The Intel Xeon E7-8800 v3 Review: The POWER8 Killer?

The Competitor: IBM's POWER8

As we briefly mentioned in the introduction, among all of the potential competitors for the Xeon E7 line, IBM's OpenPower might be the most potent competitor at this time.  So how do IBM's offerings compare to Intel's? IBM POWER 8 is a Brainiac (high IPC) design that also wants to be speed demon (high clock speeds).

The POWER8 core can decode, issue and execute and retire 8 instructions per cycle. That degree of of instruction level parallelism (ILP) can not be extracted out of (most) software. To battle the lack of ILP in software, no less than 8 threads (SMT) are active per core.  According to IBM, 

• 2-threads delivers about 45% performance more than one
• 4-threads deliver yet another 30% boost
• the last 4-threads deliver about 7%

So in total, the 8-way SMT doubles the performance of this massive core. Let us compare the two chips. 

The POWER8 looks better than Haswell-EX in almost every spec, but the devil is of course in the details. First of all, Intel's L2-cache works at the same clock as the core, IBM's L2-cache runs at a lower clock (2.2 GHz or less, depending on the model). Secondly, the POWER8's L3 eDRAM cache might be much larger, but it is so also a bit slower.  

But the main disadvantage of the POWER8 is that all this superscalar wideness and high clockspeed goodness comes with a power price. This slide from Tyan at the latest OpenPOWER conference tell us more. 

A 12 core POWER8 is "limited" to 3.1 GHz if you want to stay below the 190W TDP mark. Clockspeeds higher than 4 GHz are only possible with 8-cores and a 250W TDP. This makes us really curious what kind of power dissipation we may expect from the 4.2 GHz 10-core POWER8 inside the expensive E870 Enterprise systems (300W?).  

That is not all. Each "Jordan Creek2" memory buffer on the Intel system is limited to about 9W. IBM uses a similar but more complex "Centaur" memory buffer (including a 16 MB cache) which needs more than twice as much energy (16-20W). There are at least four of them per chip, and a high-end chip can have eight. So in total the Intel CPU plus memory buffers have a 201W TDP (165W CPU + 4x9W Jordan Creek 2), while the IBM platform has at best a 270W TDP (190W CPU+ 4x20W MBC).