You, an engineer newly added to a top-secret project in your organization, have been directed to commission a new imix node for your most important client. The directions you receive are terse, but helpful:
On Sunday, Nov 4, 2018, Director Hines wrote: Welcome to the team, need you to get started right away. The client needs an imix setup with their two apps -- ASAP. Make sure it is working, we need to keep this client happy. - DH
Hmm, ok, not a lot to go on, but luckily in orientation you learned how to flash a kernel and apps on to the imix board, so you are all set for your first assignment.
Poking around, you notice a folder called "important-client". While that is a good start, you also notice that it has two apps inside of it! "Alright!" you are thinking, "My first day is shaping up to go pretty smoothly."
After installing those two apps, which are a little mysterious still, you decide
that it would also be a good idea to install an app you are more familiar with:
the "blink" app. After doing all of that, you run
tockloader list and see the
$ tockloader list No device name specified. Using default "tock" Using "/dev/ttyUSB1 - imix IoT Module - TockOS" [App 0] Name: app2 Enabled: True Sticky: False Total Size in Flash: 16384 bytes [App 1] Name: app1 Enabled: True Sticky: False Total Size in Flash: 8192 bytes [App 2] Name: blink Enabled: True Sticky: False Total Size in Flash: 2048 bytes Finished in 1.959 seconds
Make sure you have these apps installed correctly and
tockloader listproduces similar output as shown here.
Great! Now you check that the LED is blinking, and sure enough, no problems
there. The blink app was just for testing, so you
tockloader uninstall blink
to remove that. So far, so good, Tock! But, before you prepare to head home
after a successful day, you start to wonder if maybe this was a little too easy.
Also, if you get this wrong, it's not going to look good as the new person on
Looking in the folders for the two applications, you notice a brief description of the apps, and a URL. Ok, maybe you can check if everything is working. After trying things for a little bit, everything seems to be in order. You tell the director the board is ready and head home a little early—you did just successfully complete your first project for a major client after all.
Expecting a more challenging project after how well things went yesterday, you are instead greeted by this email:
On Monday, Nov 5, 2018, Director Hines wrote: I know you are new, but what did you do?? I've been getting calls all morning from the client, the imix board you gave them ran out battery already!! Are you sure you set up the board correctly? Fix it, and get it back to me later today. - DH
Well, that's not good. You already removed the blink app, so it can't be that. What you need is some way to inspect the board and see if something looks like it is going awry. You first try:
$ tockloader listen
to see if any debugging information is being printed. A little, but nothing helpful. Before trying to look around the code, you decided to try sending the board a plea for help:
and, surprisingly, it responded! (if not, update the Tock kernel and try again!)
Welcome to the process console. Valid commands are: help status list stop start
Ok! Maybe the process console can help. Try the
Total processes: 2 Active processes: 2 Timeslice expirations: 4277
It seems this tool is actually able to inspect the current system and the active processes! But hmmm, it seems there are a lot of "timeslice expirations". From orientation, you remember that processes are allocated only a certain quantum of time to execute, and if they exceed that the kernel forces a context switch back to the kernel. If that is happening a lot, then the board is likely unable to go to sleep! That could explain why the battery is draining so fast!
But which process is at fault? Perhaps we should try another command.
PID Name Quanta Syscalls Dropped Callbacks State 00 app2 0 336 0 Yielded 01 app1 8556 1439951 0 Running
Ok! Now we have the status of individual applications. And aha! We can clearly
see the faulty application. From our testing we know that one app detects
button presses and one app is transmitting sensor data. Let's see if we can
disable the faulty app somehow and see which data packets we are still getting.
Going back to the help command, the
stop command seems promising:
stop <app name>
After debugging, we now know a couple things about the issue:
- The name of the faulty app.
- That it is functionally correct but is for some reason consuming excess CPU cycles.
Using this information, dig into the the faulty app.
To get the director off your back, you should be able to introduce a simple fix that will reduce wakeups by waiting a bit between samples.
While the quick fix will slow the number of wakeups, you know that you can do better than polling for something like a button press! Tock supports asynchronous operations allowing user processes to subscribe to interrupts.
Looking at the button interface (in button.h), it looks like we'll first have to enable interrupts and then sign up to listen to them.
Once this energy-optimal patch is in place, it'll be time to kick off a triumphant e-mail to the director, and then off to celebrate!