Memory management is the process of allocating objects and removing the unused objects so that memory can be allocated for new objects.
Heap and Nursery
Heap is created when JVM starts up and may increase or decrease in size during application runtime. Java objects reside in the heap area. Garbage collection is run when heap area becomes full in size. Objects that are no longer in use get cleared to make space free for new objects allocation.
JVM uses more memory than just heap, such as, Java methods, thread stacks and native handles are allocated in memory separate from the heap area, as well as JVM internal data structures.
You may also read Understanding Run-Time Data Areas in JVM
The heap is sometimes divided into two areas (or generations) called the nursery (or young space) and the old space (tenured).
The young generation is divided further three segments – eden, survivor1 and survivor2.
Eden space – the pool from which memory is initially allocated for most objects.
Survivor spaces – the pool containing objects that have survived the garbage collection of the Eden space. Actually objects move from Eden -> Survivor1 -> Survivor2.
Tenured space – the pool containing objects that have exist for some time in the survivor spaces.
The nursery is a part of the heap reserved for allocation of new objects. When the nursery becomes full, garbage is collected by running a special young collection, where all objects that have lived long enough in the nursery are promoted (moved) to the old or tenured space, thus freeing up the nursery for more object allocation. When the old space becomes full, garbage is collected there, a process called an old collection.
Most objects in nursery are temporary and short lived. A nursery or young collection is designed to be swift at finding newly allocated objects that are still alive and moving them away from the nursery to old or tenured space. Typically, a young collection frees a given amount of memory much faster than an old collection or a garbage collection of a single-generational heap (a heap without a nursery).
In heap, a part of the nursery is reserved as a keep area. The keep area contains the most recently allocated objects in the nursery and is not garbage collected until the next young collection. This prevents objects from being promoted just because they were allocated right before a young collection started.
Permanent Generation – This is the memory pool as the name suggests contains permanent class metadata and descriptors information. Therefore, the PermGen (Permanent Generation) space always reserved for classes and those that are tied to the classes, for example, static members.
In Java 8, PermGen is replaced by Metaspace, which is very similar to PermGen but the main difference is that Metaspace re-sizes dynamically, i.e., it can expand at runtime.
Java Metaspace – unbounded (by default). So PermSize & MaxPermSize parameters will be ignored in Java 8.
Code Cache (Virtual or reserved) – If you are using HotSpot Java VM this includes code cache area that contains memory which will be used for compilation and storage of native code.
During object allocation, JVM distinguishes between small and large objects. The limit for when an object is considered large depends on the JVM version, the heap size, the garbage collection strategy and the platform used, but is usually somewhere between 2 and 128 kB.
Small objects are allocated in thread local areas (TLAs). The thread local areas are free chunks reserved from the heap and given to a Java thread for exclusive use. The thread can then allocate objects in its TLA without synchronizing with other threads. When the TLA becomes full, the thread simply requests a new TLA. The TLAs are reserved from the nursery if such exists, otherwise they are reserved anywhere in the heap.
Large objects that don’t fit inside a TLA are allocated directly on the heap. When a nursery is used, the large objects are allocated directly in old space. Allocation of large objects requires more synchronization between the Java threads, although the JVM uses a system of caches of free chunks of different sizes to reduce the need for synchronization and improve the allocation speed.
Garbage collection is the process of freeing space in the heap or the nursery for allocation of new objects.
The Mark and Sweep Model
The JVM uses the mark and sweep garbage collection model for performing garbage collections of the whole heap. A mark and sweep garbage collection consists of two phases, the mark phase and the sweep phase.
During the mark phase all objects that are reachable from Java threads, native handles and other root sources are marked as alive, as well as the objects that are reachable from these objects and so forth. This process identifies and marks all objects that are still used, and the rest can be considered garbage.
During the sweep phase the heap is traversed to find the gaps between the live objects. These gaps are recorded in a free list and are made available for new object allocation.
The JVM uses two improved versions of the mark and sweep model. One is mostly concurrent mark and sweep and the other is parallel mark and sweep.
Mostly Concurrent Mark and Sweep
The mostly concurrent mark and sweep strategy (often simply called concurrent garbage collection) allows the Java threads to continue running during large portions of the garbage collection. The threads must however be stopped a few times for synchronization.
The mostly concurrent mark phase is divided into four parts:
Initial marking, where the root set of live objects is identified. This is done while the Java threads are paused.
Concurrent marking, where the references from the root set are followed in order to find and mark the rest of the live objects in the heap. This is done while the Java threads are running.
Precleaning, where changes in the heap during the concurrent mark phase are identified and any additional live objects are found and marked. This is done while the Java threads are running.
Final marking, where changes during the precleaning phase are identified and any additional live objects are found and marked. This is done while the Java threads are paused.
The mostly concurrent sweep phase consists of four parts:
Sweeping of one half of the heap. This is done while the Java threads are running and are allowed to allocate objects in the part of the heap that isn’t currently being swept.
A short pause to switch halves.
Sweeping of the other half of the heap. This is done while the Java threads are running and are allowed to allocate objects in the part of the heap that was swept first.
A short pause for synchronization and recording statistics.
Parallel Mark and Sweep
The parallel mark and sweep strategy (also called the parallel garbage collector) uses all available CPUs in the system for performing the garbage collection as fast as possible. All Java threads are paused during the entire parallel garbage collection.
Dynamic and Static Garbage Collection Modes
By default, the JVM uses a dynamic garbage collection mode that automatically selects a garbage collection strategy to use, aiming at optimizing the application throughput. You can also choose between two other dynamic garbage collection modes or select the garbage collection strategy statically.
The following dynamic modes are available:
throughput, which optimizes the garbage collector for maximum application throughput. This is the default mode.
pausetime, which optimizes the garbage collector for short and even pause times.
deterministic, which optimizes the garbage collector for very short and deterministic pause times.
The major static strategies are:
singlepar, which is a single-generational parallel garbage collector (same as parallel)
genpar, which is a two-generational parallel garbage collector
singlecon, which is a single-generational mostly concurrent garbage collector
gencon, which is a two-generational mostly concurrent garbage collector
Objects that are allocated next to each other will not necessarily become unreachable (“die”) at the same time. This means that the heap may become fragmented after a garbage collection, so that the free spaces in the heap are many but small, making allocation of large objects hard or even impossible. Free spaces that are smaller than the minimum thread local area (TLA) size can not be used at all, and the garbage collector discards them as dark matter until a future garbage collection frees enough space next to them to create a space large enough for a TLA.
To reduce fragmentation, the JVM compacts a part of the heap at every garbage collection (old collection). Compaction moves objects closer together and further down in the heap, thus creating larger free areas near the top of the heap. The size and position of the compaction area as well as the compaction method is selected by advanced heuristics, depending on the garbage collection mode used.
Compaction is performed at the beginning of or during the sweep phase and while all Java threads are paused.
External and Internal Compaction
The JVM uses two compaction methods called external compaction and internal compaction. External compaction moves (evacuates) the objects within the compaction area to free positions outside the compaction area and as far down in the heap as possible. Internal compaction moves the objects within the compaction area as far down in the compaction area as possible, thus moving them closer together.
The JVM selects a compaction method depending on the current garbage collection mode and the position of the compaction area. External compaction is typically used near the top of the heap, while internal compaction is used near the bottom where the density of objects is higher.
Sliding Window Schemes
The position of the compaction area changes at each garbage collection, using one or two sliding windows to determine the next position. Each sliding window moves a notch up or down in the heap at each garbage collection, until it reaches the other end of the heap or meets a sliding window that moves in the opposite direction, and starts over again. Thus the whole heap is eventually traversed by compaction over and over again.
Compaction Area Sizing
The size of the compaction area depends on the garbage collection mode used. In throughput mode the compaction area size is static, while all other modes, including the static mode, adjust the compaction area size depending on the compaction area position, aiming at keeping the compaction times equal throughout the run. The compaction time depends on the number of objects moved and the number of references to these objects. Thus the compaction area will be smaller in parts of the heap where the object density is high or where the amount of references to the objects within the area is high. Typically the object density is higher near the bottom of the heap than at the top of the heap, except at the very top where the latest allocated objects are found. Thus the compaction areas are usually smaller near the bottom of the heap than in the top half of the heap.
You can find the original documentation in Oracle Docs.
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