Under the Hood of .NET Memory Management - Simple Talk

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Chapter 7: The Windows Memory Model It is important to remember that the allocated memory is virtual, which means it doesn't directly refer to physical memory. Instead, it refers to a virtual memory address that is translated by the CPU to a physical memory address on demand. The reason for using virtual addresses is to allow for a much more flexible memory space to be constructed; one which is far larger than what is actually available in physical RAM. To make this possible, a machine's hard drive is used as a kind of secondary memory store to increase the total amount of memory available to our applications, and we will now explore that virtual/physical relationship. Virtual and Physical Memory Physical memory is simply the amount of physical RAM installed on the machine. By definition, it has a fixed size that can only be increased by opening the back of the machine and slotting more inside. Any resource with a fixed limit causes a problem; once that limit is reached – bang – there is nowhere to go. In the absence of any other kind of memory, once the physical memory is all used up, things start to go horribly wrong. Virtual memory was devised as a way around the problem: using a machine's hard drive as a kind of alternative memory store of last resort. Physical disk capacity is much greater than RAM capacity, and so it makes sense to use part of your disk storage as a way of increasing the amount of available memory. To facilitate this, a large file is created, often called a "swap" or "page" file, which is used to store physical memory sections that haven't been accessed for a while. This allows the memory manager (MM) to reuse physical memory and still have access to "paged" data when requested. I probably don't need to mention the downside of this cunning solution, but I will, anyway: accessing bits from RAM is pretty fast; getting them off a disk is really slow. 198
Chapter 7: The Windows Memory Model Virtual memory uses a combination of physical, RAM-based memory and disk-based memory to create a uniform virtual memory address space. Combining the two resources into a single pool gives applications access to a far larger and, as far as processes are concerned, homogenous memory space. In Figure 7.1 you can see a Virtual Address Space (VAS) made up of a sequence of memory blocks, or pages, which point to either physical or disk-based memory. Figure 7.1: Process Virtual Address Space. Pages The virtual memory address space is organized into chunks, or pages, and there are two types of pages (you can specify page size in VirtualAlloc): • small (4 KB) • large (16 MB). When a chunk of memory is requested by the .NET runtime, a set of virtual pages, sufficiently large to accommodate the requested size, are allocated. If you asked for 9 K, you would get three pages at 4 K each, that is, 12 K in total. 199
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Under the Hood of .NET Memory Management - Simple Talk

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