Hosting an object heap on manycore hardware: an exploration

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 manycore 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 several-core 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.