Victim replication: maximizing capacity while hiding wire delay in tiled chip multiprocessors

In this paper, we consider tiled chip multiprocessors (CMP) where each tile contains a slice of the total on-chip L2 cache storage and tiles are connected by an on-chip network. The L2 slices can be managed using two basic schemes: 1) each slice is treated as a private L2 cache for the tile; 2) all slices are treated as a single large L2 cache shared by all tiles. Private L2 caches provide the lowest hit latency but reduce the total effective cache capacity, as each tile creates local copies of any line it touches. A shared L2 cache increases the effective cache capacity for shared data, but incurs long hit latencies when L2 data is on a remote tile. We present a new cache management policy, victim replication, which combines the advantages of private and shared schemes. Victim replication is a variant of the shared scheme which attempts to keep copies of local primary cache victims within the local L2 cache slice. Hits to these replicated copies reduce the effective latency of the shared L2 cache, while retaining the benefits of a higher effective capacity for shared data. We evaluate the various schemes using full-system simulation of both single-threaded and multi-threaded benchmarks running on an 8-processor tiled CMP. We show that victim replication reduces the average memory access latency of the shared L2 cache by an average of 16% for multi-threaded benchmarks and 24% for single-threaded benchmarks, providing better overall performance than either private or shared schemes.