RFR: 8265448: (zipfs): Reduce read contention in ZipFileSystem [v3]

[Jason Zaugg]

> If the given Path represents a file, use the overload of read defined
> in FileChannel that accepts an explicit position and avoid serializing
> reads.
> 
> Note: The underlying NIO implementation is not required to implement
> FileChannel.read(ByteBuffer, long) concurrently; Windows still appears
> to lock, as it returns true for NativeDispatcher.needsPositionLock.
> 
> 
> On MacOS X, the enclosed benchmark improves from:
> 
> 
> Benchmark                    Mode  Cnt   Score   Error  Units
> ZipFileSystemBenchmark.read  avgt   10  75.311 ? 3.301  ms/op
> 
> 
> To:
> 
> 
> Benchmark                    Mode  Cnt   Score   Error  Units
> ZipFileSystemBenchmark.read  avgt   10  12.520 ? 0.875  ms/op

Jason Zaugg has updated the pull request incrementally with one additional commit since the last revision:

  Access ZipFileSystem.ch via outer pointer as it is now final

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3853/files
  - new: https://git.openjdk.java.net/jdk/pull/3853/files/0859d2d6..bccf5d3c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3853&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3853&range=01-02

  Stats: 15 lines in 1 file changed: 0 ins; 11 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3853.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3853/head:pull/3853

PR: https://git.openjdk.java.net/jdk/pull/3853