Hobbits give presents to other people on their own birthdays. Not very expensive ones, as a rule, but it was not a bad system. Actually in Hobbiton and Bywater every day in the year it was somebody's birthday, so that every hobbit in those parts had a fair chance of at least one present at least once a week.
—The Fellowship of the Ring, Book 1, Chapter 1
Linux's CFS strives to be completely fair, meaning that all processes should recieve an appropriately weighted proportion of the processor time. In particular, if there are a total of N processes in the system, then (if they have the same nice values) all processes will ideally recieve exactly 1/Nth of the processor time. In this studio we will explore the CFS from user space.
In this studio, you will:
Please complete the required exercises below, as well as any optional enrichment exercises that you wish to complete. We encourage you to please work in groups of 2 or 3 people on each studio (and the groups are allowed to change from studio to studio) though if you would prefer to complete any studio by yourself that is allowed.
As you work through these exercises, please record your answers, and when finished upload them along with the relevant source code to the appropriate spot on Canvas. If you work in a group with other people, only one of you should please upload the answers (and any other files requested) for the studio, and if you need to resubmit (e.g., if a studio were to be marked incomplete when we grade it) the same person who submitted the studio originally should please do that.
Make sure that the name of each person who worked on these exercises is listed in the first answer, and make sure you number each of your responses so it is easy to match your responses with each exercise.
/proc/sys/kernel/sched_autogroup_enabled from 1
to 0 on your Raspberry Pi. To do that, first log into your Raspberry Pi
and obtain a root terminal (sudo bash) in order to have
permission to modify the value.
Show the current value of that file using the cat command
cat /proc/sys/kernel/sched_autogroup_enabled
then modify the file's value
echo 0 > /proc/sys/kernel/sched_autogroup_enabled
and then show the file's new value
cat /proc/sys/kernel/sched_autogroup_enabled
and then exit the root terminal. As the answer to this exercise
please show the terminal output with the previous and new values of
that file.
while(1) loop, but we want a little control over
where this task executes. To do this, we will use Linux's processor
affinities.
For this exercise, write a infinite-loop program that takes a single integer argument
from the command line, which gives the processor that the program should execute upon.
In order to control where your process is allowed to run, use the function
sched_setaffinity(). You will need to specify
the set of allowable CPUs with a variable of type cpu_set_t. In
order to manipulate this data type you should use the functions documented in
man 3 CPU_SET. This is a nonstandard extension, so your program will
need to #define _GNU_SOURCE before including sched.h.
Hint: Ensure that #define _GNU_SOURCE is included
before any other header files that you include, as well.
Hint: Ensure that you include sched.h rather than linux/sched.h since this is user-space code.
Hint: make sure to use CPU_ZERO to
initialize your cpu_set_t variable, before setting the
processor on which you wish it to run.
Use top
and trace-cmd (or even better,
htop
and kernelshark) to
verify that your program runs continuously on a processor of your
choosing, and as the answer to this exercise please show the output
that confirms this is working correctly.
While those are running, run a few tasks of your own choosing (text editing, internet
browsing, etc.), and notice how well the system responds. Then use Ctrl-C
in each of the background workload windows to stop the task in each one, and see how
well the system runs without the background workload running.
As the answer to this question, describe briefly how well the system responded when the background tasks were running, versus when they were not.
dense_mm program from previous studios
to examine the behavior of CPU-bound tasks on a heavily loaded system. First,
use the command time ./dense_mm 300 to get a rough measure of
program execution time on a quiet system. Now, restart the background tasks
and run the same command again.
As the answer to this exercise, copy and paste the output from each run of
time ./dense_mm 300.
real and user timings. As the answer to
this exercise, answer the following questions: what does the previous exercise tell
you about the way that two CPU-bound tasks share a processors under the default
Linux scheduler (CFS)? What do you predict will happen to the real and
user time of dense_mm 300 if you were to increase the
number of background tasks that were competing with it?
nice priorities can influence the proportion of
time a task recieves.
Run the command time sudo nice -n -20 ./dense_mm 300. As the answer
to this exercise, say what proportion of time (user divided by
real) the task recieved.
The following tips may be helpful, so you don't have to deal with myriad terminal windows. You can detach a command from the terminal by executing it
with an ampersand (&) at the end. Also, you can terminate any task using
its PID (which you can get from top or htop) via the
kill
command. For example, if a task's PID was 2424 you could
use kill -9 2424 to terminate that task.
Once you've set up the competing workloads, execute the command
time ./dense_mm 300 and as the answer to this exercise
show the results and explain briefly whether or not they support your
prediction earlier about what will happen to the real and
user time of dense_mm 300 if you were to increase the
number of background tasks, and why they do or do not.
real and user time of dense_mm 300
as you do that.