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Part IV
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Core Facilities Roots: Static fields Method parameters Local variables CPU registers
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Managed heap A B C D
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FIguRE 21-3 Managed heap before a collection
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After a root and the objects referenced by its fields are marked, the garbage collector checks the next root and continues marking objects . If the garbage collector is going to mark an object that it previously marked, it can stop walking down that path . This behavior serves two purposes . First, performance is enhanced significantly because the garbage collector doesn t walk through a set of objects more than once . Second, infinite loops are prevented if you have any circular linked lists of objects . Once all of the roots have been checked, the heap contains a set of marked and unmarked objects . The marked objects are reachable via the application s code, and the unmarked objects are unreachable . The unreachable objects are considered garbage, and the memory that they occupy can be reclaimed . The garbage collector now starts what is called the compact phase of the collection . This is when the collector traverses the heap linearly looking for contiguous blocks of unmarked (garbage) objects . If small blocks are found, the garbage collector leaves the blocks alone . If large free contiguous blocks are found, however, the garbage collector shifts the nongarbage objects down in memory to compact the heap . Naturally, moving the objects in memory invalidates all variables and CPU registers that contain pointers to the objects . So the garbage collector must revisit all of the application s roots and modify them so that each root s value points to the objects new memory location . In addition, if any object contains a field that refers to another moved object, the garbage collector is responsible for correcting these fields as well . After the heap memory is compacted, the managed heap s NextObjPtr pointer is set to point to a location just after the last nongarbage object . Figure 21-4 shows the managed heap after a collection . As you can see, a garbage collection generates a considerable performance hit, which is the major downside of using a managed heap . But keep in mind that garbage collections occur only when generation 0 is full, and until then, the managed heap is significantly faster than a C-runtime heap . Finally, the CLR s garbage collector offers some optimizations that greatly improve the performance of garbage collection . I ll discuss these optimizations later in this chapter, in the Generations and Other Garbage Collection Features for Use with Native Resources sections .
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21 Roots: Static fields Method parameters Local variables CPU registers
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FIguRE 21-4 Managed heap after a collection
As a programmer, you should take away a couple of important points from this discussion . To start, you no longer have to implement any code to manage the lifetime of objects your application uses . And notice how the two bugs described at the beginning of this chapter no longer exist . First, it s not possible to leak objects because any object not accessible from your application s roots can be collected at some point . Second, it s not possible to access an object that is freed because the object won t be freed if it is reachable, and if it s not reachable, your application has no way to access it . Also, since a collection causes memory compaction, it is not possible for managed objects to fragment your process s virtual address space . This would sometimes be a severe problem with unmanaged heaps but is no longer an issue when using the managed heap . Using large objects (discussed later in this chapter) is an exception to this, and fragmentation of the large object heap is possible . Important A type s static field roots whatever object it refers to forever or until the
AppDomain that the types are loaded into is unloaded . A common way to leak memory is to have a static field refer to a collection object and then to keep adding items to the collection object . The static field keeps the collection object alive and the collection object keeps all its items alive . For this reason, it is best to avoid static fields whenever possible .
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