The IBM POWER8 Review: Challenging the Intel Xeon
by Johan De Gelas on November 6, 2015 8:00 AM EST- Posted in
- IT Computing
- CPUs
- Enterprise
- Enterprise CPUs
- IBM
- POWER
- POWER8
Floating Point: C-ray
Shifting over from integer to floating point benchmarks we have C-ray. C-ray is an extremely simple ray-tracer which is not representative of any real world raytracing application. In fact, it is essentially a floating point benchmark that runs out the L1-cache. Luckily it is not as synthetic and meaningless as Whetstone, as you can actually use the software to do simple raytracing. That is not the kind of benchmark we like to use for the evaluations of server CPUs, but since our first efforts to port some of our favorite applications to OpenPOWER failed, we settled for something easier. We knew we would have the POWER8 system only for a few weeks, so we had to play it safe.
First we compiled the C-ray multi-threaded version with -O3 -ffast-math. To understand the CPU performance better, we limited C-ray with taskset to one or two threads (CPU 0 and 18) on the Haswell-based Xeon and one to eight threads on the POWER8. We also kept the output resolution at 768x432 to keep the render times in check. The "sphfract" file was used as input.
Real floating point intensive applications tend to put the memory subsystem under pressure, and running a second thread makes it only worse. So we are used to seeing that many HPC applications performe worse with multi-threading on. But since C-ray runs mostly out of the L1-cache, we get different behavior. Still, 8 threads of floating action seem to be too much: the POWER8 delivers the best FP performance at 4 threads. At this point, the POWER8 core is able to deliver 20% higher floating point performance than the Haswell Xeon.
Next we used all 160 (20 x 8 threads SMT) or 72 (36 x 2 threads SMT) threads and increased the resolution to 3840x2160.
With a core count that is 80% higher, there is nothing stopping the Xeon E5-2699 v3 from taking the top spot. Still, the POWER8 delivers solid performance and outperforms the slower Xeon E5-2695 v3 by 5%. Although the real world relevance of this benchmark is small, we now have an idea of how good the "basic FP" performance is. Otherwise in real world applications, the use of AVX-2/VSX and the available bandwidth will play a role.
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LemmingOverlord - Friday, November 6, 2015 - link
Mate... Bite your tongue! Johan is THE man when it comes to Datacenter-class hardware. Obviously he doesn't get the same exposure as teh personal technology guys, but he is definitely one of the best reviewers out there (inside and outside AT).joegee - Friday, November 6, 2015 - link
He's been doing class A work since Ace's Hardware (maybe before, I found him on Ace's though.) He is a cut above the rest.nismotigerwvu - Friday, November 6, 2015 - link
Johan,I think you had a typo on the first sentence of the 3rd paragraph on page 1.
"After seeing the reader interestin POWER8 in that previous article..."
Nice read overall and if I hadn't just had my morning cup of coffee I would have missed it too.
Ryan Smith - Friday, November 6, 2015 - link
Good catch. Thanks!Essence_of_War - Friday, November 6, 2015 - link
That performance per watt, it is REALLY hard to keep up with the Xeons there!III-V - Friday, November 6, 2015 - link
IBM's L1 data cache has a 3-cycle access time, and is twice as large (64KB) as Intel's, and I think I remember it accounting for something like half the power consumption of the core.Essence_of_War - Friday, November 6, 2015 - link
Whoa, neat bit of trivia!JohanAnandtech - Saturday, November 7, 2015 - link
Interesting. Got a link/doc to back that up? I have not found such detailed architectural info.Henriok - Friday, November 6, 2015 - link
Very nice to see tests of non-x86 hardware. It's interesting too se a test of the S822L when IBM just launched two even more price competitive machines, designed and built by Wistron and Tyan, as pure OpenPOWER machines: the S812LC and S822LC. These can't run AIX, and are substantially cheaper than the IBM designed machines. They might lack some features, but they would probably fit nicely in this test. And they are sporting the single chip 12 core version of the POWER8 processor (with cores disabled).DanNeely - Friday, November 6, 2015 - link
"The server is powered by two redundant high quality Emerson 1400W PSUs."The sticker on the PSU is only 80+ (no color). Unless the hotswap support comes with a substantial penalty (if so why); this design looks to be well behind the state of the art. With data centers often being power/hvac limited these days, using a relatively low efficiency PSU in an otherwise very high end system seems bizarre to me.