3. GM Overview

GM is a message-based communication system for Myrinet. Like many messaging systems, GM's design objectives included low CPU overhead, portability, low latency, and high bandwidth. Additionally, GM has several distinguishing characteristics:

• GM has extremely low overhead of about 1 microsecond per packet on all architectures.
• GM can provide simultaneous memory-protected user-level OS-bypass network interface access to several user-level applications simultaneously. (On systems that do not support memory protection, such as VxWorks, no memory protection is provided.)
• GM provides reliable ordered delivery between hosts in the presence of network faults. GM will detect and retransmit lost and corrupted packets. GM will also reroute packets around network faults when alternate routes exist. Catastrophic network errors, such as crashed hosts or disconnected links, are nonfatal; the undeliverable packets are returned to the client with an error indication, although most client programs are unable to adapt in the presence of such severe errors.
• GM supports clusters of over 10,000 nodes.
• GM provides two levels of message priority to allow efficient deadlock-free bounded-memory forwarding.
• GM allows clients to send messages up to 2**31 - 1 bytes long, under operating systems that support sufficient amounts of DMAable memory to be allocated.
• GM automatically maps Myrinet networks.

GM is a light-weight communication layer, and as such has limitations that can be addressed by layering a heavier-weight interface over GM. Some such limitations are the following:

• GM is unable to send messages from or receive messages into nonDMAable memory.
• The GM API does not yet support any gather or scatter operations directly.

From the client's point of view, GM consists of a library, `libgm.a', and a header file, `gm.h'. All externally visible GM identifiers in these files match the regular expression `^_*[Gg][Mm]_' to minimize name space pollution.

Additionally, GM has other parts that system administrators need to be concerned about:

`gm'

The GM driver provides systems services. It is called ``gm'' under Unix, and is the `Myricom Myrinet Adapter' driver implemented in ``gm.sys'' under Windows NT.

`mapper'

The Myrinet mapper daemon maps the network. It is called ``sbin/mapper'' under Unix, and is the `Myricom Myrinet Mapper Daemon' service implemented in ``gm_mapper_service.exe'' under Windows NT.