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Q: I've been reading for years that Itanium is going to get cancelled, but Intel still keeps producing new versions of it. So my question is what, specifically, is Itanium so good at that Intel keeps it around, despite the fact that Xeon keeps getting more powerful and is much cheaper than Itanium? What kinds of applications are people using Itanium for, and why can't they just switch to Xeon instead of hassling with a different architecture?

It's not for nothing that Intel's Itanium processor family is commonly called "Itanic." Predictions of the line's demise regularly crop up in the tech press, with the most recent one coming courtesy of Oracle, which declared the Itanium line dead before canceling the future development of its popular database stack for the architecture.

Despite the perpetual doom and gloom that has become part of Itanium's lore, the processor line marches on. Intel recently unveiled Poulson, the next entry in the Itanium line—a 32nm, 3 billion-transistor monster of a chip. And then there's Kittson, Poulson's official successor and the part that's rumored to be, yet again, the "last" Itanium processor before the line gets canceled.

Intel also recently unveiled the Xeon E7, another monster of a processor about which the company's own Kirk Skaugen made the following claim at IDF 2011 Beijing this past April: "Xeon's reliability and performance is now equal [to]—and in some cases better than—Itanium, and they're going to leapfrog [each other] in performance over time."

So the question you've posed is a good one. If, by Intel's own admission, Xeon is as good as Itanium now while being drastically cheaper, why do Intel and its customers bother with Itanium?

As with so many other things in this world, the answer boils down to money. But let's break the "money" answer down further by looking at Itanium from the perspective of both Intel and its customers.

Why Intel still makes Itanium

Intel has recently become fond of pointing out a set of numbers that tells you all you need to know about why the company continues to pour development money into Itanium.

In both the aforementioned IDF presentation and at the most recent Intel Investor meeting, Intel execs pointed out that Itanium has grown into a $4 billion per year business for the chipmaker.

Now, this $4 billion number is a lot smaller than $30 billion, which is the size of Intel's Xeon business. But it's a lot larger than $1.6 billion, which was the revenue for all of AMD combined (CPUs, GPUs—the whole company) in the first quarter of 2011. Furthermore, Intel puts the size of the entire Opteron ecosystem at $2.8 billion, so Intel's Itanium revenues alone beat everyone's total revenues (AMD plus all of its OEM partners) from Opteron.

So while Itanium shipments may be so small that they number in the hundreds of thousands, the business is big enough to make it worth staying in for Intel.

Why customers still buy Itanium

The Itanium decision on the customer side is dominated by two big issues: legacy software and RAS. Let's take the last one first, since it came first temporally.

RAS is an acronym that stands for "Reliability, Availability, Serviceability," and it essentially means "all of the features that you need to ensure that your system never goes down. Ever. For any reason. Because downtime costs you millions of dollars an hour."

Because companies depend on their mainframes to have zero downtime over the course of decades, mainframe architectures like Itanium and its competitors have long had RAS features that go beyond mere support for ECC RAM. If you're interested in learning more about these features, here's a really good breakdown of Itanium RAS features from 2007; Intel is committed to seeing Xeon support all of these, so that RAS isn't part of the Xeon vs. Itanium decision going forward.

While the commodity Xeon line may be finally gaining support for mainframe-level RAS features, it will take a very long time for Xeon to displace Itanium and other mainframe architectures in the mainframe market. That's because in the mainframe market, legacy issues are an order of magnitude more important than they are in any corner of the commodity tech market.

For mainframe users, "legacy" doesn't just mean a giant, crufty, Rube Goldberg-esque conglomeration of legacy programs and libraries that keeps your specific, idiosyncratic workload running 24/7—although that's part of the picture. It also means critical vendor and contractor relationships, including support and maintenance agreements that have multiyear timespans. With so much legacy inertia standing in the way of change, mainframe users are just not interested in entertaining the terrifying prospect of porting their mission-critical mainframe systems to an entirely new hardware and OS stack, so they keep refreshing and expanding the systems that they already have. Hence the market for new mainframe systems like Itanium. In other words, this is a classic case of vendor lock-in.

Intel's Kirk Skaugen acknowledged the centrality of OS and vendor choices in his aforementioned IDF presentation. Here's the full quote for reference:

"We used to position Itanium as highest performance, highest reliability... We're still committed to Itanium. It's really now a choice of operating system. Xeon's reliability and performance is now equal [to]—and in some cases better than—Itanium, and they're going to leapfrog [each other] in performance over time." "If you like HP-UX, OpenVMS, Nonstop, and [other] mainframe operating systems, we're going to fully support you on Itanium. But now Xeon is in a space where there's no workload on the planet that Xeon can't handle."

While there may be no workload on the planet that Xeon can't handle, there are definitely some mainframe operating systems out there that it can't, and it can't because those OSes run on Itanium.

The reason Skaugen listed HP-UX first is because it's a really popular Itanium OS, and a factor in keeping customers locked into Itanium. According to a widely quoted IDC estimate, HP ships some 90 percent of Itanium systems sold, so in the vast majority of cases, choosing Itanium means choosing HP and HP-UX. And the reason customers keep buying those HP systems is because they bought HP Itanium systems in the past and they'd like to keep upgrading them and expanding them.

Whither Itanium?

While Itanium's future may not look particularly bright, it's not all that dim, either. Intel's newly announced Poulson will double the performance of the Itanium platform, and it also marks a fundamental shift in direction from the Itanium chips that have come before.

Poulson essentially gives up on the core VLIW (very long instruction word) idea that was the genesis of Intel's original IA-64 project, and it breaks apart VLIW's signature, statically scheduled instruction bundles into individual instructions before scheduling them (sometimes out-of-order).

Poulson also scales back the massively powerful floating-point hardware that (along with RAS) has historically set the Itanium line apart from Intel's x86 chips; this floating-point muscle was an artifact of the workstation era, when Intel designed Itanium to compete in the now-defunct RISC workstation market. In throwing out the monster FPU and replacing it in part with post-VLIW bookkeeping apparatus that's designed to improve Poulson's performance on the kinds of server workloads for which Itanium is actually used today, Intel is finally bringing Itanium a few major steps toward Xeon, even as it moves Xeon toward Itanium via RAS improvements.

So while Itanium and Xeon are beginning to look more and more alike, the aforementioned legacy and economic issues suggest that Itanium will stick around, at least in the medium-term, as a separate and viable product line within Intel.