As ever more computation shifts onto multicore architectures, it is increasingly critical to find effective ways of dealing with multithreaded performance bugs like true and false sharing. Previous approaches to fixing false sharing in unmanaged languages have employed highly-invasive runtime program modifications. We observe that managed language runtimes, with garbage collection and JIT code compilation, present unique opportunities to repair such bugs directly, mirroring the techniques used in manual repairs.We present REMIX, a modified version of the Oracle HotSpot JVM which can detect cache contention bugs and repair false sharing at runtime. REMIX's detection mechanism leverages recent performance counter improvements on Intel platforms, which allow for precise, unobtrusive monitoring of cache contention at the hardware level. REMIX can detect and repair known false sharing issues in the LMAX Disruptor high-performance inter-thread messaging library and the Spring Reactor event-processing framework, automatically providing 1.5-2x speedups over unoptimized code and matching the performance of handoptimization. REMIX also finds a new false sharing bug in SPECjvm2008, and uncovers a true sharing bug in the HotSpot JVM that, when fixed, improves the performance of three NAS Parallel Benchmarks by 7-25x. REMIX incurs no statistically-significant performance overhead on other benchmarks that do not exhibit cache contention, making REMIX practical for always-on use.

This paper evaluates managing the processor's datapath-width at the compiler level by means of exploiting dynamic narrow-width operands. We capitalize on the large occurrence of these operands in multimedia programs to build static narrow-width regions that may be directly exposed to the compiler. We propose to augment the ISA with instructions directly exposing the datapath and the register widths to the compiler. Simple exception management allows this exposition to be only speculative. In this way, we permit the software to speculatively accommodate the execution of a program on a narrower datapath-width in order to save energy. For this purpose, we introduce a novel register file organization, the byte-slice register file, which allows the width of the register file to be dynamically reconfigured, providing both static and dynamic energy savings. We show that by combining the advantages of the byte-slice register file with the advantages provided by clock-gating the datapath on a per-region basis, up to 17% of the datapath dynamic energy can be saved, while a 22% reduction of the register file static energy is achieved.