Evaluation and Refinement of an Explicit Virtual-Memory Primitive

The general-purpose virtual-memory subsystem of Linux does not scale to the multimillion IOP/s SSDs of today. Further, its implicit write-back and back-fill semantic hinders data-intensive applications, like databases, more than it actually helps. In contrast, explicit virtual-memory abstractions, as introduced by ExMap, provide an Exokernel-like interface to manipulate theVMsurface. Thereby, the user application remains in full control over the VM surface and can benefit from low operation latencies. In this paper, we investigate the performance of the existing ExMap abstraction and propose enhancements for its system-call interface, its process-local memory pool management, and the user-space page-state tracking. With memory-mapped I/O vectors, the global bundle list, and exported page tables, we improve the end-toend throughput of in-memory random B+tree lookups by up to 5.7 percent, while the out-of-memory case improves by up to 4.9 percent.