Itanium - is there light at the end of the tunnel?

The limits of TLP...

Hardware engineers do not believe in massive superscalar CPUs anymore^[2]. Increasing ILP a tiny bit requires exponentially bigger out-of-order hardware, which exponentially requires more power.

TLP and multi-core is hot and trendy. But the same problems that were true for squeezing ILP out of hardware are true about TLP in software. On the exception of naturally parallel applications such as rendering and database servers, getting more and more threads out of the majority of hardware will require exponentially more programming and debugging time. There are more high TLP designs such as Sun's Niagara that increase throughput, but also response time. And while the numbers of users that can update and read the database matters, the response time of the database can be important too. For example, while OLTP loads consist of many relatively simple SQL selects, Decision Support Systems (DSS or OLAP) fire off very complex queries with a high response time. To offer a good "data mining" experience, the single thread performance must not be neglected. The same can be said for some HPC and many typical workstation applications.

So, while designs that sacrifice ILP completely on the altar of TLP, such as Sun's Niagara, they may well be very popular in some markets such as webserving. Single thread performance is going to make the difference between the different multi-core solutions.

Here, the Itanium can leverage two big advantages: higher ILP and smaller cores. This last comment might seem ridiculous, given that the current Itanium Madison is about 432 mm² large. However, if we look at the core (L1 inclusive), there about 25 million transistors that take about 80 mm² die space. Note that the pictures below have been scaled and resized to reflect the relative proportions of the different cores.

Madison 9 MB die

However, about 10-15 mm^² is used for the x86 compatibility, excess baggage that most competitors don't have. Let us take a closer look at the Itanium Madison core.

Madison core parts

As you can see, the IA 32 or x86 is a big chunk of the die. If we ignore this x86 part and compare the Itanium core with the 0.13µ 190 mm² large Opteron, we can see that the Opteron core is about as large as the Itanium core.

Opteron die, rotated 90°

In the next part, we study this even more closely.


Itanium: a slim figure

We compare the different CPU cores in the table below. We consider the L1-cache being part of the core, but we list the number of transistors separately to be clear. To keep it fair, we compare all CPU using the same process technologies, 0.13µ, with the exception of the Intel Xeon.

There is no doubt that a 0.13µ Xeon MP is no match for either of the IBM Power 5, the Itanium or Opteron with both Spec FP2000 and SpecInt around 1200 for a Xeon MP 3 GHz. The Xeon MP 0.13µ is also a 32 bit CPU, so it does not belong in the list below.

The reason why I listed the 90nm Xeon (DP) is to show how complex an x86 architecture can get when it has 64 bit and an extremely deep pipeline in a quest for high clock speeds, which must negate the low ILP. With more than 50 million transistors, it is no wonder that Xeon "Irwindale/Nocona" is the hottest CPU (per core) of the bunch despite being manufactured in a more advanced process.

Why do we use Spec FP2000 and Spec Int2000^[3]? It is true that these benchmarks are close to meaningless when you want to compare server or workstation performance in the real world. Spec FP is a decent predictor of HPC/scientific performance, but fails to predict Digital Content Creation performance despite containing a few OpenGL benchmarks. The reason why we use these two benchmarks is that currently, we are evaluating the CPU architecture, its future potential and current compiler performance, and not the complete system.

CPU feature	Intel Itanium "Madison"	Intel Xeon P4 Irwindale	IBM Power 5 (+)	AMD Opteron
Process technology	0.13 µ CU	0.09 µ CU	0.13 µ CU SOI	0.13 µ CU SOI
Die Size (mm2)	432	130	389	190
Number of transistors (Million)	592	169	276	106
Number of transistors (Million) L1-cache	1.8	+/- 6	5.3	7.7
Number of transistors (Million) L2 Cache	14	113	107	57
Number of transistors (Million) L3 Cache	510	0	off die	0
Number of transistors (Million) Tag (L2 + L3)	23	4	33	4
Number of transistors (Million) Core	20	50	35*	40
Pure logic core (-L1)	18	44	30	32
Top clock speed	1600	3800	1900	2600
Best Spec FP2000 Score	2712	1898	2839	1955
Best Spec Int2000 Score	1590	1810	1470	1713
TDP	107 W	115-130 W	200 W**	<95W

* per core, two cores: about 70 million transistors
** for two cores

To calculate the cache sizes, we used the following formula:

Cache size expressed in Bytes x 9 bits per byte (8 + 1 bit ECC/parity protection) x 6 Transistors per bit (SRAM)

We calculated the Power 5 core as follows. The PowerPC 970FX, aka Apple's G5, is essentially a Power 4 core with Altivec, but without the L3 cache tag. If we subtract the number of transistors of the L2 cache (28 million) from the total number of transistors in the PowerPC 970, we end up with about 30 million transistors. The Power 5 core is a bit more complex (SMT and a few tweaks have been added), so we estimate it at about 35 million transistors.

HP and Intel have stated that the Itanium 2 core, including the L2-cache, has about 40 million transistors. If we subtract the L2 cache, we end up with about 26 million transistors, which still includes the x86 compatibility transistors (about 4 million) and the L2-tag. It wouldn't be fair to include the x86 transistors when we compare the merits of EPIC with x86 and RISC.

The Itanium core is twice as small as the Xeon's

So, we end up with 20 to 22 million transistors for the core, which is truly remarkable for a CPU that is considered the fastest FP CPU out there (together with the Power 5), and is better than all the RISC players in Spec Int. The 0.13 micron Opteron (2.6 GHz) beats the best Itanium by about 10%; still is remarkable how good 20 million transistors can perform.

And, what about the Pentium M? Well, the core is about 25 million transistors, but it is pretty hard to compare this CPU with Itanium as the Pentium M is optimised for low power consumption. If we keep it fair and compare the two cores using the same process technology, the Pentium M isn't even close when it comes to performance. A 2 GHz Pentium M scores a respectable 1500 in SpecInt, but trails far behind with a specfp score of about 1000.