fix pancake example & other FIXME

lectures/parallelism/slides.qmd

@@ -113,16 +113,15 @@ module load gcc
 ```
 2. Compile and run the serial example
 ```bash
-gcc main.c -o serial.x -lm
+gcc -O3 main.c -o serial.x -lm
 time ./serial.x   # use `time` to check the runtime
 ```
 3. Compile and run the example using OpenMP
 ```bash
-gcc -fopenmp main.c -o parallel.x -lm
-time OMP_NUM_THREADS=2 ./parallel.x
+gcc -O3 -fopenmp main.c -o parallel.x -lm
+export OMP_NUM_THREADS=2
+time ./parallel.x
 ```
-FIXME: time is often a shell builtin, so output varies and also the place
-where the env-var needs to be set.
 
 4. See next slide!
 
@@ -131,11 +130,12 @@ where the env-var needs to be set.
 4. Now compile/run with
 ```bash
 gcc -fopenmp main.c -o parallel.x -lm -DWRITE_DECOMP
-OMP_NUM_THREADS=4 time ./parallel.x
+export OMP_NUM_THREADS=4 
+time ./parallel.x
 ```
 5. What does the additional output mean?
 
-4. Now uncomment/adapt
+6. Now uncomment/adapt
    * `schedule(static,100)`
    * `schedule(static,10)`
 and interpret the results.
@@ -182,16 +182,16 @@ Images generated by Pradipta Samanta with DALL-E
 ## Strong vs weak scaling
 
 :::{.smaller}
-Starting with batter for $N$ pancakes and 1 pan, we can scale by using $P$ pans in two ways:
+Starting with 1 pan, we can (perfect) scale this by using $P$ pans in two ways:
 :::
 
 :::{.fragment}
 |Parameter / Scaling type |Strong|Weak|
 |-------|---|---|
 |Resources <br> (e.g. pans) | $P$| $P$ |
-|Total workload <br> (e.g. pancake count)| $N$ | $P \times N$ |
+|Total workload <br> (e.g. pancake count)| $N$ | $N \times P$ |
 |Workload per worker <br> (e.g. pancakes per pan) | $N/P$ | $N$ |
-|Total time | $T_1 \times N/P$ | $T_1 \times N$ |
+|Total time | $T_1 \times N/P$ | $T_1 \times N \times P$ |
 :::
 
 
@@ -203,7 +203,7 @@ Starting with batter for $N$ pancakes and 1 pan, we can scale by using $P$ pans
   * Weak scaling
   --> 
 
-# Reductions FIXME title should be more generic
+# Aggregate parallel results
 ## What is happening here?
 ```c++
     int a[] = {2, 4, 6};