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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