updated readmes for exercises

exercises/exerciseA/README.md

@@ -30,7 +30,8 @@ In `plot.py`, we have given code that will load all of the timing data in `timin
 
 **TASK**: Add code to plot of the execution duration vs. the number of threads
 
-Open a PR with your plotting code and post your plots in the conversation, don't upload binaries to the Git remote!
+**TASK**: Open a Pull Request with your plotting code and post your plots in the
+conversation. Don't upload binaries to the Git remote!
 
 **OPTIONAL TASK**: Add code to calculate and plot the speed-up time compared
 to single-threaded execution. Include your code and plot in the PR.

exercises/exerciseC/README.md

@@ -0,0 +1,38 @@
+# Exercise C: blending processes and threads
+
+Objective: investigate how the number of processes and threads impacts the
+speed-up time of a computation.
+
+## First
+
+For each of the 19 images in the folder `images/`, the `process_images.py`:
+(1) decomposes the image using a singular-value decomposition (SVD), (2) removes the
+largest singular value and (3) returns the reconstructed image. The script also measures
+the time for the computation and saves the result in `timings/`.
+
+You can change the number of processes and threads on a set of images by calling the function
+as follows:  
+```python process_images.py 3 2 images/*```
+The code above will use 3 processes and 2 threads to analyse everything in the folder `images/`.
+
+**TASKS**:  
+ 0. Familiarize yourself with the code in `process_images.py`. Where is the number of
+    threads set in the code? Why is it set there? Where is the number of processes set
+    in the code?  
+ 1. Hypothesize what would be a good number of processes and threads for this exercise. 
+ 2. Try a couple combinations of processes and threads, look at the saved timings, and see if
+    the results match your expectations.
+
+## Second
+
+This folder also includes a bash script called `run_with_all_configurations.sh`.
+
+**TASKS**:  
+ 0. Open the bash script. What does it do?  
+ 1. Execute the bash script in the terminal:  
+    `bash run_with_all_configurations.sh`  
+    Observe what's printed to screen. Does it match your expectations?
+ 2. Open `plot.py` and see what it does. Run the script and view the results. Do they
+    match your expectations?
+ 3. Add the image as a comment to the Pull Request you opened in Exercise A (or make a
+    new Pull Request if you need one).

exercises/exerciseC/process_images.py

@@ -45,7 +45,7 @@ if __name__ == '__main__':
     # we need to set the variable before numpy is imported!
     os.environ['OMP_NUM_THREADS'] = str(n_threads)
 
-    # We delay the import of numpy because we want to set OMP_NUM_THREADS.
+    # We delay the import of numpy because we have to set OMP_NUM_THREADS before import.
     # We delay the import of PIL in case it uses numpy internally.
     import numpy as np
     from PIL import Image