Assessing IBM's POWER8, Part 1: A Low Level Look at Little Endian
by Johan De Gelas on July 21, 2016 8:45 AM ESTClosing Thoughts
Testing both the IBM POWER8 and the Intel Xeon V4 with an unbiased compiler gave us answers to many of the questions we had. The bandwidth advantage of POWER8's subsystem has been quantified: IBM's most affordeable core can offer twice as much bandwidth than Intel's, at least if your application is not (perfectly) vectorized.
Despite the fact that POWER8 can sustain 8 instructions per clock versus 4 to 5 for modern Intel microarchitectures, chips based on Intel's Broadwell architecture deliver the highest instructions per clock cycle rate in most single threaded situations. The larger OoO buffers (available to a single thread!) and somewhat lower branch misprediction penalty seem to the be most likely causes.
However, the difference is not large: the POWER8 CPU inside the S812LC delivers about 87% of the Xeon's single threaded performance at the same clock. That the POWER8 would excel in memory intensive workloads is not a suprise. However, the fact that the large L2 and eDRAM-based L3 caches offer very low latency (at up to 8 MB) was a surprise to us. That the POWER8 won when using GCC to compile was the logical result but not something we expected.
The POWER8 microarchitecture is clearly built to run at least two threads. On average, two threads gives a massive 43% performance boost, with further peaks of up to 84%. This is in sharp contrast with Intel's SMT, which delivers a 18% performance boost with peaks of up to 32%. Taken further, SMT-4 on the POWER8 chip outright doubles its performance compared to single threaded situations in many of the SPEC CPU subtests.
All in all, the maximum throughput of one POWER8 core is about 43% faster than a similar Broadwell-based Xeon E5 v4. Considering that using more cores hardly ever results in perfect scaling, a POWER8 CPU should be able to keep up with a Xeon with 40 to 60% more cores.
To be fair, we have noticed that the Xeon E5 v4 (Broadwell) consumes less power than its formal TDP specification, in notable contrast to its v3 (Haswell) predecessor. So it must be said that the power consumption of the 10 core POWER8 CPU used here is much higher. On paper this is 190W + 64W Centaur chips, versus 145W for the Intel CPU. Put in practice, we measured 221W at idle on our S812LC, while a similarly equipped Xeon system idled at around 90-100W. So POWER8 should be considered in situations where performance is a higher priority than power consumption, such as databases and (big) data mining. It is not suited for applications that run close to idle much of the time and experience only brief peaks of activity. In those markets, Intel has a large performance-per-watt advantage. But there are definitely opportunities for a more power hungry chip if it can deliver significantly greater performance.
Ultimately the launch of IBM's LC servers deserves our attention: it is a monumental step forward for IBM to compete with Intel in a much larger part of the market. Those servers seem to be competitively priced with similar Xeon systems and can access the same Little Endian data as an x86 server. But can POWER8 based system really deliver a significant performance advantage in real server applications? In the next article we will explore the S812LC and its performance in a real server situations, so stay tuned.
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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.