AMD Radeon HD 7970 Review: 28nm And Graphics Core Next, Together As One
by Ryan Smith on December 22, 2011 12:00 AM EST- Posted in
- GPUs
- AMD
- Radeon
- ATI
- Radeon HD 7000
PCI Express 3.0: More Bandwidth For Compute
It may seem like it’s still fairly new, but PCI Express 2 is actually a relatively old addition to motherboards and video cards. AMD first added support for it with the Radeon HD 3870 back in 2008 so it’s been nearly 4 years since video cards made the jump. At the same time PCI Express 3.0 has been in the works for some time now and although it hasn’t been 4 years it feels like it has been much longer. PCIe 3.0 motherboards only finally became available last month with the launch of the Sandy Bridge-E platform and now the first PCIe 3.0 video cards are becoming available with Tahiti.
But at first glance it may not seem like PCIe 3.0 is all that important. Additional PCIe bandwidth has proven to be generally unnecessary when it comes to gaming, as single-GPU cards typically only benefit by a couple percent (if at all) when moving from PCIe 2.1 x8 to x16. There will of course come a time where games need more PCIe bandwidth, but right now PCIe 2.1 x16 (8GB/sec) handles the task with room to spare.
So why is PCIe 3.0 important then? It’s not the games, it’s the computing. GPUs have a great deal of internal memory bandwidth (264GB/sec; more with cache) but shuffling data between the GPU and the CPU is a high latency, heavily bottlenecked process that tops out at 8GB/sec under PCIe 2.1. And since GPUs are still specialized devices that excel at parallel code execution, a lot of workloads exist that will need to constantly move data between the GPU and the CPU to maximize parallel and serial code execution. As it stands today GPUs are really only best suited for workloads that involve sending work to the GPU and keeping it there; heterogeneous computing is a luxury there isn’t bandwidth for.
The long term solution of course is to bring the CPU and the GPU together, which is what Fusion does. CPU/GPU bandwidth just in Llano is over 20GB/sec, and latency is greatly reduced due to the CPU and GPU being on the same die. But this doesn’t preclude the fact that AMD also wants to bring some of these same benefits to discrete GPUs, which is where PCI e 3.0 comes in.
With PCIe 3.0 transport bandwidth is again being doubled, from 500MB/sec per lane bidirectional to 1GB/sec per lane bidirectional, which for an x16 device means doubling the available bandwidth from 8GB/sec to 16GB/sec. This is accomplished by increasing the frequency of the underlying bus itself from 5 GT/sec to 8 GT/sec, while decreasing overhead from 20% (8b/10b encoding) to 1% through the use of a highly efficient 128b/130b encoding scheme. Meanwhile latency doesn’t change – it’s largely a product of physics and physical distances – but merely doubling the bandwidth can greatly improve performance for bandwidth-hungry compute applications.
As with any other specialized change like this the benefit is going to heavily depend on the application being used, however AMD is confident that there are applications that will completely saturate PCIe 3.0 (and thensome), and it’s easy to imagine why.
Even among our limited selection compute benchmarks we found something that directly benefitted from PCIe 3.0. AESEncryptDecrypt, a sample application from AMD’s APP SDK, demonstrates AES encryption performance by running it on square image files. Throwing it a large 8K x 8K image not only creates a lot of work for the GPU, but a lot of PCIe traffic too. In our case simply enabling PCIe 3.0 improved performance by 9%, from 324ms down to 297ms.
Ultimately having more bandwidth is not only going to improve compute performance for AMD, but will give the company a critical edge over NVIDIA for the time being. Kepler will no doubt ship with PCIe 3.0, but that’s months down the line. In the meantime users and organizations with high bandwidth compute workloads have Tahiti.
Facebook
Twitter
RSS
292 Comments
View All Comments
chiddy - Thursday, December 22, 2011 - link
Ryan,Thanks for the great review. My only gripe - and I've been noticing this for a while - is the complete non-mention of drivers or driver releases for Linux/Unix and/or their problems.
For example, Catalyst drivers exhibit graphical corruption when using the latest version (Version 3) of Gnome Desktop Environment since its release before April. This is a major bug which required most users of AMD/ATI GPUs to either switch desktop environments, switch to Nvidia or Intel GPUs, or use the open source drivers which lack many features. A partial fix appeared in Catalyst 11.9 making Gnome3 usable but there are still elements of screen corruption on occassion. (Details in the "non-official" AMD run bugzilla http://ati.cchtml.com/show_bug.cgi?id=99 ).
AMD have numerous other issues with Linux Catalyst drivers including buggy openGL implementation, etc.
Essentially, as a hardware review, a quick once over with non-Microsoft OSs would help alot, especially for products which are marketed as supporting such platforms.
Regards,
kyuu - Thursday, December 22, 2011 - link
Why in the heck would they mention Linux drivers and their issues in an article covering the (paper) release and preliminary benchmarking of AMD's new graphics cards? It has nada to do with the subject at hand.Besides, hardly anyone cares, and those that do care already know.
chiddy - Thursday, December 22, 2011 - link
And I guess that AMD GPUs are sold as "Windows Only"?Thanks for your informative insight.
MrSpadge - Thursday, December 22, 2011 - link
There are no games for *nix and everything always depends on your distribution. The problems are so diverse and numerous.. it would take an entire article to briefly touch this field.Exagerating, but I really wouldn't be interested in endless *nix troubleshooting. Hell, I can't even get nVidia 2D acceleration in CentOS..
chiddy - Thursday, December 22, 2011 - link
You have a valid point on that front and I agree, nor would I expect such an article any time soon.However, on the other hand, one would at the very least expect a GPU using manufacturer released drivers to load a usable desktop. This is an issue that was distro agnostic and instantly noticeable, and only affected AMD hardware, as do most *nix GPU driver issues!
If all that was done during a new GPU review was fire it up in any *nix distribution of choice for just a few minutes (even Ubuntu as I think its the most popular at the moment) to ensure that the basics work it would still be a great help.
I will have to accept though that there is precious little interest!
Ryan Smith - Thursday, December 22, 2011 - link
Hi Chiddy;It's a fair request, so I'll give you a fair answer.
The fact of the matter is that Linux drivers are not a top priority for either NVIDIA or AMD. Neither party makes Linux drivers available for our launch reviews, so I wouldn't be able to test new cards at launch. Not to speak for either company, but how many users are shelling out $550 to run Linux? Cards like the 7970 have a very specifically defined role: Windows gaming video card, and their actions reflect this.
At best we'd be able to look at these issues at some point after the launch when AMD or NVIDIA have added support for the new product to their respective Linux drivers. But that far after the product's launch and for such a small category of users (there just aren't many desktop Linux users these days), I'm not sure it would be worth the effort on our part.
chiddy - Friday, December 23, 2011 - link
Hi Ryan,Thanks very much for taking the time to respond. I fully appreciate your position, particularly as the posts above very much corroborate the lack of interest!
Thanks again for the response, I very much appreciate the hard work yourself and the rest of the AT team are doing, and its quality speaks for itself in the steady increase in readers over the years.
If you do however ever find the time to do a brief piece on *nix GPU support after launch of the next generation nVidia and AMD GPUs that would be wonderful - and even though one would definately not buy a top level GPU for *nix, it would very much help those of us who are dual booting (in my case Windows for gaming / Scientific Linux for work), and somewhat remove the guessing game during purchase time. If not though I fully understand :-).
Regards,
Ali
CeriseCogburn - Thursday, March 8, 2012 - link
Nvidia consistenly wins over and over again in this area, so it's "of no interest", like PhysX...AmdInside - Thursday, December 22, 2011 - link
I won't be getting much sleep tonight since that article took a long time to read (can't imagine how long it must have taken to write up). Great article as usual. While it has some very nice features, all in all, it doesn't make me regret my purchase of a Geforce GTX 580 a couple of months ago. Especially since I mainly picked it up for Battlefield 3.ET - Thursday, December 22, 2011 - link
The Cayman GPU's got quite a performance boost from drivers over time, gaining on NVIDIA's GPU since their launce. The difference in architecture between the 79x0 and 69x0 is higher than the 69x0 and 58x0, so I'm sure there's quite a bit of room for performance improvement in games.Have to say though that I really hope AMD stops increasing the card size each generation.