contentenc: encryptBlocksParallel: explain why last part runs in new goroutine

The result is counter-intuitive, so explain it here.

1 files changed, 5 insertions, 1 deletions

diff --git a/internal/contentenc/content.go b/internal/contentenc/content.go
index 81864d4..697026a 100644
--- a/internal/contentenc/content.go
+++ b/internal/contentenc/content.go
@@ -219,7 +219,11 @@ func (be *ContentEnc) encryptBlocksParallel(plaintextBlocks [][]byte, ciphertext
 			low := i * groupSize
 			high := (i + 1) * groupSize
 			if i == ncpu-1 {
-				// Last group, pick up any left-over blocks
+				// Last part picks up any left-over blocks
+				//
+				// The last part could run in the original goroutine, but
+				// doing that complicates the code, and, surprisingly,
+				// incurs a 1 % performance penalty.
 				high = len(plaintextBlocks)
 			}
 			be.doEncryptBlocks(plaintextBlocks[low:high], ciphertextBlocks[low:high], firstBlockNo+uint64(low), fileID)