HPCwire: A Tutorial of the RDMA Model

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September 15, 2006

A Tutorial of the RDMA Model

A Response to 'A Critique of RDMA' and a Critique on Legacy Network Adapter Architectures

by Renato Recio, Chief Engineer, IBM eSystem Networks

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[Editor's note: The complete article with graphics is available at http://www.hpcwire.com/hpc/885757.html]

RDMA encompasses more than can be encapsulated by a reference to RDMA Writes and RDMA Reads. The reduction of the RDMA programming model by describing a poor mapping over MPI or for that matter Sockets indicates a limited understanding of the extensive capabilities of the full RDMA Model.

Forward thinking architects look to future trends instead of stretching to illustrate (or alluding to) similes with past implementations when gauging the value of a technology. This paper will show how current trends in basic and derivative technologies, such as the periodic doubling of CMOS density, will continue to cement RDMA's already significant traction with HPC, its deep incorporation into the OS, and its growing position in database and storage solutions.

This article begins with an overview of the RDMA Model and then describes the attributes of a well performing RDMA chip architecture. It will describe how the RDMA Model can benefit existing APIs, such as MPI and Sockets. At the dismay of legacy network adapter architectures, it is worthwhile to note that APIs also evolve to take advantage of standardized networking enhancements. This article will describe API evolutionary steps that enable even higher performance benefits.

RDMA Model Basics

The RDMA Model is more than RDMA Writes and RDMA Reads. The RDMA Model provides mechanisms that enable out of user space operations to a previously registered user space memory buffer without operating system (OS) involvement. The registrations are stored in memory translation and protection tables (MTPT), which can be stored in system or adapter memory. These registrations are then used to translate and validate data that is being transmitted from or placed into the registered buffer. The data transfer mechanisms include Sends, RDMA Writes, RDMA Reads and Atomics, some vendors may have proprietary value added mechanisms too.

The RDMA Model is more than RDMA Writes and Reads. Following are the major capabilities that define the RDMA Model and contribute to making it the highly efficient solution that it is:

* The support of many Send/Receive Work Queue (WQ) Pairs (QPs), in the order of thousands to millions.

* Memory registration of user space buffers with the adapter and a mechanism to associate a registered buffer with one or more QPs.

* QP Work Queue Elements (WQEs) that can reference any previously registered buffers.

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