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Garbage First Overview

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This chapter is from the book

This chapter is from the book

Garbage First (G1) GC

The G1 garbage collector addresses many of the shortcomings of Parallel, Serial, and CMS GC by taking a somewhat different approach. G1 divides the heap into a set of regions. Most GC operations can then be performed a region at a time rather than on the entire Java heap or an entire generation.

In G1, the young generation is just a set of regions, which means that it is not required to be a consecutive chunk of memory. Similarly, the old generation is also just a set of regions. There is no need to decide at JVM launch time which regions should be part of the old or young generation. In fact, the normal operational state for G1 is that over time the virtual memory mapped to G1 regions moves back and forth between the generations. A G1 region may be designated as young and later, after a young generation collection, become available for use elsewhere, since young generation regions are completely evacuated to unused regions.

In the remainder of this chapter, the term available region is used to identify regions that are unused and available for use by G1. An available region can be used or designated as a young or old generation region. It is possible that after a young generation collection, a young generation region can at some future time be used as an old generation region. Likewise, after collection of an old generation region, it becomes an available region that can at some future time be used as a young generation region.

G1 young collections are parallel stop-the-world collections. As mentioned earlier, parallel stop-the-world collections pause all Java application threads while the garbage collector threads execute, and the GC work is spread across multiple threads. As with the other HotSpot garbage collectors, when a young generation collection occurs, the entire young generation is collected.

Old generation G1 collections are quite different from those of the other HotSpot collectors. G1 old generation collections do not require the entire old generation to be collected in order to free space in the old generation. Instead, only a subset of the old generation regions may be collected at any one time. In addition, this subset of old generation regions is collected in conjunction with a young collection.

Similar to CMS GC, there is a fail-safe to collect and compact the entire old generation in dire situations such as when old generation space is exhausted.

A G1 old generation collection, ignoring the fail-safe type of collection, is a set of phases, some of which are parallel stop-the-world and some of which are parallel concurrent. That is, some phases are multithreaded and stop all application threads, and others are multithreaded and execute at the same time as the application threads. Chapters 2 and 3 provide more detail on each of these phases.

G1 initiates an old generation collection when a Java heap occupancy threshold is exceeded. It is important to note that the heap occupancy threshold in G1 measures the old generation occupancy compared to the entire Java heap. Readers who are familiar with CMS GC remember that CMS initiates an old generation collection using an occupancy threshold applied against the old generation space only. In G1, once the heap occupancy threshold is reached or exceeded, a parallel stop-the-world initial-mark phase is scheduled to execute.

The initial-mark phase executes at the same time as the next young GC. Once the initial-mark phase completes, a concurrent multithreaded marking phase is initiated to mark all live objects in the old generation. When the concurrent marking phase is completed, a parallel stop-the-world remark phase is scheduled to mark any objects that may have been missed due to application threads executing concurrently with the marking phase. At the end of the remark phase, G1 has full marking information on the old generation regions. If there happen to be old generation regions that do not have any live objects in them, they can be reclaimed without any additional GC work during the next phase of the concurrent cycle, the cleanup phase.

Also at the end of the remark phase, G1 can identify an optimal set of old generations to collect.

The regions selected for inclusion in a CSet are based on how much space can be freed and the G1 pause time target. After the CSet has been identified, G1 schedules a GC to collect regions in the CSet during the next several young generation GCs. That is, over the next several young GCs, a portion of the old generation will be collected in addition to the young generation. This is the mixed GC type of garbage collection event mentioned earlier.

With G1, every region that is garbage collected, regardless of whether it is young or old generation, has its live objects evacuated to an available region. Once the live objects have been evacuated, the young and/or old regions that have been collected become available regions.

An attractive outcome of evacuating live objects from old generation regions into available regions is that the evacuated objects end up next to each other in the virtual address space. There is no fragmented empty space between objects. Effectively, G1 does partial compaction of the old generation. Remember that CMS, Parallel, and Serial GC all require a full GC to compact the old generation and that compaction scans the entire old generation.

Since G1 performs GC operations on a per-region basis, it is suitable for large Java heaps. The amount of GC work can be limited to a small set of regions even though the Java heap size may be rather large.

The largest contributors to pause times in G1 are young and mixed collections, so one of the design goals of G1 is to allow the user to set a GC pause time goal. G1 attempts to meet the specified pause time goal through adaptive sizing of the Java heap. It will automatically adjust the size of the young generation and the total Java heap size based on the pause time goal. The lower the pause time goal, the smaller the young generation and the larger the total heap size, making the old generation relatively large.

A G1 design goal is to limit required tuning to setting a maximum Java heap size and specifying a GC pause time target. Otherwise, G1 is designed to dynamically tune itself using internal heuristics. At the time of writing, the heuristics within G1 are where most active HotSpot GC development is taking place. Also as of this writing, G1 may require additional tuning in some cases, but the prerequisites to building good heuristics are present and look promising. For advice on how to tune G1, see Chapter 3.

To summarize, G1 scales better than the other garbage collectors for large Java heaps by splitting the Java heap into regions. G1 deals with Java heap fragmentation with the help of partial compactions, and it does almost all its work in a multithreaded fashion.

As of this writing, G1 primarily targets the use case of large Java heaps with reasonably low pauses, and also those applications that are using CMS GC. There are plans to use G1 to also target the throughput use case, but for applications looking for high throughput that can tolerate longer GC pauses, Parallel GC is currently the better choice.

G1 Design

As mentioned earlier, G1 divides the Java heap into regions. The region size can vary depending on the size of the heap but must be a power of 2 and at least 1MB and at most 32MB. Possible region sizes are therefore 1, 2, 4, 8, 16, and 32MB. All regions are the same size, and their size does not change during execution of the JVM. The region size calculation is based on the average of the initial and maximum Java heap sizes such that there are about 2000 regions for that average heap size. As an example, for a 16GB Java heap with -Xmx16g -Xms16g command-line options, G1 will choose a region size of 16GB/2000 = 8MB.

If the initial and maximum Java heap sizes are far apart or if the heap size is very large, it is possible to have many more than 2000 regions. Similarly, a small heap size may end up with many fewer than 2000 regions.

Each region has an associated remembered set (a collection of the locations that contain pointers into the region, shortened to RSet). The total RSet size is limited but noticeable, so the number of regions has a direct effect on HotSpot’s memory footprint. The total size of the RSets heavily depends on application behavior. At the low end, RSet overhead is around 1 percent and at the high end 20 percent of the heap size.

A particular region is used for only one purpose at a time, but when the region is included in a collection, it will be completely evacuated and released as an available region.

There are several types of regions in G1. Available regions are currently unused. Eden regions constitute the young generation eden space, and survivor regions constitute the young generation survivor space. The set of all eden and survivor regions together is the young generation. The number of eden or survivor regions can change from one GC to the next, between young, mixed, or full GCs. Old generation regions comprise most of the old generation. Finally, humongous regions are considered to be part of the old generation and contain objects whose size is 50 percent or more of a region. Until a JDK 8u40 change, humongous regions were collected as part of the old generation, but in JDK 8u40 certain humongous regions are collected as part of a young collection. There is more detail on humongous regions later in this chapter.

The fact that a region can be used for any purpose means that there is no need to partition the heap into contiguous young and old generation segments. Instead, G1 heuristics estimate how many regions the young generation can consist of and still be collected within a given GC pause time target. As the application starts allocating objects, G1 chooses an available region, designates it as an eden region, and starts handing out memory chunks from it to Java threads. Once the region is full, another unused region is designated an eden region. The process continues until the maximum number of eden regions is reached, at which point a young GC is initiated.

During a young GC, all young regions, eden and survivor, are collected. All live objects in those regions are evacuated to either a new survivor region or to an old generation region. Available regions are tagged as survivor or old generation regions as needed when the current evacuation target region becomes full.

When the occupancy of the old generation space, after a GC, reaches or exceeds the initiating heap occupancy threshold, G1 initiates an old generation collection. The occupancy threshold is controlled by the command-line option -XX:InitiatingHeapOccupancyPercent, which defaults to 45 percent of the Java heap.

G1 can reclaim old generation regions early when the marking phase shows that they contain no live objects. Such regions are added to the available region set. Old regions containing live objects are scheduled to be included in a future mixed collection.

G1 uses multiple concurrent marking threads. In an attempt to avoid stealing too much CPU from application threads, marking threads do their work in bursts. They do as much work as they can fit into a given time slot and then pause for a while, allowing the Java threads to execute instead.

Humongous Objects

G1 deals specially with large object allocations, or what G1 calls “humongous objects.” As mentioned earlier, a humongous object is an object that is 50 percent or more of a region size. That size includes the Java object header. Object header sizes vary between 32- and 64-bit HotSpot VMs. The header size for a given object within a given HotSpot VM can be obtained using the Java Object Layout tool, also known as JOL. As of this writing, the Java Object Layout tool can be found on the Internet [2].

When a humongous object allocation occurs, G1 locates a set of consecutive available regions that together add up to enough memory to contain the humongous object. The first region is tagged as a “humongous start” region and the other regions are marked as “humongous continues” regions. If there are not enough consecutive available regions, G1 will do a full GC to compact the Java heap.

Humongous regions are considered part of the old generation, but they contain only one object. This property allows G1 to eagerly collect a humongous region when the concurrent marking phase detects that it is no longer live. When this happens, all the regions containing the humongous object can be reclaimed at once.

A potential challenge for G1 is that short-lived humongous objects may not be reclaimed until well past the point at which they become unreferenced. JDK 8u40 implemented a method to, in some cases, reclaim a humongous region during a young collection. Avoiding frequent humongous object allocations can be crucial to achieving application performance goals when using G1. The enhancements available in JDK 8u40 help but may not be a solution for all applications having many short-lived humongous objects.

Full Garbage Collections

Full GCs in G1 are implemented using the same algorithm as the Serial GC collector. When a full GC occurs, a full compaction of the entire Java heap is performed. This ensures that the maximum amount of free memory is available to the system. It is important to note that full GCs in G1 are single-threaded and as a result may introduce exceptionally long pause times. Also, G1 is designed such that full GCs are not expected to be necessary. G1 is expected to satisfy application performance goals without requiring a full GC and can usually be tuned such that a full GC is not needed.

Concurrent Cycle

A G1 concurrent cycle includes the activity of several phases: initial marking, concurrent root region scanning, concurrent marking, remarking, and cleanup. The beginning of a concurrent cycle is the initial mark, and the ending phase is cleanup. All these phases are considered part of “marking the live object graph” with the exception of the cleanup phase.

The purpose of the initial-mark phase is to gather all GC roots. Roots are the starting points of the object graphs. To collect root references from application threads, the application threads must be stopped; thus the initial-mark phase is stop-the-world. In G1, the initial marking is done as part of a young GC pause since a young GC must gather all roots anyway.

The marking operation must also scan and follow all references from objects in the survivor regions. This is what the concurrent root region scanning phase does. During this phase all Java threads are allowed to execute, so no application pauses occur. The only limitation is that the scanning must be completed before the next GC is allowed to start. The reason for that is that a new GC will generate a new set of survivor objects that are different from the initial mark’s survivor objects.

Most marking work is done during the concurrent marking phase. Multiple threads cooperate to mark the live object graph. All Java threads are allowed to execute at the same time as the concurrent marking threads, so there is no pause in the application, though an application may experience some throughput reduction.

After concurrent marking is done, another stop-the-world phase is needed to finalize all marking work. This phase is called the “remark phase” and is usually a very short stop-the-world pause.

The final phase of concurrent marking is the cleanup phase. In this phase, regions that were found not to contain any live objects are reclaimed. These regions are not included in a young or mixed GC since they contain no live objects. They are added to the list of available regions.

The marking phases must be completed in order to find out what objects are live so as to make informed decisions about what regions to include in the mixed GCs. Since it is the mixed GCs that are the primary mechanism for freeing up memory in G1, it is important that the marking phase finishes before G1 runs out of available regions. If the marking phase does not finish prior to running out of available regions, G1 will fall back to a full GC to free up memory. This is reliable but slow. Ensuring that the marking phases complete in time to avoid a full GC may require tuning, which is covered in detail in Chapter 3.

Heap Sizing

The Java heap size in G1 is always a multiple of the region size. Except for that limitation, G1 can grow and shrink the heap size dynamically between -Xms and -Xmx just as the other HotSpot GCs do.

G1 may increase the Java heap size for several reasons:

  1. An increase in size can occur based on heap size calculations during a full GC.
  2. When a young or mixed GC occurs, G1 calculates the time spent to perform the GC compared to the time spent executing the Java application. If too much time is spent in GC according to the command-line setting -XX:GCTimeRatio, the Java heap size is increased. The idea behind growing the Java heap size in this situation is to allow GCs to happen less frequently so that the time spent in GC compared to the time spent executing the application is reduced.

    The default value for -XX:GCTimeRatio in G1 is 9. All other HotSpot garbage collectors default to a value of 99. The larger the value for GCTimeRatio, the more aggressive the increase in Java heap size. The other HotSpot collectors are thus more aggressive in their decision to increase Java heap size and by default are targeted to spend less time in GC relative to the time spent executing the application.

  3. If an object allocation fails, even after having done a GC, rather than immediately falling back to doing a full GC, G1 will attempt to increase the heap size to satisfy the object allocation.
  4. If a humongous object allocation fails to find enough consecutive free regions to allocate the object, G1 will try to expand the Java heap to obtain more available regions rather than doing a full GC.
  5. When a GC requests a new region into which to evacuate objects, G1 will prefer to increase the size of the Java heap to obtain a new region rather than failing the GC and falling back to a full GC in an attempt to find an available region.
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