Chameleon: Operating System Support for Dynamic Processors

The rise of multi-core processors has shifted performance efforts towards parallel programs. However, single-threaded code, whether from legacy programs or ones difficult to parallelize, remains important. Proposed asymmetric multicore processors statically dedicate hardware to improve sequential performance, but at the cost of reduced parallel performance. However, several proposed mechanisms provide the best-of-both-worlds by combining multiple cores into a single, more powerful processor for sequential code. For example, Core Fusion merges multiple cores to pool caches and functional units, and Intel's Turbo Boost raises the clock speed of a core if the other cores on a chip are powered down. These reconfiguration mechanisms have two important properties. First the set of available cores and their capabilities can vary over short time scales. Current operating systems are not designed for rapidly changing hardware: the existing hotplug mechanisms for reconfiguring processors require global operations and hundreds of milliseconds to complete. Second, configurations may be mutually exclusive: using power to speed one core means it cannot be used to speed another. Current schedulers cannot manage this requirement. We present Chameleon, an extension to Linux to support dynamic processors that can reconfigure their cores at runtime. Chameleon provides processor proxies to enable rapid reconfiguration, execution objects to abstract the processing capabilities of physical CPUs, and a cluster scheduler to balance the needs of sequential and parallel programs. In experiments that emulate a dynamic processor, we find that Chameleon can reconfigure processors 100,000 times faster than Linux and allows applications full access to hardware capabilities: sequential code runs at full speed on a powerful execution context, while parallel code runs on as many cores as possible.