On Mon, 26 May 2025 17:16:02 GMT, Alan Bateman <al...@openjdk.org> wrote:
> A ForkJoinPool can be created with worker threads that clear thread locals > between tasks, thus avoiding a build up of thread locals left behind by tasks > executed in the pool. The common pool does this. Erasing thread locals (by > writing null to Thread.threadLocals) grinds with thread locals that keep > native memory alive, esp. when there isn't a Cleaner or other means to free > the memory. > > For the JDK, this is currently an issue for the NIO direct buffer cache. If a > task running on a thread in the common pool does socket or network channel > I/O then it can leak when the task terminates. Prior to JDK 24 each buffer in > the cache had a cleaner, as if allocated by ByteBuffer.allocateDirect, so it > had some chance of being released. The changes in > [JDK-8344882](https://bugs.openjdk.org/browse/JDK-8344882) mean there is no > longer is cleaner for buffers in the cache. > > The options in the short term are to restore the cleaner, register a clearer > for the buffer cache, have the FJP resetThreadLocals special case > "terminating thread locals", or move the terminating thread locals to a > different TL map. Viktor Klang, Doug Lea, and I discussed this topic recently > and agreed the best short term approach is to split the map. As terminating > thread locals are for platform threads then the map can be in the > Thread.FieldHolder and avoid adding another field to Thread. Medium to long > term require further thought in this area, including seeing what might be > useful (and safe) to expose. > > Testing 1-5. Performance testing ongoing. This pull request has now been integrated. Changeset: ac9af69e Author: Alan Bateman <al...@openjdk.org> URL: https://git.openjdk.org/jdk/commit/ac9af69eee9636ff98c2b60224964e518aebb421 Stats: 149 lines in 9 files changed: 89 ins; 37 del; 23 mod 8357637: Native resources cached in thread locals not released when FJP common pool threads clears thread locals Reviewed-by: vklang ------------- PR: https://git.openjdk.org/jdk/pull/25457
