cryptocore: add urandom + randprefetch benchmarks

The benchmark that supported the decision for 512-byte
prefetching previously lived outside the repo.

Let's add it where it belongs so it cannot get lost.

internal/cryptocore/randprefetch.go

@@ -6,20 +6,9 @@ import (
 	"sync"
 )
 
-/*
-Number of bytes to prefetch.
-
-512 looks like a good compromise between throughput and latency:
-Benchmark16-2      	 3000000	       567 ns/op	  28.18 MB/s
-Benchmark64-2      	 5000000	       293 ns/op	  54.51 MB/s
-Benchmark128-2     	10000000	       220 ns/op	  72.48 MB/s
-Benchmark256-2     	10000000	       210 ns/op	  76.17 MB/s
-Benchmark512-2     	10000000	       191 ns/op	  83.75 MB/s
-Benchmark1024-2    	10000000	       171 ns/op	  93.48 MB/s
-Benchmark2048-2    	10000000	       165 ns/op	  96.45 MB/s
-Benchmark4096-2    	10000000	       165 ns/op	  96.58 MB/s
-Benchmark40960-2   	10000000	       147 ns/op	 108.82 MB/s
-*/
+// Number of bytes to prefetch.
+// 512 looks like a good compromise between throughput and latency - see
+// randsize_test.go for numbers.
 const prefetchN = 512
 
 func init() {

internal/cryptocore/randprefetch_test.go

@@ -38,3 +38,11 @@ func TestRandPrefetch(t *testing.T) {
 		t.Errorf("random data should be incompressible, but: in=%d compressed=%d\n", p*l*l, b.Len())
 	}
 }
+
+func BenchmarkRandPrefetch(b *testing.B) {
+	// 16-byte nonces are default since gocryptfs v0.7
+	b.SetBytes(16)
+	for i := 0; i < b.N; i++ {
+		randPrefetcher.read(16)
+	}
+}

internal/cryptocore/randsize_test.go

@@ -0,0 +1,40 @@
+// +build go1.7
+
+// ^^^^^^^^^^^^ we use the "sub-benchmark" feature that was added in Go 1.7
+
+package cryptocore
+
+import (
+	"fmt"
+	"testing"
+)
+
+/*
+The troughput we get from /dev/urandom / getentropy depends a lot on the used
+block size. Results on my Pentium G630 running Linux 4.11:
+
+BenchmarkRandSize/16-2         	 3000000	       571 ns/op	  27.98 MB/s
+BenchmarkRandSize/32-2         	 3000000	       585 ns/op	  54.66 MB/s
+BenchmarkRandSize/64-2         	 2000000	       860 ns/op	  74.36 MB/s
+BenchmarkRandSize/128-2        	 1000000	      1197 ns/op	 106.90 MB/s
+BenchmarkRandSize/256-2        	 1000000	      1867 ns/op	 137.06 MB/s
+BenchmarkRandSize/512-2        	  500000	      3187 ns/op	 160.61 MB/s
+BenchmarkRandSize/1024-2       	  200000	      5888 ns/op	 173.91 MB/s
+BenchmarkRandSize/2048-2       	  100000	     11554 ns/op	 177.25 MB/s
+BenchmarkRandSize/4096-2       	  100000	     22523 ns/op	 181.86 MB/s
+BenchmarkRandSize/8192-2       	   30000	     43111 ns/op	 190.02 MB/s
+
+Results are similar when testing with dd, so this is not due to Go allocation
+overhead: dd if=/dev/urandom bs=16 count=100000 of=/dev/null
+*/
+func BenchmarkUrandomBlocksize(b *testing.B) {
+	for s := 16; s <= 8192; s *= 2 {
+		title := fmt.Sprintf("%d", s)
+		b.Run(title, func(b *testing.B) {
+			b.SetBytes(int64(s))
+			for i := 0; i < b.N; i++ {
+				RandBytes(s)
+			}
+		})
+	}
+}