AMD's Graphics Core Next Preview: AMD's New GPU, Architected For Compute
by Ryan Smith on December 21, 2011 9:38 PM ESTNot Just A New Architecture, But New Features Too
So far we’ve talked about Graphics Core Next as a new architecture, how that new architecture works, and what that new architecture does that Cayman and other VLIW architectures could not. But along with the new architecture GCN will bring with it a number of new compute features to further flesh out AMD’s GPU computing capabilities and to cement the GPU’s position as the CPU’s partner rather than a subservient peripheral.
In terms of base features the biggest change will be that GCN will implement the underlying features necessary to support C++ and other advanced languages. As a result GCN will be adding support for pointers, virtual functions, exception support, and even recursion. These underlying features mean that developers will not need to “step down” from higher languages to C to write code for the GPU, allowing them to more easily program for the GPU and CPU within the same application. For end-users the benefit won’t be immediate, but eventually it will allow for more complex and useful programs to be GPU accelerated.
Because the underlying feature set is evolving, the memory subsystem is also evolving to be able to service those features. The chief change here is that the hardware is being adapted to support an ISA that uses unified memory. This goes hand-in-hand with the earlier language features to allow programmers to write code to target both the CPU and the GPU, as programs (or rather compilers) can reference memory anywhere, without the need to explicitly copy memory from one device to the other before working on it. Now there’s still a significant performance impact when accessing off-GPU memory – particularly in the case of dGPUs where on-board memory is many times faster than system memory – so developers and compilers will still be copying data around to keep it close to the processor that’s going to use it, but this essentially becomes abstracted from developers.
Now what’s interesting is that the unified address space that will be used is the x86-64 address space. All instructions sent to a GCN GPU will be relative to the x86-64 address space, at which point the GPU will be responsible for doing address translation to local memory addresses. In fact GCN will even be incorporating an I/O Memory Mapping Unit (IOMMU) to provide this functionality; previously we’ve only seen IOMMUs used for sharing peripherals in a virtual machine environment. GCN will even be able to page fault half-way gracefully by properly stalling until the memory fetch completes. How this will work with the OS remains to be seen though, as the OS needs to be able to address the IOMMU. GCN may not be fully exploitable under Windows 7.
Finally on the memory side, AMD is adding proper ECC support to supplement their existing EDC (Error Detection & Correction) functionality, which is used to ensure the integrity of memory transmissions across the GDDR5 memory bus. Both the SRAM and VRAM memory can be ECC protected. For the SRAM this is a free operation, while for the VRAM there will be a performance overhead. We’re assuming that AMD will be using a virtual ECC scheme like NVIDIA, where ECC data is distributed across VRAM rather than using extra memory chips/controllers.
Elsewhere we’ve already mentioned FP64 support. All GCN GPUs will support FP64 in some form, making FP64 support a standard feature across the entire lineup. The actual FP64 performance is configurable – the architecture supports ½ rate FP64, but ¼ rate and 1/16 rate are also options. We expect AMD to take a page from NVIDIA here and configure lower-end consumer parts to use the slower rates since FP64 is not currently important for consumer uses.
Finally, for programmers some additional hardware changes have been made to improve debug support by allowing debugging tools to tap the GPU at additional points. The new ISA for GCN will already make debugging easier, but this will further that goal. As with other developer features this won’t directly impact end-users, but it will ultimately lead to better software sooner as the features and tools available for debugging GPU programs have been well behind the well-established tools used for debugging CPU programs.
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StormyParis - Friday, June 17, 2011 - link
Thank you for a very enlightening write up. Comments and questions:1- please add a comma in there somewhere. I had to read the sentence 4 times to understand it (page 1=: "VLIW designs will never achieve perfect efficiency in this regard, but the farther off real world utilization is the weaker the benefits of VLIW."
2- When, if ever, will we vile users see any benefits ? I get the feeling that most apps are still not optimized well, if at all, for multicore/threading. Come to think of it, most don't even use most of the x86 extensions more recent than SSE2. Now we're talking of yet another x86 extension, that is not only AMD-specific, but very task-specific. Apart from a handful of labs doing GPU computing, and the usual Photoshop filters... i'm doubtful ?
MonkeyPaw - Friday, June 17, 2011 - link
I'm not an expert in this sort of design, but is AMD setting up this architecture to replace the x86 ALU? Bulldozer is already running 2 ALUs for every 1 FPU, which is promoting ALU-heavy software design. It may take a few revisions to meld them (or phase one out), but it certainly seems like that's a heterogeneous CPU in the end.marc1000 - Friday, June 17, 2011 - link
there is a slide (on Llano article, I believe) where AMD points this. yes, they want to completely merge them, and the ALU would be one of this mergind points.A5 - Friday, June 17, 2011 - link
I think it'll be quite awhile before the monolithic cores dissolve into the heterogeneous architectures, mostly depending on how fine-grained the power gating can get. When it gets to the point where the CPU can selectively turn off components inside a given SIMD unit, I think we'll see someone go "Wait a minute, then why do we even have this big core anymore?" and it'll go away. 2018ish, maybe?jamescox - Monday, June 20, 2011 - link
ALU is generally used to refer to a very simple unit that performs arithmetic, logic, and possibly bit shift operations on integers, not floating point values. The units labeled ALU in the GPU diagrams in the article may support some integer operations, but they mainly process 32-bit floating point values, and (IMO) should not be labeled as "ALUs". FPU would probably be more accurate, but I do not know what operations these units support and whether they include a native integer ALU or just convert to integers to FP.
I don't know what you would mean by ALU-heavy software design. Bulldozer has two integer execution cores per module. Each core is composed of 2 ALUs and 2 AGUs, not shared. It also has 2 128-bit floating point (FMA) units per module shared between the two threads. This isn't really much different than an intel hyper-threaded core. Intel has, I believe, 3 ALUs, 3 AGUs, and 2 FPUs per core which is shared between 2 threads. AMDs version of multi-threading just doesn't share as much hardware between threads, which may be better than Intel's HT (2-2-1 AMD vs 1.5-1.5-1 Intel ALU-AGU-FPU). Intel's version would allow a single thread to us all of the execution resources at once, if there is no competing thread. Sharing the FPU makes a lot of sense, since most code that runs on CPUs only uses the FPU intermittently. If the code uses FP more than intermittently, then it would be a candidate for vectorization, and execution on the GPU instead.
While AMDs next generation graphics hardware may be able to execute more general code compiled from a wider range of languages, it is not an x86 processor, and it can not replace the CPU. If you look at the diagram, it has a single scalar unit to handle non-vector code in each compute unit. It also has 64 units in the 4 vector arrays of each CU. If you actually tried to compile and run the kind of branch heavy, integer code that CPUs have to deal with on a CU, then it would probably run entirely and very, very slowly on that single scalar unit.
MrSpadge - Wednesday, June 22, 2011 - link
I think you've got the right idea with this being melted into a Bulldozer-like design. however, it wouldn't replace the x86 ALUs, which are highly-optimized for high clock speed and low latency execution, as well as excellent handling of branches etc.No, it would rather replace or supplement a fat FPU shared between many "cores" (which, by then would basically mean ALUs + scheduling). Most tasks which requires massive fp number crunching can be executed well in parallel and therefore are suitable for execution on a GPU core. The question is just how to bond them together so that the software guys can actually use them..
MrS
Deleted - Thursday, December 22, 2011 - link
Basically, what we have here is a math coprocessor. Back in the day, Intel's x86 processors were very good (relatively speaking) at integer math, but choked on floating point math. So Intel created the 8087 to handle the floating point calculations while the CPU handled the integer calculations (obviously this wasn't exclusive to Intel, but I'm generalizing). Eventually, the floating point unit was merged onto the CPU, and programs began using them interchangeably.What we have today is very similar. CPUs, even with their advanced FPUs, are nowhere near as powerful as the massively parallel monstrosities we use for graphics. Eventually, they will be merged onto the CPU, and used as readily for general floating point processing tasks as FPUs are currently.
And this is the point of Fusion: to fully replace the aging floating point unit with an IGP.
A5 - Friday, June 17, 2011 - link
The benefits to home or enthusiast users of heterogeneous CPUs are still several years off. We need market penetration of hardware along with fundamental changes in software development models and smarter compilers.nedwards - Tuesday, January 28, 2014 - link
Smarter programmers would help! Let me rephrase that. Programmers thinking in a parallel mindset would help!Beenthere - Friday, June 17, 2011 - link
If AMD delivers in a timely manner they will have a bright future. This looks like a huge technological transition and I understand the need to get developers onboard now but it also tips AMD's hand to Intel who will steal any ideas that they can.Unfortunately we are still waiting for most applications to be written for 64-bit use so I'm not holding out much hope for an expeditious migration on a complex technological transition though it does appear that maybe AMD has been working on this for some time and may be able to do a better job of executing with Trinity and future products. Time will tell but I hope AMD delivers on time and they will definitely get my dime - all of them.