5.2 SVMDNS Daemon

Figure 5.1: Multiple layers for distributed simulation in SVM

SVMDNS (SVM Dynamic Naming Service) is another daemon built on top of the PVM library. It provides a higher level of interface to SVM processes. For example, in Figure 5.1 there are 4 SVM processes, each of which has a DCharts component running on it. Those DCharts components communicate with each other via ports. The SVM processes register themselves to a single SVMDNS daemon. The SVMDNS daemon invokes functions in the PVM library to create 4 PVM processes, each of them correspond to an SVM process. The location of those PVM processes depends on the configuration of the PVM daemon. In this case, PVM processes 1, 2 and 3 are located on machine 1, while PVM process 4 is located on machine 2. The PVM library hides details of this configuration, but provides a uniform API to SVMDNS.

SVMDNS provides the following functionality to an SVM process:

• Registration. Each SVM process that interacts with remote components must register itself to the SVMDNS. By default, the SVM simulator attempts to register itself to SVMDNS if and only if a model with at least one port is running in it.
• Life-time. Each SVM process registered to SVMDNS must periodically sends a keep-alive message to the SVM daemon. If the daemon does not receive such a message from an SVM process within a certain period of time (known as life-time), information about the SVM process is removed from SVMDNS' registry. The life-time can be customized in PVMUtil.py. By default it is 30 seconds. Each SVM simulator, after it registers itself, sends the keep-alive message to the SVMDNS every half life-time period.
• Component lookup. SVM processes send the name patterns or types of required components to the SVMDNS. SVMDNS is able to locate the registered components with those name patterns and types. It establishes connections between components.
• SVMDNS also maintains the connections between different components. The SVM processes are ignorant of this information. They simply use ports to identify groups of connected components in SVMDNS. SVMDNS acts as a router in inter-component communication.

To use SVMDNS, make sure the PVM requirement is satisfied, and compile the PYPVM (http://pypvm.sourceforge.net/) support with command ``make pypvm'' in the SVM directory. PYPVM enables access to the native PVM library from Python code.

Table 5.1: SVMDNS console

The script svmdns in the SVM directory starts the SVMDNS console. The SVMDNS command prompt appears, and commands can be entered. The help command displays all the available commands and their concise descriptions (Table 5.1). Initially the SVMDNS daemon is not started. To start it, execute the ``start'' command. To close the SVMDNS console, execute ``exit''. This command does not stop the SVMDNS daemon, which keeps running in the background. Other commands are explained below:

• The ``check'' command checks the current status of the daemon (whether it is started).
• The ``exec "cmd"'' command executes an arbitrary Python statement on the daemon. It can only be executed after the daemon is started with the ``start'' command. The ``exec'' command is useful for debugging. For example, ``exec "check_master_started()"'' checks the status of the daemon, and command ``exec "print SVMs"'' displays all the components registered in the daemon.
• The ``send "msg"'' command sends a message to the daemon. It can only be executed after the daemon is started with the ``start'' command. Messages received by the daemon, whether they are sent from SVM simulators or from the SVMDNS console, are displayed on the console where SVMDNS is started.
• The ``stop'' command stops the running daemon. It has no effect if the daemon is not running on the local machine.

The user may directly execute a command from the shell by running svmdns with the command as a command-line parameter. For example, to check the status, run ``svmdns check'' in the command-line; to execute an arbitrary Python statement, run ``svmdns exec "cmd"''.

File DNS in the SVM directory records the PVM ID of the SVMDNS daemon. Once the SVMDNS daemon is started, it modifies this file with its PVM ID (the return value of C function pvm_mytid()). SVM simulators use this file to locate the SVMDNS daemon. When the daemon is on another machine, the user must ensure that the content of the DNS file in its SVM directory is the same as the one on the machine where the SVM daemon is running. For a distributed simulation, there should be exactly one SVMDNS daemon running in the background. Each SVM simulator locates the same daemon with the DNS file on their own machines.