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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JohanAnandtech - Thursday, July 28, 2016 - link
Send me a mail at johan@anandtech.comabufrejoval - Thursday, August 4, 2016 - link
Hmm, a bit fuzzy after the first paragraph or so and evidently because I dislike malwaretizement: Such links should be banned!mystic-pokemon - Friday, July 22, 2016 - link
Hi floobitFor virtualization: powerVM and out of the box KVM (tested on Fedora 23, Ubuntu 15.04 / 15.10 / 16.04) work quite well. Xen doesn't work well or hasn't been officially tested / released.
tipoo - Thursday, July 21, 2016 - link
Fun! I was always curious about this processor.tipoo - Thursday, July 21, 2016 - link
Interesting that the L3 eDRAM not only allows them to pack in much more L3 (what was it, 3 SRAM transistors per eDRAM or something?), but it's also low latency which was a cited concern with eDARM by some people. Appears to be an unfounded fear.And then on top of that they put another large L4 eDRAM cache on.
Maybe Intel needs to play with eDRAM more...
tipoo - Thursday, July 21, 2016 - link
Lol, eDRAM, not eDARMKevin G - Thursday, July 21, 2016 - link
There was a change in how the L4 cache works from Broadwell to SkyLake on the mobile parts. The implication is that Intel was exploring the idea of a large L4 eDRAM for SkyLake-EP/EX parts. We'll see how that turns out as Intel also has explored using HMC as a cache for high bandwidth applications in Knights Landing. So either way, Intel has thus idea on there radar and we'll see how it pans out next year.tsk2k - Thursday, July 21, 2016 - link
Is it possible to run Windows on one of these?ZeDestructor - Thursday, July 21, 2016 - link
At the moment, a very solid no.That said, if enough partners ask for it and/or if the numbers make sense for Azure, MS will at the very least have a damn good look at porting Windows over.
DanNeely - Thursday, July 21, 2016 - link
It's probably just a case of doing QA and releasing it. They've sold a PPC build in the past; and maintain internal builds for a number of other CPU architectures to avoid accidentally baking x86isms into the core code.