How FPGAs work, and why you'll buy one (2013)

Re:How FPGAs work, and why you'll buy one (2013) by makomk on Sunday March 13, 2016 @08:18AM (#11277418)

There's an important caveat that this is missing: yes, FPGAs can do a whole bunch of multiply-accumulate operations at once, but each of those units really wants to carry out the same multiply-accumulate operation every clock cycle, with inputs coming from the same sources and outputs going to the same destinations. With DSPs you can trivially execute a completely different step of the computation every clock using the same multiplier, but on FPGAs you either have to set up all your own control logic to multiplex all the DSP units in and out at the right time or have one multiplier per stage in the computation and inevitably lose efficiency because the rate you're feeding data in is slower than the speed the FPGA can run at. And large FPGAs are relatively expensive and, well, big - the bottom-of-the-range Artix 7 with 45 multiply-accumulate slices (enough for one 45-stage computation without complex tricks) is $25 and the smallest package you can get it in is 1cm2.

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Re:How FPGAs work, and why you'll buy one (2013) by jjoonathan on Sunday March 13, 2016 @09:25AM (#11277604)

Not only that, but at all of the price points I've back-of-the-enveloped, GPUs beat FPGAs hands down at processing power. Even if they didn't, the sheer effort required to take advantage of that power would make GPUs more attractive for general purpose compute. Here's the what you generally have to put up with on a FPGA:

* Fixed point.

* Resource starved. Limited # logic blocks, using DRAM from an FPGA is a massive PITA and you pay dearly for every 100kb of SRAM.

* You figure out how to pipeline so that every cell gets used on every cycle.

* HDL languages

* Terribly long compile+route times...

* ...and sometimes routing fails even if your program is well-formed and "ought to" fit on the chip!

* Proprietary toolchains

* Expensive kits (don't let the $20 kits fool you -- you'll spend thousands if you're really using an FPGA for what they're good at)

* Even more expensive test equipment (makes the thousands you spent on the FPGA kit look cheap)

The flipside is that GPUs don't do deterministic latency or talking to hardware. If you need to ship billions of samples per second from your JESD204B ADC back across your PCIe bus (to get them into your GPU, of course) then there's only one chip that fits the bill: the mighty FPGA.

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Re:How FPGAs work, and why you'll buy one (2013) by ginko on Sunday March 13, 2016 @08:36AM (#11277467)

>And large FPGAs are relatively expensive

That's an understatement.

http://www.findchips.com/search/xcvu9p

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Re:How FPGAs work, and why you'll buy one (2013) by aristidb on Sunday March 13, 2016 @10:04AM (#11277713)

Intel and NVidia also sell chips in this price range.

http://ark.intel.com/products/84685/Intel-Xeon-Processor-E7-... http://www.amazon.com/Nvidia-Accelerator-passive-cooling-900...

Now, I guess you can decent performance out of cheaper chips already, but paying $5000 for an Intel CPU is hardly rare.

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Re:How FPGAs work, and why you'll buy one (2013) by b1340276 on Sunday March 13, 2016 @01:54PM (#11278597)

You are ignoring that the 12 core Xeons are still relatively cheap at $2000 which means it's still affordable to just buy four of them and put them into a single server.

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