On Mon, 15 Dec 2025 11:49:19 GMT, Jonas Norlinder <[email protected]> wrote:
> # Background > > When Java applications uses APIs like java.io.FileOutputStream it will hook > into native implementations in e.g. io_util_md.c for Unix/Linux. Java does > not allow reading a directory and the implementation reflect this fact. For > Unix there are three access modes O_RDONLY, O_WRONLY, O_RDWR. Moreover, on > Unix it is possible to read a directory and an extra check has been added in > the code to ensure that the user is trying to read a file (with O_RDONLY) and > not a directory. This extra check results in an additional syscall. > > This check is actually redundant in case user are using access mode O_WRONLY > or O_RDWR. If one is trying to call open on a directory with these modes the > specification in Unix and Linux specifies that EISDIR shall be returned. For > the case of Unix standard it has been part of the standard at least since > 1997 (https://pubs.opengroup.org/onlinepubs/007908799/xsh/open.html) and > Linux since at least 2004 (see v 2.0 > https://www.kernel.org/pub/linux/docs/man-pages/Archive/ ) to return error if > user is trying to write to an directory. In OpenJDK we also include AIX and > they are certified to follow the Unix standard > (https://www.opengroup.org/openbrand/register/ibm.htm). I believe that it is > therefore safe to assume that this is a well implemented aspect of the Unix > standard by now and that this technical debt can be eliminated (assuming that > this check was indeed needed at some point). > > # Performance Improvements > > A stress-test that opens a huge amount of files to trigger a syscall storm > reveals that a removal of this redundant syscall may also improve performance > during high load: > > > JDK 27 baseline > Benchmark Mode Cnt Score Error Units > FileWriteStress.test sample 8438452 3722.451 ± 2.402 ns/op > > JDK 27 patched > Benchmark Mode Cnt Score Error Units > FileWriteStress.test sample 4952304 3191.912 ± 4.011 ns/op > > > ~17% performance boost. Thanks for the review Alan. Pushed fixes. Here is a quick benchmark run with `RandomAccessFile` included. # Baseline Benchmark Mode Cnt Score Error Units OpenFileStress.testFileOutputStream sample 2471437 3812.476 ± 3.962 ns/op OpenFileStress.testRandomAccessFile sample 2468304 3817.273 ± 4.616 ns/op # Patch Benchmark Mode Cnt Score Error Units OpenFileStress.testFileOutputStream sample 1446195 3276.342 ± 7.779 ns/op OpenFileStress.testRandomAccessFile sample 1710040 3315.482 ± 6.024 ns/op (In case anyone asks: The microbenchmark checks for Windows as per discussions with @cl4es where we concluded that if we can make the tests run on any platform that is preferable–even if it really has nothing to do with this particular improvement.) ------------- PR Comment: https://git.openjdk.org/jdk/pull/28823#issuecomment-3711954521
