RFR: 8265175: (fs) Files.copy(Path, Path, CopyOption...) should use sendfile on Linux [v2]

[Brian Burkhalter]

On Wed, 14 Apr 2021 16:14:10 GMT, Brian Burkhalter <bpb at openjdk.org> wrote:

>> 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 refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:
> 
>   8265175: (fs) Files.copy(Path,Path,CopyOption...) should use sendfile on Linux

>From the notes on the man page for `sendfile(2)`:

sendfile() will transfer at most 0x7ffff000 (2,147,479,552)
bytes, returning the number of bytes actually transferred.  (This
is true on both 32-bit and 64-bit systems.)


I verified this behavior on Ubuntu. For files larger than 2147479552, it transfers chunks of size 2147479552 except possibly for the last chunk in which case the number of remaining bytes is returned. At EOF, zero is returned. Thus for such large files the cancel mechanism should work.

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