Talk by Ivan Godard – 2013-10-16 at Stanford
Mostly missless memory in the Mill CPU
Avoiding the pain of cache misses in a statically-scheduled architecture
The Mill is a new CPU architecture designed for very high single-thread performance within a very small power envelope. It achieves DSP-like power/performance on general purpose codes, without reprogramming. The Mill is a wide-issue, statically scheduled design with exposed pipeline. High-end Mills can decode, issue, and execute over thirty MIMD operations per cycle, sustained. The pipeline is very short, with a mispredict penalty of only four cycles.
It is well known that exposed-pipe static scheduling yields near-perfect code with minimal power – except when there is a miss in the cache. In a conventional VLIW, a miss stalls the whole machine, whereas an out-of-order architecture can sometimes find other useful operations to execute while waiting on the memory hierarchy. The Mill uses a novel load instruction that tolerates load misses as well as hardware out-of-order approaches can do, while avoiding the need for expensive load buffers and completely avoiding false aliasing. In addition, store misses are impossible on a Mill, and a large fraction of the memory traffic of a conventional processor can be omitted entirely.
The talk covers these and other technical aspects of the memory hierarchy in the Mill design.
Ivan Godard has designed, implemented or led the teams for 11 compilers for a variety of languages and targets, an operating system, an object-oriented database, and four instruction set architectures. He participated in the revision of Algol68 and is mentioned in its Report, was on the Green team that won the Ada language competition, designed the Mary family of system implementation languages, and was founding editor of the Machine Oriented Languages Bulletin. He is a Member Emeritus of IFIPS Working Group 2.4 (Implementation languages) and was a member of the committee that produced the IEEE and ISO floating-point standard 754-2011.
Ivan is currently CTO at Out-of-theBox Computing, a startup now emerging from stealth mode. OOTBC has developed the Mill, a clean-sheet rethink of general-purpose CPU architectures. The Mill is the subject of this talk.