We propose a simple extension to the optical network of a scalable multiprocessor that optimizes page swap outs. More specifically, we propose to extend the network with an optical ring that not only transfers swapped-out pages between the local memories and the disks of the multiprocessor but also acts as a systemwide write cache for these pages. This extended optical network confers several performance benefits: It provides a staging area where swapped-out pages can reside until the disk is free, it increases the possibility of combining several writes to disk, and it acts as a victim cache for pages that are swapped out and subsequently accessed by the same or a different processor. To evaluate the extent to which these benefits affect performance, we use detailed execution-driven simulations of several out-of-core parallel applications that run on an eight-node scalable multiprocessor. Our results demonstrate that our optical ring provides consistent performance improvements that derive mostly from faster page swap outs and victim caching. To show that our optical ring can also be applied successfully to traditional multiprocessors in which processors are interconnected with electronic networks, we evaluate its benefits for a mesh-connected multiprocessor. This latter evaluation shows that our optical ring improves performance for a traditional multiprocessor by roughly the same amount as it does for an optically interconnected multiprocessor. On the basis of these results and our parameter-space study our main conclusion is that our optical ring is highly efficient under several architectural assumptions and for most out-of-core parallel applications. Even though our study focuses on optimizing page swap outs, we believe that caching data with an optical ring can be beneficial for other types of disk-write traffic as well.
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