How to start with FPGA as "coprocessor"

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I think you are moving in the wrong direction, if you can't solve it with some numerical package like numpy/linpack then it is highly unlikely you will succeed with an FPGA based solution. What you probably want is a fast graphics card + CUDA/OpenCL which will most likely outperform your FPGA based design.

Still it will be an interesting learning exercise ;-)

Hans

Out of curiosity, what is the specific issue you encounter using a 'normal' CPU ?

As you say:

Typically, assuming a constant-time (of duration ta) atom of work and n atoms to process over p cpu, the cpu would take a time t_cpu = ta_cpu * round-up(n/p_cpu)

The only way a FPGA can beat that is if it: a) has a ta_fpga > p_cpu while retaining ta_fpga ~= ta_cpu c) has a ta_fpga > p_cpu (ideal case)

Depending on how much you're willing to spend (big FPGAs aren't cheap), the first question would be, how big can you get 'p_cpu' ? Using MPI to distribute the atoms of work over a lot of cores should not be very difficult, and a 'lot of cores' can be obtained easily from cloud providers nowadays.

FPGAs are not as easy to tryout, today I think it's pretty much Amazon F1 in the cloud - or buying.

That being said, FPGA vendors promote a lot of solutions for this particular problem, from low-level solutions (e.g. a PCIe core and a lot of hand-written Verilog/VHDL/...) to high-level solutions (e.g. , , etc.). Those solutions can be with stand-alone FPGAs or with the FPGA integrated in a SoC with normal cores (e.g. Xilinx Zynq families, among others).

There's also non-vendor solutions, mostly accelerated SoC such as or (extension to ) that can help get started.

Cordially,

--
Romain

Definitely not the right kind of problem.

An FPGA would be quite good, IMHO.

What I would need are things like

- an efficient (base 2) popcount operation

- counters in base 3, 5, 7,11, 13 and 17

- adders for all of the bases above

- efficient popcount operations for all of the bases above

plus handling of numbers in the region of 72 bits.

That is an alternative. I am also looking at that, but FPGAs seem to be more interesting, at the moment.

Certainly.

Therefore, what sort of system should I be looking for? I don't want to spend my whole time writing Linux kernel drivers or Bluetooth communication drivers for the FPGA :-)

So, something that can be interfaced easily with a computer (either on board or with a host computer running Linux) would be great.

It's too slow.

I managed to search the number space up to around 2^64 in around half a CPU year (from which you can tell that one key is to reduce the search space).

There are things that an FPGA should be able to do better than a CPU. One example is implementing a base-n counter, which is a serial operation on a CPU and can easily be done in parallel on an FPGA.

That is of course a possibility. In the CPU-based approach I simply used OpenMP with schedule(dynamic). However, for this kind of hobbyist thing, I'd rather learn something interesting than throw money at a cloud provider :-)

Thanks for the pointers.

Seems to be rather high-level, and also rather abstract (ok, so these systems are usually aimed at professionals, not at hobbyists).

I'll look around a bit and see if I can find anything that helps me, but at the moment, I have to say it all looks rather daunting :-)

OK, I must admit I didn't really look closely at the page you gave but I do know for a lot of numerical intensive calculations a modern PC + Cuda is not easily beaten by an FPGA especially in terms of cost and development time.

This is easy as most processors have a POPCNT instruction so you should be able to find some efficient RTL code on the web. In most cases it is just a bunch of counters/adders.

This I suspect will be more difficult especially if you have to deal with large word length, if not then a LUTs+adders could provide a fast solution.

No idea, perhaps converting to base2 (allowing you to instantiate optimised vendors cores), do all your operations and move back to base

3..17?

That could be a problem as 72bits adders/popcnt will not be fast, you will need to heavily pipeline and optimise your design which adds another level of complexity.

If you looked at Bluetooth I assume the data rate required is not that high. In this case I would go for a simple UART, you can easily get

1Mbits without much effort. No special drivers are required. If you need more bandwidth then have a look at the many Future Technology USB devices like the F232H which are easy to interface and could give you up to 40MByte/sec transfer speeds. The drivers are freely available for Windows and Linux. I have used them on a previous project and they worked without any issue. For anything higher get a PCIe FPGA development board which normally come with drivers to fast DMA a block of data to and from the FPGA.

Good luck, Hans

On 09/05/2021 12:50, Thomas Koenig wrote: ..snip

Ah, I assumed this was some commercial project, in that case go for it, FPGA's are the best solution :-)

Just start small, take one of your required operators, say popcnt, implement it in VHDL/(S)Verilog (or chisel/Python/C/etc) and simulate it. Next get a low cost board from eBay, download the free vendor tools and try to implement it. Depending on the prototype board you can probably use some switches and 7-segment display for I/O.

Good luck,

Regards, Hans.

For boards, there are a number of evaluation boards available for all levels of processing. It might make sense to look for one with a PCIe connector that can be just connected to a PC to be a bit easier to interface, but even a stand alone board, maybe with small embedded processor that just sends answers out the serial port may be simpler.

For ideas of how to build the computation. Thinking a bit, the idea that module-N counters are fairly simple it a good starting point. You actually don't want a 'simple' counter as that says you can't get the parrallism, But building N count by N counters sets (of 7 base-x counters, 2, 3, 5, 7, 11, 13, 17).

Such a counter probably costs 2-3 Luts per bit per base, At your approximately 72 bits numbers, we are talking about 2k luts per computation unit.

For the biggest devices, we could maybe get 1000 of these into a very largest FPGA, and likely could be processing at a few 100 MHz clock rate, so you will be works at a total processing rate in the 100s of Billion tests per second, which should allow you to make a rough estimate of the speed it will process. You may not want to plan on the very largest of FPGAs, as those ARE pricey (the board for the one I looked up for this size was about $16,000).

Hi Thomas.

If I understood you correctly, what you want would be an FPGA engine/coproc

at I have below. That is a pretty neat mathematical problem! I hope that you know a more efficient way of converting any number to a seq uence of digits of a given base than the one I have written. The convertBase() algorithm that I wrote is not exactly FPGA friendly, but it can be managed to put in a FPGA in a efficient way with Dividers and Mul tipliers in pipeline maybe...

My advice is to find some metrics that you want for a first smallish FPGA e ngine/coprocessor (like process 10M numbers per second, using up 2000LUTS,

500FFs, 2BRAMs, 4 mults 18x18). Any FPGA board should be good to start this project, but for a beginner it is better to use some streamline board. The n, it is a matter of replicating that FPGA engine/coprocessor and how much money you can afford in buying a board with big FPGA device or some cloud t ime in some FPGA cloud server. And it is possible that something that you c an put to work at 100MHz in a cheap FPGA board, may run at 400MHz in a very expensive one...

For "convertBase(m, 2); sum1 = SumArray();" you can use a pipelined 'pop count' arquitecture, for the other cases you may use pipelined tree adders (with a small numbers of bits this will be really fast). With pipeline, y ou can execute the section "SumArray();" as if it was being execute it in j ust one clock cycle at 100MHz or 200MHz or even more!

The not so FPGA friendly part is really the "convertBase()" algorithm. That loop with a division (and a multiplication) is a bit troublesome... I hope you know better algorithm to perform this part. I can think in ways of usi ng pipelined dividers... but most likely it is not the most efficient way.. .

Regards, Nelson

#include #include

// Definition of Constants #define C_VALUELIMIT_INIT 2000000000 #define C_VALUELIMIT_FINIT 2010000000 #define C_BASECONVEND 0xFF #define C_DIGITMAXSIZE 256

// Definition of Global Variables uint8_t conv[C_DIGITMAXSIZE];

// Definition of Functions void convertBase(uint64_t n, uint8_t k) { uint64_t l, j; int i = 0; if (n == 0) conv[i++] = '0'; while (n > 0) { l = n / k; j = n - k * l; conv[i] = (uint32_t) j; n = l; i++; } conv[i] = C_BASECONVEND; }

uint32_t SumArray() { uint32_t sum = 0; int i = 0; while (conv[i] != C_BASECONVEND) i++; i--; for (; i >= 0; i--) sum += conv[i]; return sum; }

int main() { uint32_t sum1, sum2; uint64_t m; for (m = C_VALUELIMIT_INIT; m < (uint64_t) C_VALUELIMIT_FINIT; m++) { convertBase(m, 2); sum1 = SumArray(); convertBase(m, 3); sum2 = SumArray(); if (sum1 != sum2) continue ; convertBase(m, 5); sum2 = SumArray(); if (sum1 != sum2) continue ; convertBase(m, 7); sum2 = SumArray(); if (sum1 != sum2) continue ; convertBase(m, 11); sum2 = SumArray(); if (sum1 != sum2) continue ; convertBase(m, 13); sum2 = SumArray(); if (sum1 != sum2) continue ; printf("Sequence number found %lld\n", m); } return 0; }

I would NOT do a convertBase() type archtecture for the FPGA. It is just too unfriendly.

My thought was to build a series of 'Base-X' counters/accumulators, in the bases, 2, 3, 5, 7, 11, 13, 17. This is a fairly simple operation, especially since the increment value will be a constant equal to the number copies of the system. Start with them all at the same value (like

0) and just increment them by the same value expressed in their base.

This becomes an easy one cycle to update system.

Nelson Ribeiro schrieb:

Yep, it's neat. What I did worked for all primes up to 13, but 17 is just too far off (so far).

In the immortal words of Henry S. Warren of "Hacker's Delight" fame: "On many computers, division is very time consuming and is to be avoided when possible."

He also gives a neat bag of tricks of calculating the division remainder of many odd constants, by selectively summing their digits. This works for numbers n where

2 ^ m = 1 (mod n)

so you can break your number into chunks of m bits, add them together and still have the same remainder.

Once you have calculated the remainder by repeated addition of these chunks to a size you can manage, you can then divide by multiplying with the modular inverse of its number.

This will give you a single digit of your base n number, to be repeated until the number has been converted to base n. For base three, 4 = 3+1, so any grouping of bits with an even number works.

I understand most FPGAs have six-bit lookup tables these days. For calculating the remainder base three, that is actually pretty handy - use 12*2 LUTs to reduce the bits from 72 to 24 in one go. Repeat, and you are left with 8 bits, which is definitely managable.

Of course, then comes the 72*72 bit multiplication, which is probably going to take some time...

Base 11 and 13 are less friendly, they would need 10 respectively

12 bit lookup tables.

That is one thing I already looked at. There is a rather elegant popcnt implementation using a 6-bit counter.

Richard Damon schrieb:

That sounds like a good possibility.

There is one important thing: It is possible to reduce the amount of work done rather dramatically, and this is also necessary.

Going to 2^64 with this problem (which I have already done) means looking at around 1.84e19 numbers. Running at 500e6 Hz and doing one test per cycle would lead to 3.7e10 seconds running time, or about 1170 years. Too long.

The serial version of the code consists of nested loops running from

0 to 16. The sum of digits reached so far is easy to calculate, just add the sum of the digits to the new one. The minimum number of digits base 17 then is that sum.

It is then possible to calculate the maximum number of bits that the binary representation in that range can have, and skip the loop if that is too large.

Example: If the sum of the first five leading digits base 17 is 85, there is no way that we will find a number with 72 bits whose popcount is equal to 85.

That has saved a _lot_ of computing effort, at the cost of adding some complexity to the program.

So, any counter will have to have some rather complicated logic to make it skip the values where there cannot possibly be a match.

Unless the problem has something I am overlooking, there is no reason to try to convert an 'aarbirary' number into the various bases.

If you start with the representation of the number X in these bases, it is very simple to compute X+N in all the bases for a fixed number N. By starting with N consecutive numbers precomputed in the bases (like the numbers 1 to N), you would then step through all numbers above that until some base overflows its storage.

No need for big multipliers or dividers, just simple constant incrementers. For example, for the base 17 digits, represented with 5 bits, you just need the current 5 bit, the 1 bit carry in, and 5 CONSTANT increment value, so it is simple lookup for each bit. Maybe to do a bit of work to optimize the carry chain for speed.

I agree with you Richard. I did not thought of that! Its definitely a very efficient way to process like 200M 72-bit numbers per second (assuming 200MHz in a cheap modern FPGA device with some pipelining) with one small FPGA engine/coprocessor.

My guess is that that would be the processing rate for a single core unit, which will take about 2k Luts.

Reasonable cheap FPGAs will likely handle a small multiple of that.

Maybe getting 10s of copies in middle sized but still reasonably priced.

This does assume that you will be just incrementing through the values.

IF you are able to skip large jumps, that might help you with a different algorithm, and perhaps that would be worth it. If it is just occational jumps of large values, perhaps giving up some number of processors to have a unit that can compute the next possible value and factor into the needed bases, and then restart there.

I suppose the big question is how big of gaps do you tend to find, If it can jumps thousands of values, it could well be worth it, and I suppose it well could be. I could see the binary represtation could establish an upper bound for the sum of digits, and if higher order digits of some base exceed that value, you know you need to increment till those change, which could be a very big jump.

Richard Damon schrieb:

You are right, the gaps are indeed huge, and the gains enormous.

If I limit myself to 72 bits, I have around 4.72237E+21 possible binary numbers, but "only" 2.91386E+18 eligible numbers base 17 which have a sum of digits of 72 or less, so this is a reduction by a factor of 1600 alone, more if you look at the actual ranges rather than the maximum as I did above. For base 13, the factor is around 120, for 11 it is 50.

It sounds like this skip will be key to processing, and I suspect that only using the highest base will probably get you enough to be practical, and will allow still good speed.

Build the system with 1 (small FPGA), 17 (medium FPGA), or 17^2 (large FPGA) of these computation cores.

The incrementer rather than being a fixed increment gets the increment to do from logic looking at the sum of the upper digits of the base 17 number of the first unit, and will add a power of 17 to the current numbers in all the bases. You will just precompute the powers of 17 in the bases you are using.

If you start at 0, then the first unit will only roll its upper digits when all the digits below that digit are zero, so we can rapidly skip by just adding repeatedly that power of 17 to the sum.

Yes, we could compute a multiple of that power to add to make that digit roll to 0, but my first guess is that this very likely will cost us more than the at most 16 cycles to wrap it (needing a number of base-k multiplies), so better just punt and just add 17^n repeatedly to do it.

If you have only 1 unit, then you could get more complicated skip logic and let the other bases inject their skips, but you need to be careful about not lettin yourself add too much and go past the roll over point as after a skip you might not be at the right nmultiple of the power of thqt base. The question becomes if it is worth the complexity.

Maybe a bit of an update here.

I have since implemented two algorithms which gave me an enormous speedup on traditional CPUs.

Key to both algorithms is a function which returns the range of the sums of digits base n between integers a and b. For base 2, this is particularly efficient.

One method is a recursive binary search - given a range between a and b, it checks if, for all bases looked at, the ranges of sums of digits intersect. If they don't, return. If they do, partition into two parts and look again for each one.

The second is the skip function you mentioned above. If it is given a base-17 number like, it looks for the next largest number with one more zero digit at the end, like this:

01 03 16 04 03 01 03 16 05 00 01 04 00 00 00 01 04 00 00 00 02 00 00 00 00

(have to watch for carries there) and tests at each stage if the sum of digits base 2 in the range between the original number and the new one is still valid.

This is _extremely_ efficient - at a high number range, this can give skips of 17^10 or so. I alternate this base 17, base 13 and base 11.

This has allowed me to find numbers which have the same number of digits in base 17, 13, 11, 5 and 5 (not 3), like

7172806004621143883825103 (which is larger than 2^82). There are very few of those, and none have so far had the same sum of digits in base 3.

A key to speed is obviously the time in which a large number in binary format can be converted into base n. Is an FPGA the right tool for that?

Well, I personally don't know any algorithm to convert a "large number in b inary format into any base n, with n being a prime number" that would be a good fit for FPGAs.

d at these operations. But that skip method seems to be very promising... but it may need a lot of investigation/exploration/analysis/research from my point of view....But I really can see that the gain in skipping values is really major, I simply cannot think right now in a good "architecture" to implemented it!

What I can show you is where FPGAs shine. I wrote a module in Verilog code that can be synthesizable at 100 MHz (barely!) for a Zynq 7020 when retimin g is used (basically I did not pipelined the design, but used some of the t ools options that tries to do it for me) and that makes use of around 2100 LUTS.

The concept idea for the system would be the following:

There would be an application running in an PC (written in C, C#, Python, w hatever language it would be preferred) that would create jobs to be distri buted to boards with FPGA devices (either through Ethernet, or simply throu gh UART). In a FPGA device it would exist at least one (Soft) processor con nected to many of these modules, to which those jobs are distributed. Thes e jobs would consist in 2 72-bit numbers, one at which the processing would start, another at which the processing would end. (The module requires tha t the Start Number would be converted to each n Base by the (Soft) Processo r before it starts processing).

The description of the module is the following:

For each base (2, 3, 5, 7, 11, 13, 17) there is a counter of that base, wh ich at every and each 1 clock cycle advances one unit. In pipeline and in p arallel with these counters there is a tree of adders ( well for base 2 the popcount module is used) to sum up all the "digits" values of that number for each base. To avoid adders with more than 7 bits, overflow flag is used whenever a sum does not fit in 7 bits. At one point every adder of each ba se n is compared with each other. If none overflowed, and if all have the s ame value then this is a relevant value, and outputs this signal.

The module that I designed is not finished, is a proof of concept, it may h ave bugs, but has been designed to show how to generate sequences A135127, A212222, A335839 and the next sequence of ?Integers whose sum of di gits in base b is the same for every prime b up to 17.? It can be found in:

For the people that will look into the code, sorry for the lack of comments ? just wanted to try out some things. For instance, a counter with

72 bits running at 100MHz is not something that I have done in the past (fo r low end FPGAs), or tree adders with like 20 7-bit adders running at the s ame 100MHz? These would ideally be pipelined by hand, but I got awa y with the tools options, after inserting a few pipeline registers. The 100 MHz objective would be easier to achieve if less bits would be used? ?

One module like this would process 100M numbers per second, and for the A3

35839 sequence would process up to 4294899857375(base 10) in half a day (A m I doing the math correctly? 4294899857375 / 100000000 /3600/ 24 = 0.497 09!)

Don?t know why your target is 72-bits and above, but with less bit s (lets say 64-bits numbers) 100MHz would be better achievable, and in a 15

0? FPGA board, it would fit 12/15 of these modules.

Regards, Nelson