Saturday 3 October 2015

Its a long way to - Python to C++

Well that took a while... Its been almost three weeks since my last post, and in that time I've been slowly working on replicating the functionality of my 'MotorTest' python script in a C++ program. Some of that time was spent learning about new features and rewriting the code to use it, and some of it was spent with an annoyingly long drawn out cold that meant that some days I just didn't want to be on the computer.

The main reasons I gave for doing this was learning more about C++ and improving the performance of the resulting program. So given that the functionally my robot is exactly the same as it was last time, was it worth the effort?

Overall... yes. Whilst its taken a while to complete I've learnt a fair bit about C++11 and the newer classes and APIs available in it (Even if I didn't end up using them in my current code). A number of pieces of functionality I've used before (such as threads and mutexs) are now built into the standard C++11 libraries, along with a few additions. I've come across a few new libraries and features (using the evdev library for input and eventfd for thread message passing). I've also read up on various articles about thread safety, and hopefully my code is thread safe..

One of the fun things I learned was that the default C++ compiler in Raspbian doesn't actually support C++11... My initial development was on an Ubuntu machine, so I was happily using these C++11 specific features until I had enough working to try running it on the Raspberry Pi... Luckily I'm not the first person who wants to use C++11 in Raspbian and I quickly found instructions to install the C++ compiler from the Jessie repository, which is newer and supports the necessary features.

The Raspberry Pi A+ I have installed on the robot is a little slow at compiling C++, so for the most part I've been developing and testing the code on a Raspberry Pi 2B and only switched over to the  A+ when I had it working. Of course this 'slowness' is one of the reasons I wanted to switch to C++, so how is the performance?

Running the 'MotorTest' python script and monitoring it with 'top' I can see that it uses ~75% CPU time and 10% memory. Running the 'OptimusPi' binary I can see that is uses ~5% CPU time and 1.5% memory, so a significant improvement that leaves me plenty of CPU for processing the input from sensors and other activities.

Speaking of which, some additional sensors did turn up last month, and I've been trying to ignore them until I got my robot moving again. With just a week until the Egham Raspberry Jam I'll have to see how many of them I can get up and running!

New stuff!

Of course there's still plenty of coding to do. I've taken a number of shortcuts to get manual control working (Input device hard coded, the main loop just feeing input into the motors) which I'll need to tidy up and extend to reach my original target, being able to select different autonomous programs to run, auto detecting the input devices etc. The current state of my code can of course be seen by following the 'Github' link on the right.

My original target for this stage.What I actually have!
The current state of my code can of course be seen by following the 'Github' link on the right.


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