On systems with limited physical memory or many processes, footprint may dictate scalability. Footprint is the working set of a process, measured in pages and cache lines. ![]() Some users are sensitive to other considerations. However, in an interactive graphics program, even short pauses may negatively affect the user experience. For example, some consider the right metric for a web server to be throughput because pauses during garbage collection may be tolerable or simply obscured by network latencies. Users have different requirements of garbage collection. Garbage collection pauses affect the responsiveness of applications. Latency is the responsiveness of an application. ![]() Throughput includes time spent in allocation (but tuning for speed of allocation generally isn't needed). Throughput is the percentage of total time not spent in garbage collection considered over long periods of time. The primary measures of garbage collection are throughput and latency. These objects are copied into the old region. Objects are copied between survivor spaces in this way until they've been copied a certain number of times or there isn't enough space left there. The one space recently filled is a source of live objects that are copied into the other survivor space. In the next garbage collection, the purpose of the two survivor spaces are exchanged. One survivor space is empty at any time, and serves as the destination of live objects in eden and the other survivor space during garbage collection after garbage collection, eden and the source survivor space are empty. Most objects are initially allocated in eden. The young generation consists of eden and two survivor spaces. The complete address space reserved for object memory can be divided into the young and old generations. The entire address space covering the Java heap is logically divided into young and old generations. Figure 3-2 shows the default arrangement of generations in the serial garbage collector:įigure 3-2 Default Arrangement of Generations in the Serial Collectorĭescription of "Figure 3-2 Default Arrangement of Generations in the Serial Collector"Īt startup, the Java HotSpot VM reserves the entire Java heap in the address space, but doesn't allocate any physical memory for it unless needed. Major collections usually last much longer than minor collections because a significantly larger number of objects are involved. Eventually, the old generation fills up and must be collected, resulting in a major collection, in which the entire heap is collected. Typically, some fraction of the surviving objects from the young generation are moved to the old generation during each minor collection. The costs of such collections are, to the first order, proportional to the number of live objects being collected a young generation full of dead objects is collected very quickly. When the young generation fills up, it causes a minor collection in which only the young generation is collected garbage in other generations isn't reclaimed. The vast majority of objects are allocated in a pool dedicated to young objects (the young generation), and most objects die there. ![]() Garbage collection occurs in each generation when the generation fills up. To optimize for this scenario, memory is managed in generations (memory pools holding objects of different ages). For example, iterator objects are often only alive for the duration of a single loop. The sharp peak at the left represents objects that can be reclaimed (in other words, have "died") shortly after being allocated. The byte count on the y-axis is the total bytes in objects with the corresponding lifetime. The x-axis shows object lifetimes measured in bytes allocated. The blue area in Figure 3-1 is a typical distribution for the lifetimes of objects. The most important of these observed properties is the weak generational hypothesis, which states that most objects survive for only a short period of time. While naive garbage collection examines every live object in the heap every time, generational collection exploits several empirically observed properties of most applications to minimize the work required to reclaim unused (garbage) objects. The Java HotSpot VM incorporates a number of different garbage collection algorithms that all use a technique called generational collection. The time this approach takes is proportional to the number of live objects, which is prohibitive for large applications maintaining lots of live data. ![]() Any leftover objects are considered garbage. An object is considered garbage and its memory can be reused by the VM when it can no longer be reached from any reference of any other live object in the running program.Ī theoretical, most straightforward garbage collection algorithm iterates over every reachable object every time it runs.
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