PscomInterface.md

API and Internals

Contents

• Initialization
• Clean up
• Connection establishment
• Initial information exchange via TCP
• Message reception
• Message sending
• Plugin Interface

A bi-directional point-to-point connection of pscom consists of the following components:

• A pscom socket (as a central data structure for establishing and handling connections) to which the connection belongs
• State of the connection
• Type of the connection (corresponding to the respective plugin, an on-demand connection, or a precon)
• Connection info of the connected peer process
• Size of the user-defined storage within this connection
• User-defined storage (user data)

For the type definition of PSCOM_connection see include/pscom.h.

Initialization

• Before any other call to the pscom library can be made, pscom_init(...) (or pscom_init_thread(...) for a thread-safe version of the library) has to be called.
• In ParaStation MPI, this call is done in the MPID_Init(...) method.

Clean-up

• At the exit of the program, pscom automatically calls the pscom_cleanup() function via an atexit handler to close all remaining sockets and clean-up all remaining data including the plugins (see below)
• For pscom versions >= 5.5.0-1 there exists an experimental config option PSP_FINALIZE_SHUTDOWN in ParaStation MPI. If this option is set to 1, all pscom sockets are closed via pscom_close_socket(NULL) in ParaStation MPI during MPID_Finalize(). This implies a synchronized shutdown of all pscom connections before the exit handler is called.

Connection establishment

• A socket is opened (per rank) using pscom_open_socket(...) and assigned a unique name using pscom_socket_set_name(...).
• For connection establishment, the following pscom functions are used:
  • pscom_open_connection(...) to create the connection (malloc)
  • pscom_connect_socket_str(...) to establish the connection, calls pscom_connect(...) or pscom_connect_ondemand(...) depending on the chosen connectivity schema
• pscom_send(...) can be used to send the init message via the established connection

Instant connectivity

• pscom_connect(...) calls pscom_con_connect(...) to establish the connection
  • either via loopback for local connection using pscom_con_connect_loopback(...)
  • or initially via TCP (pscom_con_connect_tcp(...), see below) and then use TCP to establish a better (the best) connection depending on the available plugins and their priorities
  • Plugins can use the TCP connection to communicate the required information for connection establishment in their own con_connect(...) function (part of the plugin)
• To wait for and accept incoming connections, the following function is used:
  • pscom_wait_any() to wait for progress/ activity on any communication channel (polling, see pscom_progress(...) and pscom_con_accept_tcp(...))
  • Also when accepting incoming connections, the best connection plug-in is determined

On-demand connectivity – PSP_ONDEMAND=1

• Since version >= 5.0.24, on-demand connections are supported by pscom.
• On-demand connections are enabled by setting the environment variable PSP_ONDEMAND=1
• pscom_con objects are initializedby calling pscom_connect_ondemand(...) in pscom_connect_socket_str(...) which calls pscom_con_connect_ondemand(...)
• Upon the first write attempt on this connection, the pscom_ondemand_write_start(...) function will trigger connection establishment
• If the own name is (lexicographically) lower than the remote name, indirect connection establishment is done via pscom_precon_ondemand_backconnect_tcp(...)
  • Calling pscom_precon_connect_tcp(...) to create a TCP connection
  • If the connect via TCP does not succeed, there are three possible reasons
    1. Problems to connect, caused by network congestion or busy peer (e.g., TCP backlog too small). In this case the connection should be terminated with an error.
    2. Peer is connecting to us at the same time and the listening TCP port is already closed. This is not an error and we must not terminate the connection.
    3. Peer has no receive request on this connection and is not watching for POLLIN on the listening file descriptor. This is currently unhandled and causes a connection error
  • If the TCP connection is successful, send "call-me-back" message via pscom_precon_send_PSCOM_INFO_CON_INFO_tcp(...) as a trigger
• If the own name is (lexicographically) greater or equal to the remote name direct connection establishment is done via pscom_ondemand_direct_connect(...)
  • Closing the "on demand" connection via pscom_ondemand_cleanup(...)
  • Re-opening TCP via pscom_con_connect_tcp(...) (finding the best connection/ plugin)

Initial information exchange via TCP

• After a TCP connection has been established, some initial information such as the version number or whether it is a "call-me-back" (aka "back-connect") on-demand connection is exchanged
• The sending of the info messages is done asynchronously via the handling of ufd_events
• In pscom_con_connect_tcp(...) the function pscom_precon_handshake() is called to
  1. enable receiving
  2. send initial connection information for any connection that is in the state PSCOM_CON_STATE_CONNECTING
  3. call the plugin's handshake function

Message reception

Network side – message reception from underlying transport

• Upon arrival of any new data at the underlying transport, the respective plugin calls pscom_read_done(...) or pscom_read_pending_done(...)
• Checks for a new header (header_complete(...)) and begins to consume the data via _pscom_get_recv_req(...)
  • Checks message type and returns a request that will receive the message
  • For the internal message types see lib/pscom/pscom_io.h
• In case of user message type PSCOM_MSGTYPE_USER
  • Get the respective user recv request based on callback for "receive_dispatch" (see ParaStation MPI's mpid_irecv.c); creates a user request according to information given in Xheader
  • Check for a match in the posted user requests (Queue: recvq_user)
  • If no user request can be found (= unexpected message) create a "net request" and add it to both net queues (net_recv_userq and genrecvq_any)

Application side – post a receive request

• pscom_post_recv(...) posts a non-blocking receive request
• First, the function checks whether a matching request has already been received (Queue: net_recv_userq); There are two queues for "net requests" (network side)
  • con->net_recvq_user: rank dictated
  • sock->genrecvq_any: any source
• If no matching request has already been received, the receive request is enqueued in one of the two available user request queues (application side)
  • con->recvq_user: rank dictated
  • sock->recvq_any: any source
• If a matching message has already been received (on network side, identified by ivoking the user-callback-accept function: req->pub.ops.recv_accept) in whole or partial, the respective "net request" is dequeued and merged with the user request
• Enqueue request and "activate" connections if needed

Message sending

Application side – post a send request

• There are three types of non-blocking send functions on application level (see also include/pscom.h)
  • pscom_post_send(...): requests-based
  • pscom_send(...): copy-based
  • pscom_send_inplace(...): callback-based
• For example, the PSP layer of ParaStation MPI uses
  • pscom_send(...) for control messages (for example in ParaStation MPI's MPID_PSP_SendCtrl(...))
  • pscom_post_send(...) for regular MPI payload messages
• Only pscom_post_send(...) differentiates between EAGER (direct send) and RNDV (Rendezvous) by comparing the message length with the rendezvous size (a threshold, see below)
• All functions (also pscom_send(...) and pscom_send_inplace(...)) eventually prepare an internal request and post it to the send queue (see lib/pscom/pscom_io.c) with the function pscom_post_send_direct(...)
  • Calls pscom_req_prepare_send(...) to build the header and iovec
  • Then calls _pscom_sendq_enq(...) to enqueue the request
• For queue handling, see lib/pscom/pscom_queues.c
• For possible request status, see include/pscom.h

Network side – ReNDezVous mechanism (RNDV)

The rendezvous mechanism of pscom is currently subject to revision. This documentation will be updated as soon as the revision is completed.

Plugin Interface

The pscom maintains a global list pscom_plugins that contains the available plugins. A plugin consists of:

• Name (8 characters), version and ID
• Default and user priorities
• Function pointers to init and destroy the plugin itself
• Function pointers to hook functions that are called when a socket is created or destroyed (see also pscom_sock_create() and pscom_sock_destroy()) to perform plugin-specific init / destroy steps
• Function pointer to init pscom connection
• Function pointer to perform connection handshake of pscom
• Pointer to next plugin in the list

For the type definition see lib/pscom/pscom_plugin.h.

Initialization

All enabled plugins are tried to be loaded and initialized upon the first creation of a pscom socket during pscom_plugins_sock_init(...) method (in pscom_plugins_init(...)) using the plugin's respective init() method. Plugin-specific initialization of the socket is done afterwards using the plugin's sock_init() hook function.

Clean-up

All plugins are destroyed during pscom_cleanup(...) after the plugin-specific clean-up via pscom_plugins_sock_destroy(...) has been performed for all sockets using the sock_destroy(...) hook function of the plugins. Afterwards, the plugin itself is destroyed using its destroy(...) method (in pscom_plugins_destroy(...)).

Accessing a plugin

• Access to a specific plugin from the list is provided via plugin name or ID
  • pscom_plugin_by_name(...)
  • pscom_plugin_by_archid(...)
• Access to the first or next plugin in the list is provided via
  • pscom_plugin_first()or
  • pscom_plugin_next(...) methods, respectively