Assessing IBM's POWER8, Part 1: A Low Level Look at Little Endian
by Johan De Gelas on July 21, 2016 8:45 AM ESTMemory Subsystem: Latency Measurements
There is no doubt about it: the performance of modern CPUs depends heavily on the cache subsystem. And some applications depend heavily on the DRAM subsystem too. We used LMBench in an effort to try to measure latency. Our favorite tool to do this, Tinymembench, does not support the POWER architecture yet. That is a pity, because it is a lot more accurate and modern (as it can test with two outstanding requests).
The numbers we looked at were "Random load latency stride=16 Bytes" (LMBench).
| Mem Hierarchy |
IBM POWER8 | Intel Broadwell Xeon E5-2640v4 DDR4-2133 |
Intel Broadwell Xeon E5-2699v4 DDR4-2400 |
| L1 Cache (cycles) | 3 | 4 | 4 |
| L2 Cache (cycles) | 13 | 12-15 | 12-15 |
| L3 Cache 4-8 MB(cycles) | 27-28 (8 ns) | 49-50 | 50 |
| 16 MB (ns) | 55 ns | 26 ns | 21 ns |
| 32-64 MB (ns) | 55-57 ns | 75-92 ns | 80-96 ns |
| Memory 96-128 MB (ns) | 67-74 ns | 90-91 ns | 96 ns |
| Memory 384-512 MB (ns) | 89-91 ns | 91-93 ns | 95 ns |
(Note that the numbers for Intel are higher than what we reported in our Cavium ThunderX review. The reason is that we are now using the numbers of LMBench and not those of Tinymembench.)
A 64 KB L1 cache with 4 read ports that can run at 4+ GHz speeds and still maintain a 3 cycle load latency is nothing less than the pinnacle of engineering. The L2 cache excels too, being twice as large (512 KB) and still offering the same latency as Intel's L2.
Once we get to the eDRAM L3 cache, our readings get a lot more confusing. The L3 cache is blistering fast as long as you only access the part that is closest to the core (8 MB). Go beyond that limit (16 MB), and you get a latency that is no less than 7 times worse. It looks like we actually hitting the Centaur chips, because the latency stays the same at 32 and 64 MB.
Intel has a much more predictable latency chart. Xeon's L3 cache needs about 50 cycles, and once you get into DRAM, you get a 90-96 ns latency. The "transistion phase" from 26 ns L3 to 90 ns DRAM is much smaller.
Comparatively, that "transition phase" seems relatively large on the IBM POWER8. We have to go beyond 128 MB before we get the full DRAM latency. And even then the Centaur chip seems to handle things well: the octal DDR-3 1333 MHz DRAM system delivers the same or even slightly better latency as the DDR4-2400 memory on the Xeon.
In summary, IBM's POWER8 has a twice as fast 8 MB L3, while Intel's L3 is vastly better in the 9-32 MB zone. But once you go beyond 32 MB, the IBM memory subsystem delivers better latency. At a significant power cost we must add, because those 4 memory buffers need about 64 Watts.
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nobodyblog - Thursday, July 21, 2016 - link
Please correct this error, you are saying you are comparing with BEST Intel can provide, but you did address Xeon for workloads need Xeon Phi Knight Landing which is a standalone CPU, too. If you choose correctly, the benchmark will be sooo different.IBM Power 8 is 90 GB/s, while Intel's Xeon phi knight landing (as 7290F) has a bandwidth of 400 GB/s.
IBM power 8 does above 600 gflops single precision and above 300 gflops double precision FLOPs, this is *10 in Xeon phi 7290F.
Specint: xeon phi is 1500 vs 1700 for power 8
Power and Price aside....
Thanks!
LukaP - Thursday, July 21, 2016 - link
If we start comparing different product categories, why not bring the GP100 into this as well. It will deliver 10TFLOPS of single precision and can be had for much less than any of these. But then again, there is the same caveat as the Xeon Phi. You cant actually run an OS on it, you need a host CPU and then you dispatch kernels onto the accelerator. Even if its a socketed version.smilingcrow - Thursday, July 21, 2016 - link
You can boot from newer Xeon Phi; either current or the next generation due maybe this year!LukaP - Thursday, July 21, 2016 - link
Oh really? :o that is neat, though not sure if that useful, since even highly parallel tasks usually have some IPC dependent components...Anyways have you got a source for that, would love to read more
Drumsticks - Thursday, July 21, 2016 - link
I'm a verification intern on the Phi team right now, and you can indeed boot Knight's Landing! Anandtech mentions it here: http://www.anandtech.com/show/9802/supercomputing-...nobodyblog - Friday, July 22, 2016 - link
Then you can add another xeon phi to above statistics... Xeon Phi KL is a CPU like other CPUs it does everything as mentioned even its specint is comparable, not so bad...Thanks!
tipoo - Friday, July 22, 2016 - link
Xeon Phi is x86, but it's GPU-like in nature, massively parallel for performance with low per-core performance. The IBM Power8 and other Xeons compete in highly parallel spaces like banking, but where single thread performance also still matters. Can't compare them.nobodyblog - Friday, July 22, 2016 - link
Xeon Phi Knight Landing has 3 times more single thread performance than silvermont (& knight corner).. I don't think it is so bad...The comparison is truly so, see the benchmarks, they say specint for example, or anything parallel performance, additionally, you can use a Xeon high performance with a xeon phi, there is nothing that prevents you. The benchmark is not about Database performance or parsing or anything similar, it is about this article, I don't say xeon phi is currently better positioned than xeon in these uses... But IBM's Power is not so, too, it has lots of core and lots of threads which is usable only in massive parallel uses...
Thanks!
nobodyblog - Friday, July 22, 2016 - link
On the IBM server, numactl was used to physically bind the 2, 4, or 8 copies of SPEC CPU to the first 2, 4, or 8 threads of the first core. On the Intel server, the 2 copy benchmark was bound to the first core. It is not single thread, it is a trick IBM uses to cheat in benchmarks, it is 425% percents slower than xeon in single thread.Thanks!
jospoortvliet - Tuesday, July 26, 2016 - link
The benchmarks here pit one core against one core. The IBM cores can run 1, 2, 4 or 8 threads on a single core, the Intel does 1 or 2. The 425%, not sure where that number comes from, but it isn't what shows out of these benchmarks.The benchmarks show, as described by Johan:
In single thread, the IBM does about 13% less work than the Intel core. In 2-thread mode, the IBM does about 20% more than the intel across the two threads. The intel doesn't do more than 2 threads, the IBM can and does then, on average, 43% more work across the eight threads than the Intel does with its two.
So Intel is single-thread master here, IBM is throughput king. Now if you have a HEAVILY threaded workload, with hundreds of threads and little latency requirements for each, Knights Landing or a GPU is a better choice, with their hundreds of cores. If latency is important and you can afford to use two to four threads per core the IBM performs best. If latency is everything, you keep it at 1 thread per core and the Intel Xeon is the best performer.
That is entirely ignoring cost, of course, both Intel and IBM have high and low cost solutions with their downsides and benefits. This set of benchmarks simply pitted one core against another, entirely ignoring the differences in core count (IBM 10, Intel 22) and price (Intel orders of magnitude more expensive). You'll always have to look at a bigger picture: how many cores do you get for your dollar and what are your requirements.
Performance/watt, the Intel probably wins in all area's, at least if the system is idle frequently. Without idle the IBM might be not that bad, perf/power wise.
The big take-away from this article is, though, that IBM has built a system which can be quite price-competitive with Intel in the lower-high end market. To really be able to make a choice, we'd probably need a benchmark of two price-equivalent systems. I bet the workload would make a huge difference in who wins the price/performance fight.