A Look at Altera's OpenCL SDK for FPGAs
by Rahul Garg on October 9, 2013 8:00 AM ESTConclusions: Altera's Offerings and Competitive Landscape
FPGA vendors have long preached about the efficiency of reconfigurable hardware over general purpose processors. However, FPGAs have often been rejected as an option by many due to the programming challenges associated with them. CPUs, and even GPUs, typically offered a much faster time to market and a larger talent pool of programmers. With OpenCL, FPGA vendors can play on an equal footing as far as programming is concerned. Earlier, the decision to use an FPGA over another accelerator required significant resource commitment. OpenCL allows FPGAs to be used as just another option lowering the risk and is potentially a game changer.
On a personal note, I am hoping for cheaper OpenCL capable FPGAs to hit the market. Currently, OpenCL capable FPGAs run into thousands of dollars. This is likely not an issue for the enterprise market typically targeted by FPGA vendors. However, OpenCL on FPGAs has not attracted as much mindshare as GPUs. GPU vendors have a huge advantage that anyone with a cheap laptop can start experimenting with and learning about GPUs. The easy and cheap access to GPUs enabled GPU computing to take off. Whenever computing technology has become cheaper and/or easier to program, it has enabled many creative products around it in fields not thought of by the original technology makers. FPGAs have not yet reached that stage. While there is a community of FPGA enthusiasts, enabling OpenCL on cheaper FPGAs can increase this community many-fold.
Altera's OpenCL offering effectively promises customized hardware for your OpenCL kernels and the claim is that FPGAs will be more efficient than CPUs or GPUs at many tasks. Applications that are not necessarily floating-point heavy, for example applications relying on custom integer datatypes, heavy bit-manipulation or fixed point calculations, are an area where FPGAs can shine because CPU and GPU hardware is not really tailored for such applications. The high-speed I/O connections available on an FPGA with external bandwidth far outstripping other accelerators is another advantage. I think streaming/filtering type of applications are an obvious niche that FPGAs can fulfill. On the other hand, accelerators such as Nvidia Tesla and Xeon Phi will likely continue to do well in many double-precision floating-point applications because these accelerators are heavily optimized for such use cases. Applications such as image processing or data visualization that can make use of dedicated graphics related hardware on GPUs are also best done on a GPU.
Finally, I would say I am cautiously optimistic at the prospect of using OpenCL on FPGAs. I am impressed by the theoretical potential for OpenCL on FPGAs. However, I would like to see third party studies comparing OpenCL SDKs for FPGAs and general purpose processors on various tasks to get a better understanding of performance and power consumption of various accelerator options. If you are evaluating GPUs or Xeon Phi for your application, you should definitely also consider evaluating OpenCL on FPGAs and compare their performance against other options for your application. OpenCL on FPGAs looks to be gaining steam and this will be an interesting space to watch in the near future and may very well be a turning point for wider adoption of FPGAs in various high-performance application segments.
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Atiom - Wednesday, October 9, 2013 - link
Great article. I was thinking about using FPGAs in my projects, with I mainly use microcontrolers, but I still havent done it because of the VHDL language that I havent had the time to learn. But now with the OpenCL, things my get more interesting, just hope these devices get more affordable. It would be nice if you could keep up this kind of articles.Jon Tseng - Wednesday, October 9, 2013 - link
Tx for the piece. Interesting Altera say much the same thing about high performance compute when I speak to them also.Rahul, curious on your thoughts about whether CUDA is a barrier to adoption here. NVIDIA have done a lot driving adoption and supported users. Is this a barrier to switching code to OpenCL? Or are you thinking about FPGA for stuff currently running on x86 or greenfield work?
Todd Thompson - Wednesday, October 9, 2013 - link
Rahul, thanks for this article...you did a great job of messaging the value and use-case for using an FPGA for compute. Please keep up the good work and write more about FPGAs and OpenCL!Todd Thompson - Wednesday, October 9, 2013 - link
As an aside, I'm working on the Zedboard/Zynq/ARM platform to experiment with using FPGA as a co-processor on an SOC. I will be doing some benchmarking by comparing results of b+ tree database indexing with and without Zynq as co-proc. I cannot wait for Xilinx to support OpenCL and overall OpenCL support for less expensive FPGA products.dneto - Wednesday, October 9, 2013 - link
Hi, this is David from Altera. :-)Good article, and thanks for the shout-out.
Regarding the development cycle. One of the great things about a standard like OpenCL is that you can prototype your code on a CPU or a GPU and then port it to the FPGA. You do have to watch that you use a common subset of the features available on all platforms, but this will get you a long way toward a more comfortable development flow. You focus on getting a *working* program on CPU/GPU, and then move to the Altera FPGA to run and optimize. Altera publishes a programming guide to help you optimize for our devices. For OpenCL in general, it is well known that optimizing a kernel for absolute best results often requires recoding or restructuring your device code or data.
Legalese FYI: The official name of our SDK is the "Altera SDK for OpenCL". OpenCL is a trademark of Apple, on license to Khronos.
Araemo - Wednesday, October 9, 2013 - link
I am actually really surprised I see no mention of LLVM in this article. It seems like this is the kind of job that LLVM is well-suited for, based on how many other implementations I've seen of taking one programming language in, and outputting another, more specific language.I wonder if LLVM IS involved, and they just aren't talking about it, or if LLVM isn't actually well-suited to this work, but merely easy to extend to arbitrary languages.
dneto - Wednesday, October 9, 2013 - link
David from Altera here.Yes, LLVM is part of our compiler toolchain. It's one of many technologies, open source and proprietary, used in our SDK.
LLVM is a compiler toolkit, with some finished backends. Using LLVM gets you a long way to supporting an OpenCL C compiler. But it doesn't get you the whole way.
Araemo - Wednesday, October 9, 2013 - link
Thanks for the response - I definitely understand that you still have to write significant portions of it to make it output sensible (and efficient) Verilog, but like you said, LLVM is designed with the kind of modularity that makes swapping output backends to add, say, VHDL support easier, and based on other projects I've seen that were made 'possible' by LLVM, I would have been surprised if you ignored it and rolled your own entirely. :)MrSpadge - Wednesday, October 9, 2013 - link
It could give Altera a huge push if your FPGAs could provide break-through efficiency in any BOINC projects using OpenCL. There are a few, POEM@home, Einstein@home and Collatz@home come to mind, but there are probably more. OpenCL itself is supported by BOINC and currently detects AMD, nVidia and Intel GPUs. But having integrated support for this many coprocessors I'd expect further additions to be smooth.Currently spending a few thousand bucks on hardware just for number crunching would be asking for a lot. Current GPUs only cost hundreds of $/€.. but there are quite a few people out there buying significantly more than 1 of them. So the money is there. And electricity cost is a serious concern: e.g. in Germany you pay approximately as much as the GPU cost each year just to keep it crunching 24/7.
So if Altera can be more efficient than GPUs they could offer cheaper and smaller FPGAs, which might cost 100 - 500 $/€, perform as fast as a GPU (the chip could be smaller for a healthy profit margin, if the algorithm is suitable) and thereby consume significantly less energy.. they'd have a winner!
MrSpadge - Wednesday, October 9, 2013 - link
BTW: if the larger FPGAs could thereby be made cheaper there'd very probably also be a market for them. People are even buying Titans just for BOINC, despite them being significantly worse in cost per performance than smaller nVidias.