| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-Id: <20240121153649.2733274-4-visitorckw@gmail.com>
Date: Sun, 21 Jan 2024 23:36:47 +0800
From: Kuan-Wei Chiu <visitorckw@...il.com>
To: colyli@...e.de,
kent.overstreet@...ux.dev
Cc: bfoster@...hat.com,
jserv@...s.ncku.edu.tw,
linux-bcache@...r.kernel.org,
linux-kernel@...r.kernel.org,
linux-bcachefs@...r.kernel.org,
Kuan-Wei Chiu <visitorckw@...il.com>
Subject: [PATCH 3/5] bcachefs: Optimize sort_cmp_size() using bottom-up heapsort
This optimization reduces the average number of comparisons required
from 2*n*log2(n) - 3*n + o(n) to n*log2(n) + 0.37*n + o(n). When n is
sufficiently large, it results in approximately 50% fewer comparisons.
Currently, sort_cmp_size employs the textbook version of heapsort,
where during the heapify process, each level requires two comparisons
to determine the maximum among three elements. In contrast, the
bottom-up heapsort, during heapify, only compares two children at each
level until reaching a leaf node. Then, it backtracks from the leaf
node to find the correct position. Since heapify typically continues
until very close to the leaf node, the standard heapify requires about
2*log2(n) comparisons, while the bottom-up variant only needs log2(n)
comparisons.
Note: This patch depends on patch "bcachefs: Introduce parent function
for sort_cmp_size()".
The experimental data presented below is based on an array generated
by get_random_u32().
| N | comparisons(old) | comparisons(new) | time(old) | time(new) |
|-------|------------------|------------------|-----------|-----------|
| 10000 | 235498 | 136592 | 17954 us | 14834 us |
| 20000 | 510666 | 293254 | 23549 us | 18364 us |
| 30000 | 800461 | 457351 | 33361 us | 19493 us |
| 40000 | 1101317 | 626661 | 33672 us | 26810 us |
| 50000 | 1409743 | 799745 | 42634 us | 34694 us |
| 60000 | 1721165 | 974737 | 51414 us | 41367 us |
| 70000 | 2037972 | 1152111 | 63084 us | 49146 us |
| 80000 | 2362590 | 1333270 | 73802 us | 54715 us |
| 90000 | 2690155 | 1516148 | 82693 us | 63070 us |
| 100000| 3019730 | 1699757 | 88981 us | 70367 us |
Refs:
BOTTOM-UP-HEAPSORT, a new variant of HEAPSORT beating, on an average,
QUICKSORT (if n is not very small)
Ingo Wegener
Theoretical Computer Science, 118(1); Pages 81-98, 13 September 1993
https://doi.org/10.1016/0304-3975(93)90364-Y
Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
---
This patch has undergone unit testing and micro benchmarking using the
following code [1].
[1]:
static long long int cmp_count = 0;
static int mycmp(const void *a, const void *b, size_t size)
{
u32 _a = *(u32 *)a;
u32 _b = *(u32 *)b;
cmp_count++;
if (_a < _b)
return -1;
else if (_a > _b)
return 1;
else
return 0;
}
static int test(void)
{
size_t N, i;
ktime_t start, end;
s64 delta;
u32 *arr;
for (N = 10000; N <= 100000; N += 10000) {
arr = kmalloc_array(N, sizeof(u32), GFP_KERNEL);
cmp_count = 0;
for (i = 0; i < N; i++)
arr[i] = get_random_u32();
start = ktime_get();
sort_cmp_size(arr, N, sizeof(u32), mycmp, NULL);
end = ktime_get();
delta = ktime_us_delta(end, start);
printk(KERN_INFO "time: %lld\n", delta);
printk(KERN_INFO "comparisons: %lld\n", cmp_count);
for (i = 0; i < N - 1; i++) {
if (arr[i] > arr[i+1]) {
kfree(arr);
return -1;
}
}
kfree(arr);
}
return 0;
}
fs/bcachefs/util.c | 52 +++++++++++++++++++++++++++++++---------------
1 file changed, 35 insertions(+), 17 deletions(-)
diff --git a/fs/bcachefs/util.c b/fs/bcachefs/util.c
index f5bbf96df2ce..4dacb2b9a0a5 100644
--- a/fs/bcachefs/util.c
+++ b/fs/bcachefs/util.c
@@ -979,7 +979,8 @@ void sort_cmp_size(void *base, size_t num, size_t size,
void (*swap_func)(void *, void *, size_t size))
{
/* pre-scale counters for performance */
- int i = (num/2 - 1) * size, n = num * size, c, r;
+ int i = (num / 2 - 1) * size, n = num * size, j, k;
+ const size_t lsbit = size & -size;
if (!swap_func) {
if (size == 4 && alignment_ok(base, 4))
@@ -992,28 +993,45 @@ void sort_cmp_size(void *base, size_t num, size_t size,
/* heapify */
for ( ; i >= 0; i -= size) {
- for (r = i; r * 2 + size < n; r = c) {
- c = r * 2 + size;
- if (c < n - size &&
- cmp_func(base + c, base + c + size, size) < 0)
- c += size;
- if (cmp_func(base + r, base + c, size) >= 0)
- break;
- swap_func(base + r, base + c, size);
+ /* Find the sift-down path all the way to the leaves. */
+ for (j = i; k = j * 2 + size, k + size < n;)
+ j = cmp_func(base + k, base + k + size, size) > 0 ? k : k + size;
+
+ /* Special case for the last leaf with no sibling. */
+ if (j * 2 + size * 2 == n)
+ j = j * 2 + size;
+
+ /* Backtrack to the correct location. */
+ while (j != i && cmp_func(base + i, base + j, size) >= 0)
+ j = parent(j, lsbit, size);
+
+ /* Shift the element into its correct place. */
+ for (k = j; j != i;) {
+ j = parent(j, lsbit, size);
+ swap_func(base + j, base + k, size);
}
}
/* sort */
for (i = n - size; i > 0; i -= size) {
swap_func(base, base + i, size);
- for (r = 0; r * 2 + size < i; r = c) {
- c = r * 2 + size;
- if (c < i - size &&
- cmp_func(base + c, base + c + size, size) < 0)
- c += size;
- if (cmp_func(base + r, base + c, size) >= 0)
- break;
- swap_func(base + r, base + c, size);
+
+ /* Find the sift-down path all the way to the leaves. */
+ for (j = 0; k = j * 2 + size, k + size < i;)
+ j = cmp_func(base + k, base + k + size, size) > 0 ? k : k + size;
+
+ /* Special case for the last leaf with no sibling. */
+ if (j * 2 + size * 2 == i)
+ j = j * 2 + size;
+
+ /* Backtrack to the correct location. */
+ while (j && cmp_func(base, base + j, size) >= 0)
+ j = parent(j, lsbit, size);
+
+ /* Shift the element into its correct place. */
+ for (k = j; j;) {
+ j = parent(j, lsbit, size);
+ swap_func(base + j, base + k, size);
}
}
}
--
2.25.1
Powered by blists - more mailing lists