Update allocation results #157
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dnadoba
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Sep 6, 2023
simonjbeaumont
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Nov 16, 2023
…tring].joined` (#161) ### Motivation The specialization for `[Substring].joined` was dropped in Swift 5.9[^1] and consequently we regressed in allocations[^2]. ### Modifications Workaround the lack of specialized `[Substring].joined` by handwriting the comma-separated concatenation of the SSH algorithms. ### Result Reduces the allocations to below what they were prior to the toolchain regression. ### Notes As a baseline, I was using `d23c142` of this repo, which is a commit from around the time of the toolchain regression. The allocation results are as follows: | swift-nio-ssh | swift toolchain | allocs | |------------------:|:-----------------|----------:| | d23c142 (Aug 11) | 5.9-2023-07-25 | 1,007,000 | | | 5.9-2023-07-29 | 1,100,000 | | | 5.9-RELEASE | 1,100,000 | | d23c142 + patch | 5.9-2023-07-25 | 989,000 | | | 5.9-2023-07-29 | 989,000 | | | 5.9-RELEASE | 989,000 | | d33c701 (main) | 5.9-RELEASE | 1,060,000 | | main + patch | 5.9-RELEASE | 949,000 | From this we infer: 1. The allocation regression was caused by a regression in Swift 5.9 between nightly versions `2023-07-25` and `2023-07-29`. 2. The same regression is still present in `5.9-RELEASE`. 3. The patch seems to mitigate the regression we're seeing as a result of the new 5.9 behavior. 4. Since the original regression we actually saved some 40k allocations by some other change in the code (either here, or more likely in a dependency). For more information on the Swift issue, including reproducer package and `heaptrack` outputs, see the Swift issue[^1]. [^1]: swiftlang/swift#69883 [^2]: #157
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While we should investigate why these regressed, for now let's unblock development.