The K Computer — Japan Back on Top of the TOP500

The K computer (Japanese: 京, kei) was a supercomputer developed jointly by RIKEN and Fujitsu. On 20 June 2011 the 37th TOP500 list was announced at the ISC'11 conference in Hamburg, Germany, and the K computer took first place with 8.162 petaflops (8.162 quadrillion floating-point operations per second) on the LINPACK benchmark. It was the first time a Japanese machine had led the TOP500 since June 2004, seven years earlier. And the machine was not even finished: the record came from 672 of the eventual 864 racks.

That November, fully built, it reached 10.51 petaflops and kept first place for a second list in a row. TOP500 described it as the first system ever to exceed 10 petaflops. The name kei is the Japanese unit for 10 to the 16th power, which is 10 peta.

What the TOP500 measures

The TOP500 ranks the world's supercomputers by their speed on LINPACK. It began in 1993 and is published twice a year, in June and November. LINPACK measures how fast a machine solves a large dense system of linear equations; it was developed by Jack Dongarra of the University of Tennessee.

In June 2011 TOP500 noted that the K computer's result was more than three times that of the next system and that it used almost twice as many cores as any other machine on the list. It also stressed that, unlike many recent very large systems, the K computer used no graphics processors (GPUs) or other accelerators. According to the TOP500 system record, its processors were Fujitsu's SPARC64 VIIIfx (eight cores at 2 GHz), its nodes were linked by the custom Tofu interconnect, it ran Linux, and in its completed configuration it drew about 12.66 megawatts.

RIKEN called the 93.0% efficiency remarkably high for a top-class large system, and described the K computer as a fully domestic machine, with everything from processor research and development to system design and manufacturing done in Japan.

First place after the earthquake

The ranking came just over three months after the Great East Japan Earthquake of March 2011. In the announcement, RIKEN President Ryoji Noyori thanked the companies that had kept building the machine despite the hardship, and Fujitsu Chairman Michiyoshi Mazuka wrote that partner companies in the Tohoku region had kept supplying parts even though they themselves had been hit by the disaster.

The spending screening and "second place"

The K computer had become a household name in Japan before it ever topped the list. In November 2009, the budget for the next-generation supercomputer came up in the spending screening (jigyō shiwake) run by the Government Revitalization Unit of the Democratic Party of Japan government. A written question submitted in the House of Councillors in July 2012 by Eriko Yamatani quotes Renho, a member of the House of Councillors who served as one of the screeners, as asking what reason there was to be number one in the world, and whether second place would not do.

The same written question says that the fiscal 2010 budget, after being effectively frozen, was eventually funded at 4 billion yen less than requested; that is the questioner's account. Development continued, and about a year and a half after the screening the K computer was ranked first in the world.

Completion, second place and shutdown

According to RIKEN, in 2011 the K computer not only led two consecutive TOP500 lists but also took first place in all four HPC Challenge categories and won the Gordon Bell Prize, which RIKEN presented as evidence that it performed on real applications and not just on LINPACK. It was built under the Ministry of Education's programme to create an Innovative High Performance Computing Infrastructure (HPCI) and installed at the RIKEN Advanced Institute for Computational Science in Kobe (later the RIKEN Center for Computational Science).

When it was completed, RIKEN listed expected uses including analysis of next-generation semiconductor materials, searching for drug candidates, earthquake and tsunami simulation, and global atmospheric models at 800-metre resolution to predict localised torrential rain.

Its time at the top lasted one year: in June 2012 it was overtaken by IBM's Sequoia in the United States. Even so, the K computer served many research institutions and companies for nearly seven years. Shared use ended on 16 August 2019, and the machine was switched off at a shutdown ceremony on 30 August. Its role passed to Fugaku, which RIKEN and Fujitsu had been developing since 2014 and which switched from SPARC to the Arm instruction set (see Arm architecture).

Other computing milestones of the era are on the hardware timeline.