Hosting an object heap on manycore hardware: An exploration

Abstract

In order to construct a test-bed for investigating new programming paradigms for future "manycore" systems (i.e. those with at least a thousand cores), we are building a Smalltalk virtual machine that attempts to efficiently use a collection of 56-on-chip caches of 64KB each to host a multi-megabyte object heap. In addition to the cost of inter core communication, two hardware characteristics influenced our design: the absence of hardware-provided cache-coherence, and the inability to move a single object from one core's cache to another's without changing its address. Our design relies on an object table, and the exploitation of a user-managed caching regime for read mostly objects. At almost every stage of our process, we obtained measurements in order to guide the evolution of our system. The architecture and performance characteristics of a many core platform confound old intuitions by deviating from both traditional multicore systems and from distributed systems. The implementor confronts a wide variety of design choices, such as when to share address space, when to share memory as opposed to sending a message, and how to eke out the most performance from a memory system that is far more tightly integrated than a distributed system yet far less centralized than in a severalcore system. Our system is far from complete, let alone optimal, but our experiences have helped us develop new intuitions needed to rise to the manycore software challenge. Copyright © 2009.