Introduction to G1
G1 (Garbage-First) is a new generation of garbage collector. Its core idea is to divide the heap memory into multiple regions (Regions) of equal size. Each region can be Eden, Survivor or Old. Some new algorithms for efficient garbage collection.
G1's Garbage Collection Mechanism
- Initial mark phase (Initial Mark): Mark all surviving objects, and record which objects and which Regions in the old generation need to be recycled.
- Root Region Scanning: Scan all root regions and mark all objects referenced by root regions.
- Concurrent Mark phase (Concurrent Mark): Starting from all objects marked in 2, use the "concurrent mark" method to traverse the entire heap and mark all surviving objects. Entire processes and applications are executed concurrently without blocking the main process.
- Remark phase (Remark): When the application is suspended, remark all objects associated with the marked objects in the old generation, and update the mark.
- Cleanup phase (Cleanup): According to the marking results, reclaim all the memory in the Region marked as garbage, and move the living objects to the new Region at the same time to reduce memory fragmentation.
Summarize
The core idea of G1 is to divide the heap memory into multiple small areas, and only process a part of them at a time. At the same time, it can optimize the problem of memory organization and fragmentation, so that it can handle large heap memory more efficiently. At the same time, G1 also adds some means in the implementation process, such as concurrent marking and mixed garbage collection mechanism, to improve efficiency and performance.