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Computer Architecture

Software Optimization

Previous research work has identified memory bandwidth as the main bottleneck of the ubiquitous Sparse Matrix-Vector Multiplication kernel. To attack this problem, we aim at reducing the overall data volume of the algorithm. Typical sparse matrix representation schemes store only the non-zero elements of the matrix and employ additional indexing information to properly iterate over these elements. In this paper we propose two distinct compression methods targeting index and numerical values respectively. We perform a set of experiments on a large real-world matrix set and demonstrate that the index compression method can be applied successfully to a wide range of matrices. Moreover, the value compression method is able to achieve impressive speedups in a more limited, yet important, class of sparse matrices that contain a small number of distinct values.

Operating Systems

Efficient sharing of block devices over an interconnection network is an important step in deploying a shared-disk parallel file system on a cluster of SMPs. We present gmbock, a client/server system for network sharing of storage devices over Myrinet, which uses an optimized data path in order to transfer data directly from the storage medium to the NIC, bypassing the host CPU and main memory bus. Its design enhances existing programming abstractions, combining the user level networking characteristics of Myrinet with Linux's virtual memory infrastructure, in order to construct the datapath in a way that is independent of the type of block device used. Experimental evaluation of a prototype system shows that remote I/O bandwidth can improve up to 36%, compared to an RDMA-based implementation. Moreover, interference on the main memory bus of the host is minimized, leading to an up to 41% improvement in the execution time of memory-intensive applications.

Providing scalable clustered storage in a cost-effective way depends on the availability of an efficient network block device (nbd) layer. To overcome the architectural limitation of a low number of outstanding requests in gmblock, we focus on overlapping read and network I/O for a single request, in order to improve throughput. To this end, we introduce the concept of synchronized send operations and present an implementation on Myrinet/GM, based on custom modifications to the NIC firmware and associated userspace library. Compared to a network block sharing system over standard GM and the base version of gmblock, our enhanced implementation supporting synchronized sends delivers 81% and 44% higher throughput for streaming block I/O, respectively.

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Topic revision: r6 - 2008-03-11 - NikosAnastopoulos

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