RFR: 8265175: (fs) Files.copy(Path, Path, CopyOption...) should use sendfile on Linux [v3]

[Brian Burkhalter]

> Please consider this request to change the underlying native implementation of `java.nio.file.Files.copy(Path,Path,CopyOption...)` on Linux only to perform zero-copy via `sendfile(2)`. The `sendfile()` system call is already used in `java.nio.channels.FIleChannel.transferTo​(long,long,WritableByteChannel)`. It is intentionally _not_ proposed to use `sendfile()` in the native macOS implementation as on macOS the function requires that the destination file descriptor is for a socket.
> 
> This change showed some performance improvement as measured by JMH.
> 
> **Before: user-space buffers (read() + write())**
> 
> Benchmark           (size)   Mode  Cnt      Score     Error  Units
> FilesCopy.copy       10240  thrpt    5  39167.400 ± 683.887  ops/s
> FilesCopy.copy       51200  thrpt    5  20782.622 ± 558.031  ops/s
> FilesCopy.copy      102400  thrpt    5  13260.709 ± 176.673  ops/s
> FilesCopy.copy      512000  thrpt    5   3171.837 ± 175.803  ops/s
> FilesCopy.copy     1048568  thrpt    5   1654.253 ±  39.419  ops/s
> FilesCopy.copy    10485760  thrpt    5    145.328 ±   7.192  ops/s
> FilesCopy.copy   104857600  thrpt    5     12.440 ±   2.275  ops/s
> FilesCopy.copy  1073741824  thrpt    5      1.073 ±   0.081  ops/s
> 
> **After: zero-copy (sendfile())**
> 
> Benchmark           (size)   Mode  Cnt      Score     Error  Units
> FilesCopy.copy       10240  thrpt    5  40571.516 ± 548.977  ops/s
> FilesCopy.copy       51200  thrpt    5  23993.334 ± 506.817  ops/s
> FilesCopy.copy      102400  thrpt    5  15927.485 ± 309.081  ops/s
> FilesCopy.copy      512000  thrpt    5   4207.129 ±  95.454  ops/s
> FilesCopy.copy     1048568  thrpt    5   2147.046 ±  33.446  ops/s
> FilesCopy.copy    10485760  thrpt    5    148.798 ±   1.329  ops/s
> FilesCopy.copy   104857600  thrpt    5     14.541 ±   0.675  ops/s
> FilesCopy.copy  1073741824  thrpt    5      1.270 ±   0.029  ops/s

Brian Burkhalter has updated the pull request incrementally with three additional commits since the last revision:

 - Merge
 - 8265175: Delete fstat() calls, read to EOF, change sendfile() to sendfile64(), handle EINTR case
 - 8265175: (fs) Files.copy(Path,Path,CopyOption...) should use sendfile on Linux

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3476/files
  - new: https://git.openjdk.java.net/jdk/pull/3476/files/b180a4c4..ccdbe5e7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3476&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3476&range=01-02

  Stats: 27 lines in 1 file changed: 4 ins; 16 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3476.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3476/head:pull/3476

PR: https://git.openjdk.java.net/jdk/pull/3476