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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abufrejoval - Thursday, August 4, 2016 - link
I believe "heavily threaded" is somewhat imprecise here: Knights Landing (KNL) is really more about vectorized workloads, or one very loopy and computationally expensive problem, which has been partitioned into lots of chunks, but has high locality. Same code, related data, far more computational throughput than data flowthrough.Power8 will do better on such workloads than perhaps Intel, but never as good as a GPU or KNL.
However it does evidently better per core on highly threaded workloads, where lots of execution threads share the same code but distinct or less related datasets, less scientific and more commercial workloads, more data flowing through.
Funnily KNL might even do well there, beating its Xeon-D sibling in every benchmark, even in terms of energy efficience.
But I'm afraid that's because most of the KNL surface area would remain dark on such workload while the invests would burn through any budget.
KNL is an odd beast designed for a rather specific job and only earn its money there, even if you can run Minecraft or Office on it.
Kevin G - Friday, July 22, 2016 - link
I do think comparison with Xeon Phi is fair since it can run/boot itself now with Knight's Landing. Software parity with the normal x86 ecosystem is now there so it can run off the shelf binaries.I am very curious how well such a dense number of cores perform for workloads that don't need high single threaded performance.
Another interest factor would be memory bandwidth performance as Xeon Phi has plenty. The HMC only further enhances that metric and worth exploring it as both a cache and main memory region for benchmarks.
Ratman6161 - Thursday, July 21, 2016 - link
Will you be addressing virtualization in a future article. I ask this because you are saying the lower cost Power8 systems are intended to compete with the Dell's, HP's, Lenovo etc x86 servers. But these days, a very high percentage of x86 work loads are virutalized either on VMWare or competing products. In 2009 Gartner had it at about 50% and by 2014 it was at 70%. I didn't find a number for '15 or '16 but I expect the percentage would have continued to rise. So if they want to take the place of x86 boxes, they have to be able to do the tasks those boxes do...which tends to largely be to run virtual machines that do the actual workloads.And, what about all the x86 boxes running Windows Server or more commonly Windows Server Virtual machines? Windows Server shops aren't likely to ditch windows in favor of Linux solely for the privilege of running on Power8?
One last thing to consider regarding price. These days we can buy quite robust Intel based server for around $10K. So, supposing I can buy a Power8 system for about the same price? Essentially the hardware has gotten so cheap compared to the licensing and support costs for the software we are running that its a drop in the bucket. If we needed 10 Intel servers or 6 Power 8's to do the same job (assuming the Power8's could run all our VM's), the Power8's could come out lower priced hardware wise, but the difference is, as I said, a drop in the bucket in the overall scheme of things. Performance wise, with the x86 boxes, you just throw more cores at it.
aryonoco - Friday, July 22, 2016 - link
KVM works well on POWER.No idea about proprietary things like VMWare. But that would be up to them to port.
Ratman6161 - Friday, July 22, 2016 - link
Near as I can tell, there is a PowerKVM that runs on Power 8 but that doesn't allow you to run Windows Server VM's - seems to support only Linux guests.Zetbo - Saturday, July 23, 2016 - link
Windows does not support POWER, so there is no point of using POWER if you need Windows!utroz - Thursday, July 21, 2016 - link
AMD should have used IBM's 22nm SOI to make cpu's so that they would not have been totally dead in the performance and server cpu market for years. GF now owns this process as they "bought" IBM's fabs and tech. I think that 22nm SOI might be better for high speed cpu's than the 14nm LPP FinFet that AMD is using for ZEN at the cost of die size.amagriva - Thursday, July 21, 2016 - link
How much you payed your cristal ball?spikebike - Thursday, July 21, 2016 - link
So a single socket Power8 is somewhat faster than the intel chip. But is being compared in a single socket configuration where the intel is designed for a two socket. Unless the power8 is cheaper than an intel dual socket seems most fare to compare both CPU as they are designed to be used.SarahKerrigan - Friday, July 22, 2016 - link
Power is designed for systems up to 16 sockets (IBM E880.) One socket is just the entry point.