A light-Weight Communication System for a High Performance System Area Network Amelia De Vivo
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Thesis OrganisationThis thesis is structured as follows. Chapter 2 provides an overview of research activities about user-level communication systems. We describe some of the most significant user-level communication systems developed in the last years, discussing and evaluating their design choices. Chapter 3 is the focal point of this thesis. It gives a detailed description of the QNIX communication system, discusses related design choices and illustrates work in progress and future extensions. In Chapter 4 we show the first experimental results, achieved by a preliminary implementation of the QNIX communication system. Chapter 2 User-level Communication Systems SANs exhibit great potential for HPC because of their physical features, that are high bandwidth, low latency and high reliability, but the real reason making them so useful is the user-level communication system support. Indeed, as we saw in the previous chapter, if all network access is through the operating system, a large overhead is added to both the transmission and the receive path. In this case only a low percentage of the interconnection network performance is delivered to user applications, so that it would make no distinction to have a SAN or another kind of network. User-level communication systems, instead, allow to exploit much more hardware capabilities and for this reason have been widely investigated. In the last years several research efforts have been spent for user-level communication system development and the major industrial companies have been interested in this argument. As a consequence the present scenario is very variegated and continuously in evolution. Most user-level communication systems have been implemented on the Myrinet network because of Myricom open source politics and user programmability of the LANai processor, and several comparative studies are available in literature, such as [BBR98] and [ABD+98]. Anyway it is difficult to decide which is the best because they support different communication paradigms, employ a variety of different implementation tradeoffs and exhibit specific architectural choices. Depending on some conditions, one can achieve better performance than another, so that it is not very significant to compare numbers. Rather it can be useful to try a classification of some of the most famous user-level communication systems, based on some important design issues. We choose the six issues emphasized in [BBR98]: data transfer between host and NIC (DMA or programmed I/O), address translation, protection in multi-user environment, control transfer (interrupt or polling), reliability and multicast support. The following table gives such a classification for 10 among the most significant user-level communication systems.
The research efforts about user-level communication systems achieved their best acknowledgment in 1997, when Compaq, Intel and Microsoft defined the VIA specification [CIM97] as the first attempt for a common standard. At the moment VIA is hardware supported in some SANs, such as cLAN [Gig99] and ServerNet II [Com00], and various software implementations have been realised both as research experiments and industrial products. Anyway in the rapid evolution of this field VIA, even being an important step, is substantially one among the others. Currently its promoters and a lot of other companies are working for the definition of Infiniband [Inf01], a new, broader spectrum communication standard attempt. In this chapter we try to give an overview of the user-level communication system world. For this purpose we choose to describe in details the VIA specification and the four research systems that mainly contributed to its definition: Active Messages, Illinois Fast Messages, U-Net and VMMC. The chapter is structured as follows. Active Messages is illustrated in Section 2.1, Illinois Fast Messages in Section 2.2, U-Net in Section 2.3, VMMC in Section 2.4 and the VIA specification in Section 2.5.
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