The IB ACM library implements and provides a framework for name, address, and route resolution services over InfiniBand. The IB ACM provides information needed to establish a connection, but does not implement the CM protocol.
IB ACM services are used by librdmacm to implement the rdma_resolve_addr, rdma_resolve_route, and rdma_getaddrinfo routines.
The IB ACM is focused on being scalable and efficient. The current implementation limits network traffic, SA interactions, and centralized services. ACM supports multiple resolution protocols in order to handle different fabric topologies.
This release is limited in its handling of dynamic changes.
The IB ACM package is comprised of two components: the ibacm service and a test/configuration utility - ib_acme.
The ib_acme program serves a dual role. It acts as a utility to test ibacm operation and help verify if the ibacm service and selected protocol is usable for a given cluster configuration. Additionally, it automatically generates ibacm configuration files to assist with or eliminate manual setup.
The ibacm service relies on two configuration files.
The acm_addr.cfg file contains name and address mappings for each IB <device, port, pkey> endpoint. Although the names in the acm_addr.cfg file can be anything, ib_acme maps the host name and IP addresses to the IB endpoints.
The acm_opts.cfg file provides a set of configurable options for the ibacm service, such as timeout, number of retries, logging level, etc. ib_acme generates the acm_opts.cfg file using static information. A future enhancement would adjust options based on the current system and cluster size.
The ibacm service is responsible for resolving names and addresses to InfiniBand path information and caching such data. It is implemented as a daemon that execute with administrative privileges.
The ibacm implements a client interface over TCP sockets, which is abstracted by the librdmacm library. One or more back-end protocols are used by the ibacm service to satisfy user requests. Although the ibacm supports standard SA path record queries on the back-end, it provides an experimental multicast resolution protocol in hope of achieving greater scalability. The latter is not usable on all fabric topologies, specifically ones that may not have reversible paths. Users should use the ib_acme utility to verify that multicast protocol is usable before running other applications.
Conceptually, the ibacm service implements an ARP like protocol and either uses IB multicast records to construct path record data or queries the SA directly, depending on the selected route protocol. By default, the ibacm services uses and caches SA path record queries.
Specifically, all IB endpoints join a number of multicast groups. Multicast groups differ based on rates, mtu, sl, etc., and are prioritized. All participating endpoints must be able to communicate on the lowest priority multicast group. The ibacm assigns one or more names/addresses to each IB endpoint using the acm_addr.cfg file. Clients provide source and destination names or addresses as input to the service, and receive as output path record data.
The service maps a client's source name/address to a local IB endpoint. If a client does not provide a source address, then the ibacm service will select one based on the destination and local routing tables. If the destination name/address is not cached locally, it sends a multicast request out on the lowest priority multicast group on the local endpoint. The request carries a list of multicast groups that the sender can use. The recipient of the request selects the highest priority multicast group that it can use as well and returns that information directly to the sender. The request data is cached by all endpoints that receive the multicast request message. The source endpoint also caches the response and uses the multicast group that was selected to construct or obtain path record data, which is returned to the client.
The current implementation of the IB ACM has several additional restrictions:
- The ibacm is limited in its handling of dynamic changes; the ibacm should be stopped and restarted if a cluster is reconfigured.
- Support for IPv6 has not been verified.
- The number of addresses that can be assigned to a single endpoint is limited to 4.
- The number of multicast groups that an endpoint can support is limited to 2.
The ibacm contains several internal caches. These include caches for GID and LID destination addresses. These caches can be optionally preloaded. ibacm supports the OpenSM dump_pr plugin "full" PathRecord format which is used to preload these caches. The file format is specified in the ibacm_opts.cfg file via the route_preload setting which should be set to opensm_full_v1 for this file format. Default format is none which does not preload these caches. See dump_pr.notes.txt in dump_pr for more information on the opensm_full_v1 file format and how to configure OpenSM to generate this file.
Additionally, the name, IPv4, and IPv6 caches can be be preloaded by using the addr_preload option. The default is none which does not preload these caches. To preload these caches, set this option to acm_hosts and configure the addr_data_file appropriately.