On Mon, 5 Jan 2026 20:19:43 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. > > Jonas Norlinder has updated the pull request incrementally with one > additional commit since the last revision: > > Fixes from review Marked as reviewed by alanb (Reviewer). ------------- PR Review: https://git.openjdk.org/jdk/pull/28823#pullrequestreview-3629926450
