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Infiniband has become rapidly a widely accepted medium for internode networks. The specification was finished in June 2001. From 2002 on a number of vendors has started to offer their products based on the Infiniband standard. A very complete description (1200 pages) can be found in [35]. Infiniband is employed to connect various system components within a system. Via Host Channel Adapters (HCAs) the Infiniband fabric can be used for interprocessor networks, attaching I/O subsystems, or to multi-protocol switches like Gbit Ethernet switches, etc. Because of this versatility, the market is not limited just to the interprocessor network segment and so Infiniband is expected to become relatively inexpensive because a higher volume of sellings can be realised. The characteristics of Infiniband are rather nice: there are product definitions both for copper and glass fiber connections, switch and router properties are defined and for high bandwidth multiple connections can be employed. Also the way messages are broken up in packets and reassembled as well as routing, prioritising, and error handling are all described in the standard. This makes Infiniband independent of a particular technology and it is, because of its completeness, a good basis to implement a communication library (like MPI) on top of it. Conceptually, Infiniband knows of two types of connectors to the system components, the Host Channel Adapters (HCAs), already mentioned, and Target Channel Adapters (TCAs). The latter are typically used to connect to I/O susbsystems while HCAs does more concern us as these are the connectors used in interprocessor communication. Infiniband defines a basic link speed of 2.5 Gb/s (312.5 MB/s) but also a 4× and 12× speed of 1.25 GB/s and 3.75 GB/s, respectively. Also HCAs and TCAs can have multiple ports that are independent and allow for higher reliability and speed. Messages can be sent on the basis of Remote Memory Direct Access (RDMA) from one HCA/TCA to another: a HCA/TCA is permitted to read/write the memory of another HCA/TCA. This enables very fast transfer once permission and a write/read location are given. A port together with its HCA/TCA provide a message with a 128-bit header which is IPv6 compliant and that is used to direct it to its destination via cut-through wormhole routing: In each switching stage the routing to the next stage is decoded and send on. Short messages of 32 B can be embedded in control messages which cuts down on the negotiation time for control messages.
Infiniband switches for HPC are offered with 8--288 ports and presently mostly
at a speed of 1.25 GB/s. However, recently the first switches and HCAs
accomodating double this speed have become available. Obviously, to take
advantage of this speed at least PCI Express must be present at the nodes to
which the HCAs are connected. The switches can be configured in any desired
topology but in practice a fat tree topology
is almost always preferred. It obviously depends on the quality of the MPI
implementation put on top of the Infiniband specifications how much of the raw
speed can be realised. A Ping-Pong experiment on an Infiniband-based cluster
has shown a bandwidth of around 850 MB/s and an MPI latency of < 6 µs for
small messages. The in-switch latency is typically about 200 ns. For the 2.5
GB/s products no reliable MPI bandwidth and latency data are available
yet. |