A good question to ask yourself before using the Raspberry Pi GPIO pins for time sensitive things is exactly how fast can I read and write to these pins? Writing from, or sending signals, from the GPIO pins has already been investigated, but the other way around has been more obscure. Here I answer the question and find some interesting, unexpected results!
All the following tests were done as root in a Raspberry Pi Model B+ using a Jessie build of Raspbian (Raspberry Pi Model 2 is generally 2-3 times faster). To calculate result rates, the commands were run using time (e.g.
time program) and the
real time result was used to divide into the total number of cycles or iterations to figure out the resulting rate.
How fast can you write to GPIO pins for outputing a signal?
I decided to test the two fastest languages - Python and Native C - to see how fast it could be done. Also, I included Go in the experiments, which is a language that has not been tested for generating square waves yet. Here are the results.
|Language (linked to code)||Tested||Result (cycle rate)|
My results for Python and C are similar to what was already investigated, but the Go result is new! The Go result is pretty unexpected - it seems to be the fastest language for writing to pins with the exception of C. I think this is thanks to a great library by Stian Eikeland.
How fast can you read from GPIO pins to memory?
Another question is how fast can you read? In this case I want to read a pin many times and store the value in virtual memory so there is no hard-disk storage here (which may be a bottleneck). I’m going to focus on the two fastest languages - Go and native C.
Is reading harder or easier than writing for a given language? For C, reading actually seems to be harder, as it reads about half the rate it can write a cycle - so detecting pin changes is 4x slower than writing pin changes. However, Go can read faster than it can write! That’s pretty unexpected. Go seems like a good alternative to C for reading GPIO pins.
These read speeds are effectively the fastest you can possibly read. These programs read into memory and do nothing with the result. In a practical case you’d want to use the result - most likely by writing it to disk, thus you’ll have a bottleneck there immediately. This brings me to the final question:
How fast can you read from GPIO pins to disk?
This experiment is similar to the previous, except that here I use Go and C to compile the individual bits into bytes which are written to disk as a block every 1000 Bytes. The results are surprising:
|Language (linked to code)||Tested||Result (read + write-to-disk rate)|
Here was the real surprise - Go creeps up on C in terms of speed! Go seems to be optimized for both reading the GPIO pin as well as continuous writing to disk. Given the ease of coding, it seems like the Go may be the best way to go for reading GPIO pins.
Use Go for reading GPIO pins because its about as fast as C for practical uses and a bit easier to code in, but use C if you want to write to pins (max cycle rate 15 MHz) or if you just want to keep things in memory.