RFR: 8314569: (fs) Improve normalization of UnixPath for directories [v2]

[Alan Bateman]

On Mon, 21 Aug 2023 10:43:00 GMT, Sergey Tsypanov <stsypanov at openjdk.org> wrote:

>> Avoiding `String.substring()` call in `UnixPath.normalize()` can significantly reduce normalization costs for directories on Linux:
>> 
>> @BenchmarkMode(Mode.AverageTime)
>> @OutputTimeUnit(TimeUnit.NANOSECONDS)
>> @Warmup(time = 2, iterations = 5)
>> @Measurement(time = 2, iterations = 5)
>> @Fork(value = 4, jvmArgs = "-Xmx1g")
>> public class FileToPathBenchmark {
>> 
>>     @Benchmark
>>     public Path toDirectoryPath(Data data) {
>>         return FileSystems.getDefault().getPath(data.directoryPath);
>>     }
>> 
>>     @State(Scope.Thread)
>>     public static class Data {
>>         private String directoryPath = "/tmp/tmp/tmp/";
>>     }
>> }
>> 
>> Results:
>> 
>> baseline
>> 
>> Benchmark                                                             Mode  Cnt     Score     Error   Units
>> FileToPathBenchmark.toDirectoryPath                                   avgt   50    40.524 ±   0.940   ns/op
>> FileToPathBenchmark.toDirectoryPath:·gc.alloc.rate                    avgt   50  3269.167 ± 101.018  MB/sec
>> FileToPathBenchmark.toDirectoryPath:·gc.alloc.rate.norm               avgt   50   175.213 ±   1.200    B/op
>> FileToPathBenchmark.toDirectoryPath:·gc.churn.G1_Eden_Space           avgt   50  3279.401 ± 104.252  MB/sec
>> FileToPathBenchmark.toDirectoryPath:·gc.churn.G1_Eden_Space.norm      avgt   50   175.756 ±   1.681    B/op
>> FileToPathBenchmark.toDirectoryPath:·gc.churn.G1_Survivor_Space       avgt   50     0.006 ±   0.001  MB/sec
>> FileToPathBenchmark.toDirectoryPath:·gc.churn.G1_Survivor_Space.norm  avgt   50    ≈ 10⁻³              B/op
>> FileToPathBenchmark.toDirectoryPath:·gc.count                         avgt   50  1412.000            counts
>> FileToPathBenchmark.toDirectoryPath:·gc.time                          avgt   50  4144.000                ms
>> 
>> patched
>> 
>> Benchmark                                                             Mode  Cnt     Score     Error   Units
>> FileToPathBenchmark.toDirectoryPath                                   avgt   50    33.134 ±   2.313   ns/op
>> FileToPathBenchmark.toDirectoryPath:·gc.alloc.rate                    avgt   50  2652.997 ± 152.758  MB/sec
>> FileToPathBenchmark.toDirectoryPath:·gc.alloc.rate.norm               avgt   50   115.210 ±   1.960    B/op
>> FileToPathBenchmark.toDirectoryPath:·gc.churn.G1_Eden_Space           avgt   50  2654.258 ± 157.862  MB/sec
>> FileToPathBenchmark.toDirectoryPath:·gc.churn.G1_Eden_Space.norm      avgt   50   115.200 ±   1.960    B/op
>> FileToPathBenchmark.toDirectoryPath:·gc.churn.G...
>
> Sergey Tsypanov has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Merge branch 'master' into unixpath-check-nn
>  - Further improvement
>  - Improve UnixPath normalization

src/java.base/unix/classes/sun/nio/fs/UnixPath.java line 96:

> 94:             if (n == 1) return "/";
> 95:             return input.substring(0, n - 1);
> 96:         }

I think the changes in this PR are okay as they don't impact the common case. Style wise I think I'd prefer if L95-L95 were changed to `return (n == 1) ? "/"  : input.substring(0, n - 1);`

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PR Review Comment: https://git.openjdk.org/jdk/pull/15342#discussion_r1306440305