+ lib-sort-optimize-heapsort-with-double-pop-variation.patch added to mm-nonmm-unstable branch

+ lib-sort-optimize-heapsort-with-double-pop-variation.patch added to mm-nonmm-unstable branch

[Andrew Morton]

The patch titled
     Subject: lib/sort: Optimize heapsort with double-pop variation
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     lib-sort-optimize-heapsort-with-double-pop-variation.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-sort-optimize-heapsort-with-double-pop-variation.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Kuan-Wei Chiu <visitorckw@gmail.com>
Subject: lib/sort: Optimize heapsort with double-pop variation
Date: Sat, 13 Jan 2024 11:13:52 +0800

Instead of popping only the maximum element from the heap during each
iteration, we now pop the two largest elements at once.  Although this
introduces an additional comparison to determine the second largest
element, it enables a reduction in the height of the tree by one during
the heapify operations starting from root's left/right child.  This
reduction in tree height by one leads to a decrease of one comparison and
one swap.

This optimization results in saving approximately 0.5 * n swaps without
increasing the number of comparisons.  Additionally, the heap size during
heapify is now one less than the original size, offering a chance for
further reduction in comparisons and swaps.

The following experimental data is based on the array generated using
get_random_u32().

| N     | swaps (old) | swaps (new) | comparisons (old) | comparisons (new) |
|-------|-------------|-------------|-------------------|-------------------|
| 1000  | 9054        | 8569        | 10328             | 10320             |
| 2000  | 20137       | 19182       | 22634             | 22587             |
| 3000  | 32062       | 30623       | 35833             | 35752             |
| 4000  | 44274       | 42282       | 49332             | 49306             |
| 5000  | 57195       | 54676       | 63300             | 63294             |
| 6000  | 70205       | 67202       | 77599             | 77557             |
| 7000  | 83276       | 79831       | 92113             | 92032             |
| 8000  | 96630       | 92678       | 106635            | 106617            |
| 9000  | 110349      | 105883      | 121505            | 121404            |
| 10000 | 124165      | 119202      | 136628            | 136617            |


Link: https://lkml.kernel.org/r/20240113031352.2395118-3-visitorckw@gmail.com
Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Cc: George Spelvin <lkml@sdf.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/sort.c |   18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

--- a/lib/sort.c~lib-sort-optimize-heapsort-with-double-pop-variation
+++ a/lib/sort.c
@@ -215,6 +215,7 @@ void sort_r(void *base, size_t num, size
 	/* pre-scale counters for performance */
 	size_t n = num * size, a = (num/2) * size;
 	const unsigned int lsbit = size & -size;  /* Used to find parent */
+	size_t shift = 0;
 
 	if (!a)		/* num < 2 || size == 0 */
 		return;
@@ -242,12 +243,21 @@ void sort_r(void *base, size_t num, size
 	for (;;) {
 		size_t b, c, d;
 
-		if (a)			/* Building heap: sift down --a */
-			a -= size;
-		else if (n -= size)	/* Sorting: Extract root to --n */
+		if (a)			/* Building heap: sift down a */
+			a -= size << shift;
+		else if (n > 3 * size) { /* Sorting: Extract two largest elements */
+			n -= size;
 			do_swap(base, base + n, size, swap_func, priv);
-		else			/* Sort complete */
+			shift = do_cmp(base + size, base + 2 * size, cmp_func, priv) <= 0;
+			a = size << shift;
+			n -= size;
+			do_swap(base + a, base + n, size, swap_func, priv);
+		} else if (n > size) {	/* Sorting: Extract root */
+			n -= size;
+			do_swap(base, base + n, size, swap_func, priv);
+		} else	{		/* Sort complete */
 			break;
+		}
 
 		/*
 		 * Sift element at "a" down into heap.  This is the
_

Patches currently in -mm which might be from visitorckw@gmail.com are

lib-min_heap-optimize-number-of-calls-to-min_heapify.patch
lib-min_heap-optimize-number-of-comparisons-in-min_heapify.patch
lib-sort-optimize-heapsort-for-equal-elements-in-sift-down-path.patch
lib-sort-optimize-heapsort-with-double-pop-variation.patch