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This paper describes the Blue Genet/L advanced diagnostics environment (ADE) used throughout all aspects of the Blue Gene/L project, including design, logic verification, bringup, diagnostics, and manufacturing test. The Blue Gene/L ADE consists of a lightweight multithreaded coherence-managed kernel, runtime libraries, device drivers, system programming interfaces, compilers, and host-based development tools. It provides complete and flexible access to all features of the Blue Gene/L hardware. Prior to the existence of hardware, ADE was used on Very highspeed integrated circuit Hardware Description Language (VHDL) models, not only for logic verification, but also for performance measurements, code-path analysis, and evaluation of architectural tradeoffs. During early hardware bring-up, the ability to run in a cycle-reproducible manner on both hardware and VHDL proved invaluable in fault isolation and analysis. However, ADE is also capable of supporting high-performance applications and parallel test cases, thereby permitting us to stress the hardware to the limits of its capabilities. This paper also provides insights into systemlevel and device-level programming of Blue Gene/L to assist developers of high-performance applications to more fully exploit the performance of the machine.

The Blue Genet/L compute chip is a dual-processor system-on-a-chip capable of delivering an arithmetic peak performance of 5.6 gigaflops. To match the memory speed to the high compute performance, the system implements an aggressive three-level on-chip cache hierarchy. The implemented hierarchy offers high bandwidth and integrated prefetching on cache hierarchy levels 2 and 3 (L2 and L3) to reduce memory access time. A Gigabit Ethernet interface driven by direct memory access (DMA) is integrated in the cache hierarchy, requiring only an external physical link layer chip to connect to the media. The integrated L3 cache stores a total of 4 MB of data, using multibank embedded dynamic random access memory (DRAM). The 1,024-bit-wide data port of the embedded DRAM provides 22.4 GB/s bandwidth to serve the speculative prefetching demands of the two processor cores and the Gigabit Ethernet DMA engine. To reduce hardware overhead due to cache coherence intervention requests, memory coherence is maintained by software. This is particularly efficient for regular highly parallel applications with partitionable working sets. The system further integrates an on-chip double-data-rate (DDR) DRAM controller for direct attachment of main memory modules to optimize overall memory performance and cost. For booting the system and low-latency interprocessor communication and synchronization, a 16-KB static random access memory (SRAM) and hardware locks have been added to the design. IBM J. RES. & DEV. VOL. 49 NO. 2/3 MARCH/MAY 2005 M. OHMACHT ET AL.