5.1 Converse Client-Server Interface

The Converse Client-Server (CCS) module enables Converse [#!InterOpIPPS96!#] programs to act as parallel servers, responding to requests from non-Converse programs. The CCS module is split into two parts - client and server. The server side is used by a Converse program while the client side is used by arbitrary non-Converse programs. A CCS client accesses a running Converse program by talking to a server-host which receives the CCS requests and relays them to the appropriate processor. The server-host is charmrun [#!charmman!#] for net-versions and is the first processor for all other versions.

In the case of the net-version of CHARM++, a Converse program is started as a server by running the CHARM++ program using the additional runtime option ``++server''. This opens the CCS server on any TCP port number. The TCP port number can be specified using the command-line option ``server-port''. A CCS client connects to a CCS server, asks a server PE to execute a pre-registered handler and receives the response data. The function CcsConnect takes a pointer to a CcsServer as an argument and connects to the given CCS server. The functions CcsNumNodes, CcsNumPes, CcsNodeSize implemented as part of the client interface in CHARM++ returns information about the parallel machine. The function CcsSendRequest takes a handler ID and the destination processor number as arguments and asks the server to execute the particular handler on the specified processor. CcsRecvResponse receives a response to the previous request in-place. A timeout is also specified which gives the number of seconds to wait till the function returns a 0, otherwise the number of bytes received is returned.

Once a request arrives on a CCS server socket, the CCS server runtime looks up the appropriate registered handler and calls it. If no handler is found the runtime prints a diagnostic and ignores the message. If the CCS module is disabled in the core, all CCS routines become macros returning 0. The function CcsRegisterHandler is used to register handlers in the CCS server. A handler ID string and a function pointer are passed as parameters. A table of strings corresponding to appropriate function pointers is created. Various built-in functions are provided which can be called from within a CCS handler. The debugger behaves as a CCS client invoking appropriate handlers which makes use of some of these functions. Some of the built-in functions are as follows.

• CcsSendReply - This function sends the data provided as an argument back to the client as a reply. This function can only be called from a CCS handler invoked remotely.

• CcsDelayReply - This call is made to allow a CCS reply to be delayed until after the handler has completed.

The CCS runtime system provides several built-in CCS handlers, which are available to any Converse program. All CHARM++ programs are essentially Converse programs. ccs_getinfo takes an empty message and responds with information about the parallel job. Similarly the handler ccs_killport allows a client to be notified when a parallel run exits.