Enabling Transparent Memory-Compression for Commodity Memory Systems

Transparent Memory-Compression (TMC) allows the system to obtain the bandwidth benefits of memory compression in an OS-transparent manner. Unfortunately, prior designs for TMC (MemZip) rely on using non-commodity memory modules, which can limit their adoption. We show that TMC can be implemented with commodity memories by storing multiple compressed lines in a single memory location and retrieving all these lines in a single memory access, thereby increasing the effective memory bandwidth. TMC requires metadata to specify the compressibility and location of the line. Unfortunately, even with dedicated metadata caches, maintaining and accessing this metadata incurs significant bandwidth overheads and causes slowdown. Our goal is to enable TMC for commodity memories by eliminating the bandwidth overheads of metadata accesses. This paper proposes PTMC (Practical and Transparent Memory-Compression), a simple design for obtaining bandwidth benefits of memory compression while relying only on commodity (nonECC) memory modules and avoiding any OS support. Our design uses a novel inline-metadata mechanism, whereby the compressibility of the line can be determined by scanning the line for a special marker word, eliminating the overheads of metadata access. We also develop a low-cost Line Location Predictor (LLP) that can determine the location of the line with 98% accuracy and a dynamic solution that disables compression if the benefits of compression are smaller than the overheads. Our evaluations show that PTMC provides a speedup of up to 73%, is robust (no slowdown for any workload), and can be implemented with a total storage overhead of less than 300 bytes.