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One of the key design goals of G1 was to make the duration and distribution of stop-the-world pauses due to garbage collection predictable and configurable. In fact, Garbage-First is a soft real-time garbage collector, meaning that you can set specific performance goals to it. You can request the stop-the-world pauses to be no longer than x milliseconds within any given y-millisecond long time range, e.g. no more than 5 milliseconds in any given second. Garbage-First GC will do its best to meet this goal with high probability (but not with certainty, that would be hard real-time).

To achieve this, G1 builds upon a number of insights. First, the heap does not have to be split into contiguous Young and Old generation. Instead, the heap is split into a number (typically about 2048) smaller heap regions that can house objects. Each region may be an Eden region, a Survivor region or an Old region. The logical union of all Eden and Survivor regions is the Young Generation, and all the Old regions put together is the Old Generation:

G1 Heap Regions

This allows the GC to avoid collecting the entire heap at once, and instead approach the problem incrementally: only a subset of the regions, called the collection set will be considered at a time. All the Young regions are collected during each pause, but some Old regions may be included as well:

G1 Collection Set

Another novelty of G1 is that during the concurrent phase it estimates the amount of live data that each region contains. This is used in building the collection set: the regions that contain the most garbage are collected first. Hence the name: garbage-first collection.

To run the JVM with the G1 collector enabled, run your application as

java -XX:+UseG1GC com.mypackages.MyExecutableClass

Evacuation Pause: Fully Young

In the beginning of the application’s lifecycle, G1 does not have any additional information from the not-yet-executed concurrent phases, so it initially functions in the fully-young mode. When the Young Generation fills up, the application threads are stopped, and the live data inside the Young regions is copied to Survivor regions, or any free regions that thereby become Survivor.

The process of copying these is called Evacuation, and it works in pretty much the same way as the other Young collectors we have seen before. The full logs of evacuation pauses are rather large, so, for simplicity’s sake we will leave out a couple of small bits that are irrelevant in the first fully-young evacuation pause. We will get back to them after the concurrent phases are explained in greater detail. In addition, due to the sheer size of the log record, the parallel phase details and “Other” phase details are extracted to separate sections:

0.134: [GC pause (G1 Evacuation Pause) (young), 0.0144119 secs]1
    [Parallel Time: 13.9 ms, GC Workers: 8]2
    [Code Root Fixup: 0.0 ms]4
    [Code Root Purge: 0.0 ms]5
    [Clear CT: 0.1 ms]
    [Other: 0.4 ms]6
    [Eden: 24.0M(24.0M)->0.0B(13.0M) 8Survivors: 0.0B->3072.0K 9Heap: 24.0M(256.0M)->21.9M(256.0M)]10
     [Times: user=0.04 sys=0.04, real=0.02 secs] 11

  1. 0.134: [GC pause (G1 Evacuation Pause) (young), 0.0144119 secs] – G1 pause cleaning only (young) regions. The pause started 134ms after the JVM startup and the duration of the pause was 0.0144 seconds measured in wall clock time.
  2. [Parallel Time: 13.9 ms, GC Workers: 8] – Indicating that for 13.9 ms (real time) the following activities were carried out by 8 threads in parallel
  3. – Cut for brevity, see the following section below for the details.
  4. [Code Root Fixup: 0.0 ms] – Freeing up the data structures used for managing the parallel activities. Should always be near-zero. This is done sequentially.
  5. [Code Root Purge: 0.0 ms] – Cleaning up more data structures, should also be very fast, but non necessarily almost zero. This is done sequentially.
  6. [Other: 0.4 ms] – Miscellaneous other activities, many of which are also parallelized
  7. – See the section below for details
  8. [Eden: 24.0M(24.0M)->0.0B(13.0M) – Eden usage and capacity before and after the pause
  9. Survivors: 0.0B->3072.0K – Space used by Survivor regions before and after the pause
  10. Heap: 24.0M(256.0M)->21.9M(256.0M)] – Total heap usage and capacity before and after the pause.
  11. [Times: user=0.04 sys=0.04, real=0.02 secs] – Duration of the GC event, measured in different categories:
    • user – Total CPU time that was consumed by Garbage Collector threads during this collection
    • sys – Time spent in OS calls or waiting for system event
    • real – Clock time for which your application was stopped. With the parallelizable activities during GC this number is ideally close to (user time + system time) divided by the number of threads used by Garbage Collector. In this particular case 8 threads were used. Note that due to some activities not being parallelizable, it always exceeds the ratio by a certain amount.

Most of the heavy-lifting is done by multiple dedicated GC worker threads. Their activities are described in the following section of the log:

[Parallel Time: 13.9 ms, GC Workers: 8]1
     [GC Worker Start (ms)2: Min: 134.0, Avg: 134.1, Max: 134.1, Diff: 0.1]
    [Ext Root Scanning (ms)3: Min: 0.1, Avg: 0.2, Max: 0.3, Diff: 0.2, Sum: 1.2]
    [Update RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
        [Processed Buffers: Min: 0, Avg: 0.0, Max: 0, Diff: 0, Sum: 0]
    [Scan RS (ms): Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.0]
    [Code Root Scanning (ms)4: Min: 0.0, Avg: 0.0, Max: 0.2, Diff: 0.2, Sum: 0.2]
    [Object Copy (ms)5: Min: 10.8, Avg: 12.1, Max: 12.6, Diff: 1.9, Sum: 96.5]
    [Termination (ms)6: Min: 0.8, Avg: 1.5, Max: 2.8, Diff: 1.9, Sum: 12.2]
        [Termination Attempts7: Min: 173, Avg: 293.2, Max: 362, Diff: 189, Sum: 2346]
    [GC Worker Other (ms)8: Min: 0.0, Avg: 0.0, Max: 0.0, Diff: 0.0, Sum: 0.1]
    GC Worker Total (ms)9: Min: 13.7, Avg: 13.8, Max: 13.8, Diff: 0.1, Sum: 110.2]
    [GC Worker End (ms)10: Min: 147.8, Avg: 147.8, Max: 147.8, Diff: 0.0]

  1. [Parallel Time: 13.9 ms, GC Workers: 8] – Indicating that for 13.9 ms (clock time) the following activities were carried out by 8 threads in parallel
  2. [GC Worker Start (ms) – The moment in time at which the workers started their activity, matching the timestamp at the beginning of the pause. If Min and Max differ a lot, then it may be an indication that too many threads are used or other processes on the machine are stealing CPU time from the garbage collection process inside the JVM
  3. [Ext Root Scanning (ms) – How long it took to scan the external (non-heap) roots such as classloaders, JNI references, JVM system roots, etc. Shows elapsed time, “Sum” is CPU time
  4. [Code Root Scanning (ms) – How long it took to scan the roots that came from the actual code: local vars, etc.
  5. [Object Copy (ms) – How long it took to copy the live objects away from the collected regions.
  6. [Termination (ms) – How long it took for the worker threads to ensure that they can safely stop and that there’s no more work to be done, and then actually terminate
  7. [Termination Attempts – How many attempts worker threads took to try and terminate. An attempt is failed if the worker discovers that there’s in fact more work to be done, and it’s too early to terminate.
  8. [GC Worker Other (ms) – Other miscellaneous small activities that do not deserve a separate section in the logs.
  9. GC Worker Total (ms) – How long the worker threads have worked for in total
  10. [GC Worker End (ms) – The timestamp at which the workers have finished their jobs. Normally they should be roughly equal, otherwise it may be an indication of too many threads hanging around or a noisy neighbor

Additionally, there are some miscellaneous activities that are performed during the Evacuation pause. We will only cover a part of them in this section. The rest will be covered later.

[Other: 0.4 ms]1
    [Choose CSet: 0.0 ms]
    [Ref Proc: 0.2 ms]2
    [Ref Enq: 0.0 ms]3
    [Redirty Cards: 0.1 ms]
    [Humongous Register: 0.0 ms]
    [Humongous Reclaim: 0.0 ms]
    [Free CSet: 0.0 ms]4

  1. [Other: 0.4 ms] – Miscellaneous other activities, many of which are also parallelized
  2. [Ref Proc: 0.2 ms] – The time it took to process non-strong references: clear them or determine that no clearing is needed.
  3. [Ref Enq: 0.0 ms] – The time it took to enqueue the remaining non-strong references to the appropriate ReferenceQueue
  4. [Free CSet: 0.0 ms] – The time it takes to return the freed regions in the collection set so that they are available for new allocations.

Concurrent Marking

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]

Evacuation Pause: Mixed

It’s a pleasant case when concurrent cleanup can free up entire regions in Old Generation, but it may not always be the case. After Concurrent Marking has successfully completed, G1 will schedule a mixed collection that will not only get the garbage away from the young regions, but also throw in a bunch of Old regions to the collection set.

A mixed Evacuation pause does not always immediately follow the end of the concurrent marking phase. There is a number of rules and heuristics that affect this. For instance, if it was possible to free up a large portion of the Old regions concurrently, then there is no need to do it.

There may, therefore, easily be a number of fully-young evacuation pauses between the end of concurrent marking and a mixed evacuation pause.

The exact number of Old regions to be added to the collection set, and the order in which they are added, is also selected based on a number of rules. These include the soft real-time performance goals specified for the application, the liveness and gc efficiency data collected during concurrent marking, and a number of configurable JVM options. The process of a mixed collection is largely the same as we have already reviewed earlier for fully-young gc, but this time we will also cover the subject of remembered sets.

Remembered sets are what allows the independent collection of different heap regions. For instance, when collecting region A,B and C, we have to know whether or not there are references to them from regions D and E to determine their liveness. But traversing the whole heap graph would take quite a while and ruin the whole point of incremental collection, therefore an optimization is employed. Much like we have the Card Table for independently collecting Young regions in other GC algorithms, we have Remembered Sets in G1.

As shown in the illustration below, each region has a remembered set that lists the references pointing to this region from the outside. These references will then be regarded as additional GC roots. Note that objects in Old regions that were determined to be garbage during concurrent marking will be ignored even if there are outside references to them: the referents are also garbage in that case.

Mixed Evacuation Pause: beginning

What happens next is the same as what other collectors do: multiple parallel GC threads figure out what objects are live and which ones are garbage:

Mixed Evacuation Pause: determining live objects

And, finally, the live objects are moved to survivor regions, creating new if necessary. The now empty regions are freed and can be used for storing objects in again.


To maintain the remembered sets, during the runtime of the application, a Post-Write Barrier is issued whenever a write to a field is performed. If the resulting reference is cross-region, i.e. pointing from one region to another, a corresponding entry will appear in the Remembered Set of the target region. To reduce the overhead that the Write Barrier introduces, the process of putting the cards into the Remembered Set is asynchronous and features quite a number of optimizations. But basically it boils down to the Write Barrier putting the dirty card information into a local buffer, and a specialized GC thread picking it up and propagating the information to the remembered set of the referred region.

In the mixed mode, the logs publish certain new interesting aspects when compared to the fully young mode:

[Update RS (ms)1: Min: 0.7, Avg: 0.8, Max: 0.9, Diff: 0.2, Sum: 6.1]
[Processed Buffers2: Min: 0, Avg: 2.2, Max: 5, Diff: 5, Sum: 18]
[Scan RS (ms)3: Min: 0.0, Avg: 0.1, Max: 0.2, Diff: 0.2, Sum: 0.8]
[Clear CT: 0.2 ms]4
[Redirty Cards: 0.1 ms]5

  1. [Update RS (ms) – Since the Remembered Sets are processed concurrently, we have to make sure that the still-buffered cards are processed before the actual collection begins. If this number is high, then the concurrent GC threads are unable to handle the load. It may be, e.g., because of an overwhelming number of incoming field modifications, or insufficient CPU resources.
  2. [Processed Buffers – How many local buffers each worker thread has processed.
  3. [Scan RS (ms) – How long it took to scan the references coming in from remembered sets.
  4. [Clear CT: 0.2 ms] – Time to clean the cards in the card table. Cleaning simply removes the “dirty” status that was put there to signify that a field was updated, to be used for Remembered Sets.
  5. [Redirty Cards: 0.1 ms] – The time it takes to mark the appropriate locations in the card table as dirty. Appropriate locations are defined by the mutations to the heap that GC does itself, e.g. while enqueuing references.


This should give one a sufficient basic understanding of how G1 functions. There are, of course, still quite some implementation details that we have left out for brevity, like dealing with humongous objects. All things considered, G1 is the most technologically advanced production-ready collector available in HotSpot. On top of that, it is being relentlessly improved by the HotSpot Engineers, with new optimizations or features coming in with newer java versions.

As we have seen, G1 addressed a wide range of problems that CMS has, starting from pause predictability and ending with heap fragmentation. Given an application not constrained by CPU utilization, but very sensitive to the latency of individual operations, G1 is very likely to be the best available choice for HotSpot users, especially when running the latest versions of Java. However, these latency improvements do not come for free: throughput overhead of G1 is larger thanks to the additional write barriers and more active background threads. So, if the application is throughput-bound or is consuming 100% of CPU, and does not care as much about individual pause durations, then CMS or even Parallel may be better choices.

The only viable way to select the right GC algorithm and settings is through trial and errors, but we do give the general guidelines in the next chapter.

Note that G1 will probably be the default GC for Java 9: http://openjdk.java.net/jeps/248