GM is the proprietary protocol and API used by Myricom on their boards.
As it comes, it is configured to used at most 256 bytes tranfers between the chipset and the Myrinet board. With the 64-bit PCI bus, as it exists on our nodes, this is not sufficient to exploit the available bus bandwidth (it will perform just 32 data transfers before to relinquish the bus ). With these short burst transfers, we were able to measure a PCI bandwidth of only 147MB per second. By increasing the PCI burst length to 512 bytes (64 transfers), we obtained a PCI bandwith of 227 MB/s reading from the board and of 315 MB/s writing to the Myrinet board. (you need to patch the drivers/linux/gm/gmarch.c file )