Saturday, November 13, 2010

GeForce GTX 580, Radeon HD 6970, Delays and Rumors

Unless you've been sleeping under a rock the last few days, you must have heard about the GTX 580. This new graphics card from NVIDIA is based on a "new" chip, called GF110. It's basically a fixed GF100, fully enabled, with higher clocks and slightly lower power draw—definitely a significant step in the right direction.

If you haven't already, I suggest you take a look at some of the following reviews: The Tech ReportAnandtech and That last one is in French, but Damien Triolet is one of the best reviewers out there, so it's definitely worth a look, even if it means running it through Google Translate. Plus, the charts speak for themselves anyway.

The consensus is that the card is "what Fermi should have been from the beginning" and that's definitely the feeling I got, except for the power draw, which is still freakishly high. But at least, now it's not disproportionate with regard to performance. In other words, the GTX 580 is more or less on the HD 5970's level in performance/watt, which isn't bad at all.

That said, don't run out to the store just yet, because NVIDIA's latest and greatest has two things going against it:
  • at $500 or more, it's very expensive;
  • AMD's Radeon HD 6970 is just a month away.
AMD is usually quite aggressive with pricing when introducing new graphics cards, so basically, three things can happen, the HD 6970 can be slower and much cheaper than the GTX 580, about equal and a bit cheaper, or faster and roughly the same price, perhaps a tad more expensive. Whichever of these options turns out to be true, you can bet that NVIDIA will cut the GTX 580's price, so even if you're dead set on the green team, you'd be well inspired to wait a bit.

The HD 6970 will be based on a new GPU called Cayman, which seems to feature completely new Stream Processing Units, as AMD likes to call them. Though rumors had been floating around for a while, most of what we know, we know from Gipsel's posts here. Follow that thread for all the gory details and ISA code galore.

If Gipsel's information does pertain to Cayman—and that is almost certain—then AMD has decided to move from a VLIW5 architecture to a VLIW4 one. Now you might be thinking "wait, 4<5, so how is that better?". First, let's take a look at the current VLIW5 setup, featured on Cypress (HD 5800 series):

As you can see, each VLIW5 unit features 4 small, simple, or "thin" Stream Cores, also called Streaming Processing Units (SPUs) and a complex "fat" one; plus a branch unit. This fat unit handles transcendental instructions, such as exp, log, trigonometry, etc. The problem is that transcendentals are not all that common, and this "fat" unit is… well, fat. It takes a lot of space, and often just sits there doing nothing, except leaking power.

So for Cayman, AMD has apparently removed this unit, and improved the thin ones so that they can handle transcendentals together. In practice, when a complex instruction is encountered, 3 out of the 4 SPUs combine to handle it. Obviously, that means reduced throughput for transcendental-heavy code, but that's OK because first, transcendentals are relatively rare, and second, the resulting VLIW4 unit is significantly smaller than the original VLIW5 one. If it's about 10% smaller (completely arbitrary figure) then you can put about 11% more of them in the same die area (assuming decoupled TMUs). Since your new VLIW4 unit is going to be almost as fast as the VLIW5 one in most cases, the net result is improved performance, and probably improved performance/watt as well.

We'll find out exactly how much of an improvement this is when Cayman is released, around December, 13. If you've been paying attention to rumors, then you've probably heard that it was originally scheduled for November, but AMD has decided to delay it. There has been some speculation as to why, and different theories have been put forth, which I will now discuss.

Some have speculated that AMD was surprised by the GTX 580's performance, and decided to raise clock speeds on Cayman in order to be more competitive. I find that unlikely, because the GTX 580 is "only" about 15% faster than the GTX 480 on average, which is really the least you could expect out of a "new" generation, so it's hard to believe that AMD would be surprised by that. Moreover, Dave Baumann, product manager for Radeon graphics cards, has pretty much dismissed this idea.

Other have said that poor yields—in the single digits!—could be at fault. But this is AMD's third, or arguably even fourth 40nm architecture, and by now, they should really have a good handle on TSMC's admittedly questionable 40nm process. Of course, when you're putting billions of nanoscale transistors on a few mm² of silicon, things can always go wrong, but I really doubt it in this case. Plus, this is not the sort of thing you notice at the last minute.

The most believable theory is, in my opinion, the one about shortages for a new so-called driver-MOSFET made by Texas Instruments and used on the HD 6800 and 6900 series. More here. Indeed, the recent shortages and price hikes for the HD 6800 series seem to support this.

But whatever the reason may be, Cayman should hit the market in mid-December and have a very positive impact, especially for people looking for high-end graphics. Stay tuned.

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