An application's roots include static fields, local variables on a thread's stack, CPU registers, GC handles, and the finalize queue. It determines which objects are no longer being used by examining the application's roots. When the garbage collector performs a collection, it releases the memory for objects that are no longer being used by the application. The garbage collector's optimizing engine determines the best time to perform a collection based on the allocations being made. In addition, because new objects that are allocated consecutively are stored contiguously in the managed heap, an application can access the objects quickly. Because the runtime allocates memory for an object by adding a value to a pointer, it's almost as fast as allocating memory from the stack. As long as address space is available, the runtime continues to allocate space for new objects in this manner.Īllocating memory from the managed heap is faster than unmanaged memory allocation. When the application creates the next object, the runtime allocates memory for it in the address space immediately following the first object. When an application creates the first reference type, memory is allocated for the type at the base address of the managed heap. All reference types are allocated on the managed heap. Initially, this pointer is set to the managed heap's base address. The managed heap maintains a pointer to the address where the next object in the heap will be allocated. This reserved address space is called the managed heap. When you initialize a new process, the runtime reserves a contiguous region of address space for the process. The data isn't paged until it's needed, so it's possible to encounter paging in situations where the physical memory pressure is low. The first time that physical memory pressure is high, the operating system must make room in physical memory to store data, and it backs up some of the data that's in physical memory to the page file. The page file is used even if physical memory pressure (demand for physical memory) is low. You can run out of memory if there isn't enough virtual address space to reserve or physical space to commit. Even if you have 2 GB of free space, an allocation that requires 2 GB will be unsuccessful unless all of that free space is in a single address block. When a virtual memory allocation is requested, the virtual memory manager has to find a single free block that is large enough to satisfy the allocation request. Virtual address space can get fragmented, which means that there are free blocks known as holes in the address space. The block of memory is assigned to physical storage. However, you can't store data to this memory block until it's committed. The block of memory is available for your use and can't be used for any other allocation request. The block of memory has no references to it and is available for allocation. Virtual memory can be in three states: State These functions allocate and free virtual memory for you on native heaps. If you're writing native code, you use Windows functions to work with the virtual address space. The garbage collector allocates and frees virtual memory for you on the managed heap. All processes on the same computer share the same physical memory and the page file, if there's one.īy default, on 32-bit computers, each process has a 2-GB user-mode virtual address space.Īs an application developer, you work only with virtual address space and never manipulate physical memory directly. The following list summarizes important CLR memory concepts:Įach process has its own, separate virtual address space. Provides memory safety by making sure that an object can't use for itself the memory allocated for another object. Managed objects automatically get clean content to start with, so their constructors don't have to initialize every data field. Reclaims objects that are no longer being used, clears their memory, and keeps the memory available for future allocations. The garbage collector provides the following benefits:įrees developers from having to manually release memory.Īllocates objects on the managed heap efficiently. This article describes the core concepts of garbage collection. Automatic memory management can eliminate common problems such as forgetting to free an object and causing a memory leak or attempting to access freed memory for an object that's already been freed. Therefore, developers working with managed code don't have to write code to perform memory management tasks. The garbage collector manages the allocation and release of memory for an application. In the common language runtime (CLR), the garbage collector (GC) serves as an automatic memory manager.
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