4. Definitions

This document attaches special meaning to a few commonly used words. The meaning of each of these words in the context of this document is defined here. In particular, please note the special meanings of the words "size" and "length." Understanding the special meaning of these terms is critical to understanding this document.

aligned

A value is said to be aligned if it is a multiple of the required GM alignment. The required GM alignment is 1 on LANai7 hardware, 4 on LANai4 hardware, and 8 on LANai5 hardware. Pointers to memory allocated by GM are always automatically aligned.

client software

client

The client software or simply client is the non-GM software that uses GM to provide a reliable ordered message delivery service. It can be an application, or a higher level networking layer, such as the MPICH over GM implementation provided by Myricom.

message

A message is an aligned array of bytes in DMAable memory.

buffer

A buffer is a contiguous region of DMAable memory into which a message may be copied. All GM buffers must be aligned.

length

The length of a message is the number of bytes of data that comprise the message. There is no alignment restriction on the length of any GM message.

The length of a receive buffer is the number of bytes that may be safely copied into the buffer.

packet

A packet is an aggregation of bytes sent over the network. Packet lengths are limited to just over gm_mtu(port) (usually 4096 bytes) to bound the time any packet can monopolize network resource. Note that multiple packets are required to send large messages over the network, but the segmentation of messages into packets and reassembly of packets into messages is performed automatically by GM.

size

The size of the message is any integer greater than or equal to

log (length + 8) 2

where length is the length of the message.

The size of a receive buffer is any positive integer less than or equal to

log (length + 8) 2

where length is the length of the buffer. Consequently, a buffer of size size must have a length of at least

size 2 - 8.

A buffer having a longer length serves no useful purpose in GM, but is allowed.

The function gm_min_size_for_length(length) can be used to compute the minimum size for any length, and the function gm_max_length_for_size(size) can be used to compute the maximum length for any size.

port

A port is a GM communication endpoint, and serves as the interface between the client software and the network.

user

A human using an application that uses GM.

user virtual memory

Memory directly accessible by software running in a user application.

kernel virtual memory

Memory directly accessible by the GM driver.