Modified permuteExistingNumbers() to pre-allocate sufficiently large slice for all...

Based on profiling, significant time was spent in growSlice() and memMove() as
a result of the per-number slice append(). Consolidating the allocations resulted
in significant speed and memory usage improvements.

Benchmarking command:
    time ./ch1-breeding-numbers -generations 13 -silent
    go tool pprof ch1-breeding-numbers 13gen-memprofile-gen12.mprof
        top25

Before:
    71.978u 3.726s 0:31.79 238.0%   0+0k 40+75io 353pf+0w
    Showing nodes accounting for 3.70GB, 100% of 3.70GB total

After:
    38.303u 2.233s 0:23.41 173.1%   0+0k 40+62io 354pf+0w
    Showing nodes accounting for 2.29GB, 100% of 2.29GB total

diff --git a/chapter-1-experiments/ch1-breeding-numbers.go b/chapter-1-experiments/ch1-breeding-numbers.go
index 58576cc..fc1c397 100644 (file)
--- a/chapter-1-experiments/ch1-breeding-numbers.go
+++ b/chapter-1-experiments/ch1-breeding-numbers.go
@@ -300,7 +300,11 @@ func main() {
 // right sets. This covers all the reduced form permutations. Any longer
 // permutations will be similar to one of the reduced forms.
 func permuteExistingNumbers(generation int, universe surrealUniverse) []surrealNumber {
 // right sets. This covers all the reduced form permutations. Any longer
 // permutations will be similar to one of the reduced forms.
 func permuteExistingNumbers(generation int, universe surrealUniverse) []surrealNumber {

-    var numbers []surrealNumber
+    // Profiling indicates that most calls to growSlice() and memMove()
+    // originate in this function. Allocating a sufficiently large slice
+    // upfront results in 2x speed improvement and 1/3 reduction in heap usage.
+    numberOfPermutations := ((universe.cardinality()+1) * (universe.cardinality()+1)) - 1
+    numbers := make([]surrealNumber, 0, numberOfPermutations)

 
     // First build permutations with 1 element in each set.
     for i := 0; i < universe.cardinality(); i++ {
 
     // First build permutations with 1 element in each set.
     for i := 0; i < universe.cardinality(); i++ {