  Ookami is a computer technology testbed supported by the United States
National Science Foundation. It provides researchers with access to the A64FX
processor developed by Fujitsu in collaboration with RIK{\Xi}N for the Japanese
path to exascale computing, as deployed in Fugaku, the fastest computer in the
world. By focusing on crucial architectural details, the ARM-based, multi-core,
512-bit SIMD-vector processor with ultrahigh-bandwidth memory promises to
retain familiar and successful programming models while achieving very high
performance for a wide range of applications. We review relevant technology and
system details, and the main body of the paper focuses on initial experiences
with the hardware and software ecosystem for micro-benchmarks, mini-apps, and
full applications, and starts to answer questions about where such technologies
fit into the NSF ecosystem.
