MiSAR: Minimalistic synchronization accelerator with resource overflow management

While numerous hardware synchronization mechanisms have been proposed, they either no longer function or suffer great performance loss when their hardware resources are exceeded, or they add significant complexity and cost to handle such resource overflows. Additionally, prior hardware synchronization proposals focus on one type (barrier or lock) of synchronization, so several mechanisms are likely to be needed to support real applications, many of which use locks, barriers, and/or condition variables. This paper proposes MiSAR, a minimalistic synchronization accelerator (MSA) that supports all three commonly used types of synchronization (locks, barriers, and condition variables), and a novel overflow management unit (OMU) that dynamically manages its (very) limited hardware synchronization resources. The OMU allows safe and efficient dynamic transitions between using hardware (MSA) and software synchronization implementations. This allows the MSA's resources to be used only for currently-active synchronization operations, providing significant performance benefits even when the number of synchronization variables used in the program is much larger than the MSA's resources. Because it allows a safe transition between hardware and software synchronization, the OMU also facilitates thread suspend/resume, migration, and other thread-management activities. Finally, the MSA/OMU combination decouples the instruction set support (how the program invokes hardware-supported synchronization) from the actual implementation of the accelerator, allowing different accelerators (or even wholesale removal of the accelerator) in the future without changes to OMU-compatible application or system code. We show that, even with only 2 MSA entries in each tile, the MSA/OMU combination on average performs within 3% of ideal (zero-latency) synchronization, and achieves a speedup of 1.43X over the software (pthreads) implementation.