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IBM mainframes get CPU, OS makeover

Posted: 16 Apr 2008 ?? ?Print Version ?Bookmark and Share

Keywords:IBM mainframe? CPU? processor? operating system? multicore?

IBM Corp. is exploring hybrid computing concepts and expanding open-source software support for its mainframes as two ways to kick new life into the venerable systems. News of the work came as IBM unveiled its System z10 sporting 4.4GHz processors, the first ground-up CPU redesign in an IBM mainframe in a decade.

"The hybrid platform is one of the big challenges we are tackling," said Charles Webb, an IBM fellow and chief architect of the processor in the z10. "We see more and more hybrid computing in the future, and there are many forms it can take."

In a separate effort, IBM is preparing the z10 to be its first mainframe to run OpenSolaris, the open-source OS from competitor Sun Microsystems Inc.

IBM developed a custom capability for an online gaming company in Brazil to use its zSeries mainframes along with the Cell processor IBM co-developed for the Sony Playstation. IBM is researching ways to run a broad set of accelerators in tandem with the mainframes.

"Cell is a good starting point, but it is not the only thing we think we will use long term," said Webb.

Accelerators could run jobs such as options pricing and risk analysis using transaction data processed on the mainframe's general-purpose CPUs, Webb said. But the challenges to building such systems are many.

Engineers have to define which accelerators best serve which applications. They must find ways to package flexibly the accelerators with the mainframes. Most important, they need to write management, virtualization and other middleware that hides from users the complexity of splitting applications across separate pools of processors.

"There's a lot of work to do in that area," said Webb.

Some see the new direction as a strategic growth driver for mainframe computers, which are generally on the decline as users shift many jobs to mainstream PC servers. Market watcher Gartner Inc. estimated that mainframe sales declined nearly 12 percent last year, in part a cyclical downturn. IBM, the largest vendor of all types of servers, saw its mainframe business drop 9.6 percent in 2007, as its PC server business grew 10 percent.

Mainframes "are in all the biggest companies, and they are not going away anytime soon," said Carl Greiner, a senior analyst with Ovum Ltd. "The question is, will they attract any new users? As they move to more-hybrid systems, I think they will."

When it came to the design of the microprocessors inside its z10, IBM took a more-conventional approach. Engineers redesigned the 10-year-old chip platform to ratchet frequency to a whopping 4.4GHz, up from 1.7GHz on the z9.

Each z10 module packs five quad-core processors running at 4.4GHz as well as two storage controllers.

Power efficiency
Unlike many mainstream CPUs, the zSeries chips had plenty of headroom for the shift. In part, that's because mainframes use refrigeration modules to cool the multichip modules that house the processors, keeping leakage current to less than half the total power dissipation in the z10 CPUs.

"At the system level, this was the right thing to do," said Webb. "At the chip level, it's not the most power-efficient design, but it's not clear we could have gotten the per-core performance boosts we wanted any other way."

IBM estimates the System z10 delivers twice the performance of the previous z9 on some workloads but only a 15 percent improvement in performance per watt.

"This is the first machine I've worked on where power really was something we had to worry about, but we were able to manage it," said Webb. "The real challenge for us going forward is dealing with power density, which looks like it could grow uncomfortably."

Indeed, most mainstream server CPU designers have already hit the so-called power wall, forcing them to move to multicore and multithreaded architectures.

"The rate of growth in per-processor performance has been declining and will continue to decline, primarily due to power issues," Webb said. "We are not at the end of advances in per-processor performance, but we see it ending eventually."

IBM considered and rejected multithreading for the z10 and earlier z9. The approach does not deliver a consistent performance increase on mainframe-class transaction processing, Webb said.

The z10 packs up to 80 cores in 20 quad-core processors, made in 65nm technology. Only 64 of those cores are available to the end user, with the rest mainly dedicated to I/O jobs. That's up from 54 cores available for end-user jobs in the dual-core, 90nm z9 chips.

"Even at 64 CPUs, a lot of effects pop up in the software, and we have been systematically identifying and addressing them all," Webb said. "We don't see any showstoppers ahead as we increase the number of CPUs, but there will be some software subsystems that will need to be overhauled."

Future mainframes will make stepwise increases in the number of processors and amount of cache to bolster performance. IBM added a private 3Mbyte Level 2 cache for each core in the z10. It increased Level 3 cache from 40Mbytes to 48Mbytes."For the workloads we do, we are seeing a lot of pressure on the cache subsystems," said Webb.

- Rick Merritt
EE Times

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