> # 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.
Jonas Norlinder has updated the pull request incrementally with one additional commit since the last revision: Fixes from review ------------- Changes: - all: https://git.openjdk.org/jdk/pull/28823/files - new: https://git.openjdk.org/jdk/pull/28823/files/efca77c5..458e1c34 Webrevs: - full: https://webrevs.openjdk.org/?repo=jdk&pr=28823&range=01 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28823&range=00-01 Stats: 120 lines in 3 files changed: 60 ins; 56 del; 4 mod Patch: https://git.openjdk.org/jdk/pull/28823.diff Fetch: git fetch https://git.openjdk.org/jdk.git pull/28823/head:pull/28823 PR: https://git.openjdk.org/jdk/pull/28823
