Fixed the pivot selection in the group quick-sort #3800
Merged
Conversation
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
rdhabalia
approved these changes
Feb 23, 2023
Codecov Report
@@ Coverage Diff @@
## master #3800 +/- ##
=============================================
+ Coverage 56.41% 68.44% +12.03%
- Complexity 5531 6754 +1223
=============================================
Files 473 473
Lines 40934 40936 +2
Branches 5235 5235
=============================================
+ Hits 23092 28020 +4928
+ Misses 15744 10665 -5079
- Partials 2098 2251 +153
Flags with carried forward coverage won't be shown. Click here to find out more.
📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more |
hangc0276
approved these changes
Feb 23, 2023
zymap
approved these changes
Feb 23, 2023
|
Test errors: |
horizonzy
reviewed
Feb 23, 2023
| @@ -61,11 +61,13 @@ private void quickSort(long[] array, int low, int high) { | |||
| } | |||
|
|
|||
| private int partition(long[] array, int low, int high) { | |||
| int pivotIdx = high; | |||
| int mid = low + (high - low) / 2; | |||
There was a problem hiding this comment.
Here the mid may not be a multiple of groupSize, that will introduce incorrect swap.
There was a problem hiding this comment.
Ouch.. That's correct. I'll fix it.
zymap
pushed a commit
that referenced
this pull request
Dec 6, 2023
* Fixed the pivot selection in the group quick-sort * Fixed formatting * Fixed alignment (cherry picked from commit 7f263fa)
Ghatage
pushed a commit
to sijie/bookkeeper
that referenced
this pull request
Jul 12, 2024
* Fixed the pivot selection in the group quick-sort * Fixed formatting * Fixed alignment
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Motivation
We have a custom implementation of quick-sort in the
WriteCachethat is used to sort an array of long that are grouped in 4: (ledgerId, entryId, offset, length).Because of the choice of the partition pivot, the sort results in very poor performance where the items are already pre-sorted, or mostly-sorted. Similarly, it happens when they are reverse-sorted. One of the side effects are also stack-overflow when the array is big and pre-sorted.
The problem is that we're always picking the last item in the segment as the pivot, leading to always the worst case partitioning (1, N-1) items.
The simplest solution is to pick the mid item as pivot, which works well for sorted, reverse and random scenarios. Later we can try more sophisticated approaches, such as the ones from
Arrays.sort().Benchmarks
I've compared the different cases with the baseline of
Arrays.sort():Before:
After:
The
preSortedGroupSortgoes from 6.2 to 1978.3 ops/s.