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The G1 collector builds up on many concepts of CMS from the previous section, so it is a good idea to make sure that you have a sufficient understanding of it before proceeding. Even though it differs in a number of ways, the goals of the Concurrent Marking are very similar.  G1 Concurrent Marking uses the Snapshot-At-The-Beginning approach that marks all the objects that were live at the beginning of the marking cycle, even if they have turned into garbage meanwhile. The information on which objects are live allows to build up the liveness stats for each region so that the collection set could be efficiently chosen afterwards.

This information is then used to perform garbage collection in the Old regions. It can happen fully concurrently, if the marking determines that a region contains only garbage, or during a stop-the-world evacuation pause for Old regions that contain both garbage and live objects.

Concurrent Marking starts when the overall occupancy of the heap is large enough. By default, it is 45%, but this can be changed by the InitiatingHeapOccupancyPercent JVM option. Like in CMS, Concurrent Marking in G1 consists of a number of phases, some of them fully concurrent, and some of them requiring the application threads to be stopped.

Phase 1: Initial Mark. This phase marks all the objects directly reachable from the GC roots. In CMS, it required a separate stop-the world pause, but in G1 it is typically piggy-backed on an Evacuation Pause, so its overhead is minimal. You can notice this pause in GC logs by the “(initial-mark)” addition in the first line of an Evacuation Pause:

1.631: [GC pause (G1 Evacuation Pause) (young) (initial-mark), 0.0062656 secs]

Phase 2: Root Region Scan. This phase marks all the live objects reachable from the so-called root regions, i.e. the ones that are not empty and that we might end up having to collect in the middle of the marking cycle. Since moving stuff around in the middle of concurrent marking will cause trouble, this phase has to complete before the next evacuation pause starts. If it has to start earlier, it will request an early abort of root region scan, and then wait for it to finish. In the current implementation, the root regions are the survivor regions: they are the bits of Young Generation that will definitely be collected in the next Evacuation Pause.

1.362: [GC concurrent-root-region-scan-start]
1.364: [GC concurrent-root-region-scan-end, 0.0028513 secs]

Phase 3. Concurrent Mark. This phase is very much similar to that of CMS: it simply walks the object graph and marks the visited objects in a special bitmap. To ensure that the semantics of snapshot-at-the beginning are met, G1 GC requires that all the concurrent updates to the object graph made by the application threads leave the previous reference known for marking purposes.

This is achieved by the use of the Pre-Write barriers (not to be confused with Post-Write barriers discussed later and memory barriers that relate to multithreaded programming). Their function is to, whenever you write to a field while G1 Concurrent Marking is active, store the previous referee in the so-called log buffers, to be processed by the concurrent marking threads.

1.364: [GC concurrent-mark-start]
1.645: [GC concurrent-mark-end, 0.2803470 secs]

Phase 4. Remark. This is a stop-the-world pause that, like previously seen in CMS, completes the marking process. For G1, it briefly stops the application threads to stop the inflow of the concurrent update logs and processes the little amount of them that is left over, and marks whatever still-unmarked objects that were live when the concurrent marking cycle was initiated. This phase also performs some additional cleaning, e.g. reference processing (see the Evacuation Pause log) or class unloading.

1.645: [GC remark 1.645: [Finalize Marking, 0.0009461 secs] 1.646: [GC ref-proc, 0.0000417 secs] 1.646: [Unloading, 0.0011301 secs], 0.0074056 secs]
[Times: user=0.01 sys=0.00, real=0.01 secs]

Phase 5. Cleanup. This final phase prepares the ground for the upcoming evacuation phase, counting all the live objects in the heap regions, and sorting these regions by expected GC efficiency. It also performs all the house-keeping activities required to maintain the internal state for the next iteration of concurrent marking.

Last but not least, the regions that contain no live objects at all are reclaimed in this phase. Some parts of this phase are concurrent, such as the empty region reclamation and most of the liveness calculation, but it also requires a short stop-the-world pause to finalize the picture while the application threads are not interfering. The logs for such stop-the-world pauses would be similar to:

1.652: [GC cleanup 1213M->1213M(1885M), 0.0030492 secs]
[Times: user=0.01 sys=0.00, real=0.00 secs]

In case when some heap regions that only contain garbage were discovered, the pause format can look a bit different, similar to:

1.872: [GC cleanup 1357M->173M(1996M), 0.0015664 secs]
[Times: user=0.01 sys=0.00, real=0.01 secs]
1.874: [GC concurrent-cleanup-start]
1.876: [GC concurrent-cleanup-end, 0.0014846 secs]