instance descriptors, the desc and instance commands can be
used. Metrics may be identified either by name, or using the ``dotted'' notation to
specify the domain, cluster and item fields of a PMID. Instance domains must be
identified using a ``dotted'' notation to specify the domain and serial fields. The syntax
for most commands will be displayed if the command is given without any arguments:

dbpmda> desc 253.0.0
PMID: 253.0.0
Data Type: 32-bit unsigned int InDom: PM_INDOM_NULL 0xffffffff
Semantics: instant Units: none
dbpmda> instance
instance indom# [ number | name | "name" ]
dbpmda> instance 253.0
pmInDom: 253.0
[ 0] inst: 0 name: "red"
[ 1] inst: 1 name: "green"
[ 2] inst: 2 name: "blue"

To test the most important component of a PMDA, the fetch, it is often useful to determine
the time it takes the PMDA to respond. The timer may be turned on before giving a fetch:

dbpmda> timer on
dbpmda> fetch simple.numfetch 253.0.1
PMID(s): 253.0.0 253.0.1
pmResult dump from 0x100078e0 timestamp: 0.000000 11:00:00.000 numpmid: 2
253.0.0 (simple.numfetch): numval: 1 valfmt: 0 vlist[]:
value 1 1.4012985e-45 0x1
253.0.1 (simple.color): numval: 3 valfmt: 0 vlist[]:
inst [0 or ???] value 1 1 1.4012985e-45 0x1
inst [1 or ???] value 101 1.4153114e-43 0x65
inst [2 or ???] value 201 2.8166099e-43 0xc9
Timer: 0.003921 seconds
dbpmda> timer off

The integer, floating point and hex translations of the values in the pmResult structure
are dumped if getdesc is set to off (the default). Setting getdesc to on would result in
only integer values being dumped in the above fetch as the descriptor describes the
metrics of 32-bit unsigned integers.

The simple PMDA also supports the store operation which can be tested with subsequent
fetch commands:

dbpmda> store simple.numfetch "42"
PMID: 253.0.0
Getting description...
Getting Result Structure...
253.0.0: 2 -> 42
dbpmda> fetch simple.numfetch
PMID(s): 253.0.0
pmResult dump from 0x100078e0 timestamp: 0.000000 11:00:00.000 numpmid: 1
253.0.0 (simple.numfetch): numval: 1 valfmt: 0 vlist[]:
value 43

The value argument in the store command must be a string, which is enclosed in either
single quotes (') or double quotes (").

A profile can be specified for each instance domain which includes all, some or no

dbpmda> help profile

profile indom# [ all | none ]
profile indom# [ add | delete ] number

For the instance domain specified, the profile may be changed to
include 'all' instances, no instances, add an instance or delete
an instance.

dbpmda> profile 253.0 none
dbpmda> getdesc on
dbpmda> fetch 253.0.1
PMID(s): 253.0.1
pmResult dump from 0x100078e0 timestamp: 0.000000 11:00:00.000 numpmid: 1
253.0.1 (simple.color): No values returned!
dbpmda> profile 253.0 add 2
dbpmda> fetch 253.0.1
PMID(s): 253.0.1
pmResult dump from 0x100078e0 timestamp: 0.000000 11:00:00.000 numpmid: 1
253.0.1 (simple.color): numval: 1 valfmt: 0 vlist[]:
value 202
dbpmda> profile 253.0 add 0
dbpmda> fetch 253.0.1
PMID(s): 253.0.1
pmResult dump from 0x100078e0 timestamp: 0.000000 11:00:00.000 numpmid: 1
253.0.1 (simple.color): numval: 2 valfmt: 0 vlist[]:
inst [0 or ???] value 2
inst [2 or ???] value 203
dbpmda> status

PMDA = pmda_simple.so
Connection = dso
pmDebug = 32768 ( libpmda )
Timer = off

Dump Instance Profile state=INCLUDE, 1 profiles
Profile [0] indom=1061158913 [253.0] state=EXCLUDE 2 instances
Instances: [2] [0]
dbpmda> quit

The watch command (usage: watch filename ) opens an xwsh window which tails the specified
log file. This window must be closed by the user when no longer required.

The wait command is equivalent to sleep (1) and takes a single integer argument.

The introduction of dynamic subtrees in the PMNS and PMDA_INTERFACE_4 in libpcp_pmda has
led to additional commands being supported in dbpmda to exercise the associated dynamic
PMNS services. The examples below are based on the sample PMDA.

$ dbpmda
dbpmda> open pipe /var/lib/pcp/pmdas/sample/pmdasample -d 29
Start pmdasample PMDA: /var/lib/pcp/pmdas/sample/pmdasample -d 29
dbpmda> children sample.secret
Metric: sample.secret
non-leaf foo
leaf bar
dbpmda> traverse sample.secret.foo
Metric: sample.secret.foo
dbpmda> pmid sample.secret.foo.bar.four
Metric: sample.secret.foo.bar.four
dbpmda> name 29.0.1006
PMID: 29.0.1006

The children command returns the next name component for all the direct descendants of a
node within a dynamic subtree of the PMNS. The related traverse command returns the full
metric names for all leaf nodes in the PMNS below the specified non-leaf node in a dynamic
subtree of the PMNS.

The name and pmid commands exercise the translation of metric names to PMIDs (and vice
versa) for metrics within a dynamic subtree of the PMNS.

If the commands children, traverse, pmid or name are used with a PMDA that is not using
PMDA_INTERFACE_4 or with performance metric names that are not part of a dynamic subtree
of the PMNS, then the PMDA would be expected to return errors (PM_ERR_NAME or PM_ERR_PMID)
to reflect the fact that the operation is in error (outside a dynamic subtree of the PMNS
it is pmcd(1) and not the PMDA that is responsible for implementing these functions).

Client authentication mechanisms have been incorporated into the PMCS, providing per-user
(and per-connection) information that is available to PMDAs. A PMDA using
PMDA_INTERFACE_6 or later in libpcp_pmda is able to make use of the "attribute" method to
gain visibility into these authenticated connections, with access to information including
user and group identifiers, user name, and so on. The need to exercise and debug this
interface has led to a new dbpmda command. The following example is based on the sample

$ dbpmda
dbpmda> open pipe pmdasample -D AUTH -l logfile
Start pmdasample PMDA: pmdasample -D AUTH -l logfile
dbpmda> attr "username" "tanya"
Attribute: username=tanya
dbpmda> attr 11 "0"
Attribute: userid=0

The attr command passes connection attributes (PCP_ATTR keys) and their values into a PMDA
in much the same way that PMCD would for a client connection. dbpmda always passes a
client context identifier of zero, and while no validity checking on values is performed
only recognised attributes can be set.

In the example above the AUTH debug flag is set for the PMDA, which uses this in its
attribute callback and records each attribute and value pair sent to it in its logfile.

Note that authentication checks have already been performed by PMCD by the time a PMDA is
presented with these attributes, so no further verification is necessary by the PMDA.


A value cannot be stored into metrics of type PM_TYPE_AGGREGATE or PM_TYPE_EVENT.

dbpmda uses fork(2) and exec(2) to attach to daemon PMDAs. dbpmda makes no attempt to
detect the termination of the daemon PMDA process, so it is possible for a PMDA to exit
unexpectedly without any notification. However, any further communication attempts with
the PMDA will result in errors which will indicate that the PMDA is no longer responding.

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