Current developments in microprocessor design favor increased core counts over frequency scaling to improve processor performance and energy efficiency. Coupling this architectural trend with a message-passing protocol helps realize a data-center-on-a-die. The prototype chip (Figs. 5.7.1 and 5.7.7) described in this paper integrates 48 Pentium ™ class IA-32 cores [1] on a 6×4 2D-mesh network of tiled core clusters with high-speed I/Os on the periphery. The chip contains 1.3B transistors. Each core has a private 256KB L2 cache (12MB total on-die) and is optimized to support a message-passing-programming model whereby cores communicate through shared memory. A 16KB message-passing buffer (MPB) is present in every tile, giving a total of 384KB on-die shared memory, for increased performance. Power is kept at a minimum by transmitting dynamic, fine-grained voltage-change commands over the network to an on-die voltage-regulator controller (VRC). Further power savings are achieved through active frequency scaling at the tile granularity. Memory accesses are distributed over four on-die DDR3 controllers for an aggregate peak memory bandwidth of 21GB/s at 4× burst. Additionally, an 8-byte bidirectional system interface (SIF) provides 6.4GB/s of I/O bandwidth. The die area is 567mm 2 and is implemented in 45nm high-κ metal-gate CMOS [2].The design is organized in a 6×4 2D-array of tiles [3] to increase scalability. Each tile is a cluster of two enhanced IA-32 cores sharing a router for inter-tile communication. Cores operate in-order and are two-way superscalar. A 256 entry lookup table (LUT) extension of the 64-entry TLB translates 32-bit virtual addresses to 36-bit physical addresses. The separate L1 instruction and data caches are upsized to 16KB and support both write-through and write-back. Each L1 cache is reinforced by a unified 256KB 4-way write-back L2 cache. The L2 uses a 32-byte line size, matching the cache line size internal to the core, and has a 10-cycle hit latency. The L2 also uses in-line double-error-detection and single-error-correction for improved performance and several programmable sleep modes for power reduction. The L2 cache controller features a time-outand-retry mechanism for increased system reliability.Shared memory coherency is maintained through software protocols, such as MPI and OpenMP [4], in an effort to eliminate the communication and hardware overhead required for a memory coherent 2D-mesh. A new message-passing memory type (MPMT) is introduced as an architectural enhancement to optimize data sharing using these software procedures. A single bit in a core's TLB designates MPMT cache lines. The MPMT retains all the performance benefits of a conventional cache line, but distinguishes itself by addressing non-coherent shared memory. All MPMT cache lines are invalidated before reads/writes to the shared memory to prevent a core from working on stale data. A new instruction, MBINV, is added to the core to invalidate all MPMT cache entries in a single cycle. Subsequent reads/writes to inval...

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The number of cores integrated onto a single die is expected to climb steadily in the foreseeable future. This move to many-core chips is driven by a need to optimize performance per watt. How best to connect these cores and how to program the resulting many-core processor, however, is an open research question. Designs vary from GPUs to cache-coherent shared memory multiprocessors to pure distributed memory chips. The 48-core SCC processor reported in this paper is an intermediate case, sharing traits of message passing and shared memory architectures. The hardware has been described elsewhere. In this paper, we describe the programmer's view of this chip. In particular we describe RCCE: the native message passing model created for the SCC processor.Index Terms-many-core processors, message passing APIs, non-cache-coherent shared memory.

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