For the umpteenth time, I had to deal with the dreaded Java OutOfMemoryError, this time when trying to run Fisheye Subversion browser. My problem deals with a known issue in Fisheye where a background indexing task sometimes runs causes an OOM error. Of course, an OOM does not just effect the indexing task, it can also impact other operations unrelated to the task with the memory leak. While diagnosing this problem, it got me thinking about how this could be better managed.
The JVM defines a single heap for all objects created across all threads in the java process. A consequence of this is that if one processing thread has a memory leak, any thread on the JVM may start suffering from OOM errors. However, not all threads are equally important to my application. I may not mind too much if a batch processing thread fails with an OOM but I may care very much if the lack of memory causes all of my Tomcat threads to no longer be able to process web requests.
What I would like is to be able to segment my heap so that I can dedicate portions of the heap (either by percentage of the total heap or by absolute number of bytes) to a specific set of work. That way if an OOM occurs, I can contain its impact to a certain set of threads while other threads continue processing. I would see this being configured as JVM runtime -X arguments, something like -XHeapSegment:Name=MyMemHeap,Size=25%,Thread=<ThreadNameRegex>
I could see the partitioning being relative (by percentage) or absolute (total bytes allocated). I could also see linking it to ThreadGroups as opposed to just Threads.
- Do others see value in this proposal?
- I am not an expert at JVM internals. Is there a fundamental reason this would not work?
- Any other suggestions?
- Should we allow this segmentation to be defined at compile-time as well through annotations?
- For the purposes of the problem I defined, I do not have a requirement that the young generations also be segmented. However, if it is impractical to keep a single set of young generations and segmented older generations, I can deal with having segmentation in the young generations as well.
- Obviously, this would in no way address all OutOfMemoryErrors. We can still look forward to running out of PermGenSpace