They Got it Wrong – Intel Defends Claims of Xeon Phi Incompatibility

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Hobbyist site claims the new accelerator breaks compatibility, but Intel explains why it’s a better processor.

by Andy Patrizio

Intel has disputed claims on an enthusiast site that its new high-performance computing (HPC) accelerator card Xeon Phi breaks compatibility with CPU extensions, including 64-bit support.

Enthusiast sites run contrary to the trend in technology. While Apple has led a move away from speeds and feeds, gigahertz and gigabytes, enthusiast/hobbyist sites gleefully dive into technology and go right down to the transistors on details.

But in this case, Bright Side of News missed by a mile in a harsh assessment of the Xeon Phi and Intel is disputing claims of incompatibility in the card with x86 applications.

Xeon Phi is the descendent of the Larrabee project, one of the bumpier projects Intel has had in recent years. It attempted to make a GPU out of an old x86 design but after many years, Intel threw in the towel on making a GPU and instead repositioned the technology as an accelerator for HPC applications.

Like Larrabee, Xeon Phi (developed under the codename Knight’s Corner) is built on the P5 microarchitecture, with many cores tied together by a high-speed interconnect. This core dates back to 1993. It had no vector or 64-bit support, so Intel had to do considerable modification to it, said James Reinders, director of parallel programming evangelism at Intel.

The BSN article complains that the Instruction Set Reference Manual for Xeon Phi lists the registers and instructions are not supported in the Knights Corner architecture, which includes MMX, SSE 1-4 and AVX, the latter of which was added with the Sandy Bridge generation of processors. All of these instructions were added to the x86 architecture after the P5 shipped.

Reinders acknowledges this, but says that since Intel had a core with no extensions to begin with, it added something newer and even more advanced than MMX, SSE and AVX. The old extensions are 64-bit to 256-bit vector units, but Xeon Phi has a 512-bit vector unit, the IMCI that is faster and more power efficient. Complaining about the lack of slower registers when you get a faster one is akin to buying a modern PC and wondering where’s the floppy drive, tape backup unit and VGA port.

And MMX, SSE and AVX were all graphics-related, which Intel deemed unnecessary. IMCI is designed for high-performance computing, with things like double precision performance, the size of key elements in design, and more on bandwidth and pipelining issues.

“I suppose you could complain. What he’s saying is that since we don’t support SSE, we can’t run software that assumes SSE, when the operating system will replace IMCI for SSE at the time of compile,” said Reinders. Compilers looking for MMX or SSE will use IMCI instead, which is a much wider unit.

Another BSN complaint is that Intel breaks quite a few conventions related to AMD64, AMD’s own 64-bit software extensions. The x87 FPU has been pushed aside in favor of to pass floating point values back and forth. Without SSE2, that would make binaries incompatible with Xeon Phi.

Again, Reinders notes a recompile will fix that. “We make up for the lack of MMX and SSE by passing data back and forth in IMCI. So it uses less power anyway,” he said.

“I would describe [Xeon Phi] as a compatible device but I would also say that the binaries aren’t strictly compatible because when has there ever been a Pentium-style core with 64-bit capabilities and 512-bit wide vectors? The answer is never. So what binaries were you trying to run?” he added.

Developers have always rewritten and recompiled for a new processor just to take advantage of the new extensions. This was true for MMX, SSE and AVX. What is being asked of developers to support Xeon Phi is nothing new.

It shouldn’t come as a surprise that BSN never asked Intel for its side of things. A very well-known industry analyst put it this way: “Do you think for one nanosecond that Intel is populated with a bunch of idiots and only [BSN] really know what’s going on?  Intel has thousands of engineers and all they do all day and night is sweat this stuff. Their job depends on it. So let’s get real here.”



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