[numba.md] numba_ex3: run the solution at the n the exercise asks for

lectures/numba.md

@@ -687,19 +687,20 @@ effort to compute the constant $\pi$ by Monte Carlo.
 
 Now try adding parallelization and see if you get further speed gains.
 
-You should not expect huge gains here because, while there are many
-independent tasks (draw point and test if in circle), each one has low
-execution time.
+There are many independent tasks here (draw a point and test whether it falls
+in the circle), but each one has very low execution time.
 
-Generally speaking, parallelization is less effective when the individual
-tasks to be parallelized are very small relative to total execution time.
+Generally speaking, parallelization is less effective when the individual tasks
+are very small relative to the overheads of spreading them across multiple CPUs.
 
-This is due to overheads associated with spreading all of these small tasks across multiple CPUs.
+The way around this is to give each thread enough work to make those overheads
+worth paying.
 
-Nevertheless, with suitable hardware, it is possible to get nontrivial speed gains in this exercise.
+So, for the size of the Monte Carlo simulation, use something substantial, such
+as `n = 100_000_000`.
 
-For the size of the Monte Carlo simulation, use something substantial, such as
-`n = 100_000_000`.
+At that scale, and with suitable hardware, you should see a clear gain over the
+serial version.
 ```
 
 ```{solution-start} numba_ex3
@@ -723,36 +724,65 @@ def calculate_pi_parallel(u_draws, v_draws):
     return area_estimate * 4  # dividing by radius**2
 ```
 
-Now let's see how fast it runs:
+Parallelization pays off when each thread has enough work to overcome the overhead costs while
+breaking the problem into parts for work to happen simultaneously.
+
+Let's draw a fresh, much larger set of points rather than reusing the arrays from above.
+
+```{note}
+The two arrays below occupy about 1.6 GB of memory — reduce `n` if your
+machine is short on RAM.
+```
+
+```{code-cell} ipython3
+n = 100_000_000
+rng = np.random.default_rng()
+u_big = rng.uniform(size=n)
+v_big = rng.uniform(size=n)
+```
+
+Now let's see how fast it runs (the second call measures runtime without
+compilation time):
 
 ```{code-cell} ipython3
 with qe.Timer():
-    calculate_pi_parallel(u_draws, v_draws)
+    calculate_pi_parallel(u_big, v_big)
 ```
 
 ```{code-cell} ipython3
 with qe.Timer():
-    calculate_pi_parallel(u_draws, v_draws)
+    calculate_pi_parallel(u_big, v_big)
 ```
 
-By switching parallelization on and off (selecting `True` or
-`False` in the `@jit` annotation), we can test the speed gain that
-multithreading provides on top of JIT compilation.
+For comparison, here is the serial jitted version from {ref}`speed_ex1` on the
+same points:
+
+```{code-cell} ipython3
+with qe.Timer():
+    calculate_pi(u_big, v_big)
+```
 
-On our workstation, we find that parallelization provides a modest but
-worthwhile speed gain here.
+Comparing the last two timings, multithreading provides a substantial speed
+gain on top of JIT compilation.
 
 (If you are executing locally, you will get different results, depending mainly
-on the number of CPUs on your machine.)
+on the number of CPUs on your machine — and at small sample sizes the
+parallel version can even be slower, because the gains cannot cover the cost of
+distributing work across threads.)
+
+These two arrays are large and we are finished with them, so we release the
+memory before moving on.
+
+```{code-cell} ipython3
+del u_big, v_big
+```
 
 Notice that we drew all of the random points *before* the loop and passed them in
 as arrays, so the parallel loop only *reads* from memory.
 
 Drawing the points *inside* the parallel loop instead is surprisingly delicate.
 
-
-We investigate why, and how to do it safely, in
-{ref}`numba_ex_race`.
+We investigate why, and how to do it safely, in {ref}`numba_ex_race`.
 
 ```{solution-end}
 ```