Assessing IBM's POWER8, Part 1: A Low Level Look at Little Endian
by Johan De Gelas on July 21, 2016 8:45 AM ESTMulti-Threaded Integer Performance on one core: SPEC CPU2006
Broadly speaking, the value of SPEC CPU2006's int rate test is questionable, as it puts too much emphasis on bandwidth and way too little emphasis on data synchronization. However, it does give some indication of the total "raw" integer compute power available.
We will make an attempt to understand the differences between IBM and Intel, but to be really accurate we would need to profile the software and runs dozens of tests while looking at the performance counters. That would have set back this article a bit too much. So we can only make an educated guess based upon what the existing academic literature says and our experiences with both architectures.
The Intel CPU performance is the 100% baseline in each column.
| Subtest SPEC CPU2006 Integer |
Application Type |
IBM POWER8 vs Xeon E5-2699v4 Single Thread |
IBM POWER8 vs Xeon E5-2699v4 Max Thread |
IBM POWER8 vs Xeon E5-2699v4 Top performance |
| 400.perlbench | Spam filter | N/A | N/A | N/A |
| 401.bzip2 | Compress | 91% | 139% | 139% |
| 403.gcc | Compiling | 111% | 185% | 185% |
| 429.mcf | Vehicle scheduling | 121% | 167% | 167% |
| 445.gobmk | Game AI | 90% | 156% | 156% |
| 456.hmmer | Protein seq. analyses | 79% | 79% | 101% |
| 458.sjeng | Chess | 69% | 117% | 117% |
| 462.libquantum | Quantum sim |
76% | 160% | 162% |
| 464.h264ref | Video encoding | 80% | 120% | 131% |
| 471.omnetpp | Network sim |
100% | 141% | 141% |
| 473.astar | Pathfinding | 87% | 156% | 156% |
| 483.xalancbmk | XML processing | 70% | 116% | 116% |
On (geometric) average, a single thread running on the IBM POWER8 core runs about 13% slower than on an Intel Broadwell architecture core. So our suspicion that Intel is still a bit better at extracting parallelism when running a single thread is confirmed.
Intel gains the upper-hand in the applications where branch prediction plays an important role: chess (sjeng), pathfinding (astar), protein seq. analysis (hmmer), and AI (gobmk). Intel's branch misprediction penalty is lower if the other branch is available in the µop cache (the Decode Stream Buffer) and Intel has a few clever tricks that the IBM core does not have like the loop stream detector.
Where the POWER8 core shines is in the benchmarks where memory latency is important and where the load units are a bottleneck, like vehicle scheduling (mcf). This is also true, but in lesser degree, for the network simulation (omnetpp). The reason might be that omnetpp puts a lot of pressure on the OoO buffers, and Intel's architecture offers more room with its unified buffers, whereas IBM POWER8's buffers are more partitioned (see for example the issue queue). Meanwhile XML processing does a lot of pointer chasing, but quick profiling has shown that this benchmark mostly hits the L2, and somewhat the L3. So there's no disadvantage for Intel there. On the flip side, Xalancbmk is the benchmark with the highest pressure on the ROB. Again, the larger OOO buffers for one thread might help Intel to do better.
POWER8 also does well in GCC, which has a high percentage of branches in the instruction mix, but very few branch mispredictions. GCC compiling is latency sensitive, so a 3 cycle L1, a 13 cycle L2, and the fast 8MB L3 help.
Finally, the pathfinding (astar) benchmark does some intensive pointer chasing, but it misses the L1- and L2-cache much less often than xalancbmk, and has the highest amount of branch misprediction. So the impact of the pointer chasing and memory latency is thus minimal.
Once all threads are active, the IBM POWER8 core is able to outperform the Intel CPU by 41% (geomean average).
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tipoo - Thursday, July 21, 2016 - link
They made PowerPC Windows? Source? I remember the Powermac G5s were the early dev kits for the xbox 360 due to the architecture similarity, but I assumed those stories meant they were just working in OSX or Linux on them.thunderbird32 - Thursday, July 21, 2016 - link
AFAIK, the last build of Windows for PPC was NT 4. So, it's been a while.Sunner - Thursday, July 21, 2016 - link
There were early builds of Windows 2000 for the RISC's as well, during the times when it was still called NT5. I had one of those from WinHEC, but alas I lost it when moving at some point. :(yuhong - Thursday, July 21, 2016 - link
AFAIK, the little endian PowerPC mode that NT4 used was killed when they went to 64-bit and is different from today's POWER8 little endian mode that was only recently introduced.Kevin G - Thursday, July 21, 2016 - link
I used to have such a disc for Windows NT4. That disk also had binaries for DEC Alpha and MIPS.BillyONeal - Thursday, July 21, 2016 - link
The Xbox 360 is a PPC machine, and runs a (heavily modified) version of Windows. My understanding is that most x86 assumptions had to be ferreted out to run on Itanium (early) and then on ARM (later).Einy0 - Thursday, July 21, 2016 - link
MS has builds that will run on anything. The real question is why would you want to? These chips are designed from the ground up to run massive work loads. It's a completely different style of computing than a Windows machine. Even MS server OSes aren't designed for this type of work. We are talking Banking, ERP and other big data applications. MS is still dreaming about scaling on that level. Right now their answer is clustering but that comes with it's own obstacles too.abufrejoval - Thursday, August 4, 2016 - link
Well there is always QEMU.And IBM has a much better binary translator from when they bought QuickTransit. That one originally translated Power to x86 for the Mac, then Sparc to x86 for Quicktransit and eventually x86 to Power for IBM so they could run Linux workloads on AIX.
Then what exactly do you mean with Windows (assuming this is actually a reasonable question)?
Server applications or desktop?
.NET has been ported to Linux and I guess could be made to run on Power. A Power runtime could certainly be done by Microsoft, if they wanted to.
I don't see why anyone would want to run Windows desktop workloads on this hardware, other than to show that it can be done: QEMU to that!
BedfordTim - Thursday, July 21, 2016 - link
I was intrigued to see how little effect hyper-threading with your Xeon. My own experience is that it gives a 50% boost although I appreciate there are many variables.Taracta - Thursday, July 21, 2016 - link
Something seems to be wrong with the Mem Hierarchy charts in the Intel L3 and 16MB section.