Reserve Virtual Memory: Allocating and Deallocating Physical Memory Demand

In modern computing, the concept of memory management is crucial for efficient and optimal use of available resources. Virtual memory is a valuable tool that enables a program to use more memory than what is physically available on the system. This article aims to provide detailed context on the topic of reserve virtual memory, focusing on the global topic of allocating and deallocating physical memory demand.

What is Virtual Memory?

Virtual memory is a memory management technique that creates an illusion to users of a very large (main) memory. The main memory is partitioned into fixed-sized blocks called pages. The size of a page is usually power of 2, ranging from 512 bytes to 8192 bytes. The working set of a process is the set of pages that are currently in the main memory. If the working set of a process exceeds the size of the main memory, then some pages must be moved out of the main memory to make room for new pages. This process of moving pages between the main memory and the secondary memory (usually a hard disk) is called paging.

Why Reserve Virtual Memory?

Reserving virtual memory allows a program to reserve a large contiguous buffer that might be larger than the available physical memory. This can be useful for a program that needs to process large amounts of data, such as video editing software or scientific simulations. By reserving virtual memory, a program can ensure that it has access to a large enough buffer to handle its data processing needs, without worrying about whether or not the physical memory is available.

How to Reserve Virtual Memory?

Reserving virtual memory can be done using the VirtualAlloc() function in Windows or the mmap() function in Unix-based systems. These functions allow a program to reserve a block of virtual memory, which can then be used to allocate physical memory as needed.

// Reserve a block of virtual memory
LPVOID pVirtualMem = VirtualAlloc(NULL, dwVirtualMemSize, MEM_RESERVE, PAGE_READWRITE);

In the example above, the VirtualAlloc() function is used to reserve a block of virtual memory of size dwVirtualMemSize. The MEM_RESERVE flag indicates that the memory should be reserved, but not committed. The PAGE_READWRITE flag specifies that the memory should be allocated as read/write memory.

Allocating and Deallocating Physical Memory

Once a block of virtual memory has been reserved, physical memory can be allocated and deallocated as needed within that block. This can be done using the VirtualAlloc() function with the MEM_COMMIT flag, which commits a portion of the reserved virtual memory and allocates physical memory for it.

// Allocate physical memory within the reserved block
LPVOID pPhysicalMem = VirtualAlloc(pVirtualMem, dwPhysicalMemSize, MEM_COMMIT, PAGE_READWRITE);

In the example above, the VirtualAlloc() function is used to allocate a block of physical memory of size dwPhysicalMemSize within the previously reserved block of virtual memory. The MEM_COMMIT flag indicates that the memory should be committed, i.e., physical memory should be allocated for it.

To deallocate physical memory, the VirtualFree() function can be used with the MEM_DECOMMIT flag.

// Deallocate physical memory
VirtualFree(pPhysicalMem, dwPhysicalMemSize, MEM_DECOMMIT);

In the example above, the VirtualFree() function is used to deallocate the physical memory previously allocated with the VirtualAlloc() function. The MEM_DECOMMIT flag indicates that the memory should be deallocated, but the virtual memory should remain reserved.

• Virtual memory is a valuable tool that enables a program to use more memory than what is physically available on the system.
• Reserving virtual memory allows a program to ensure that it has access to a large enough buffer to handle its data processing needs.
• The VirtualAlloc() function in Windows or the mmap() function in Unix-based systems can be used to reserve virtual memory.
• Physical memory can be allocated and deallocated as needed within a reserved block of virtual memory.
• The VirtualAlloc() function can be used to allocate physical memory with the MEM_COMMIT flag, and to deallocate physical memory with the MEM_DECOMMIT flag.

References

• Tanenbaum, A. S. (2009). Modern Operating Systems (3rd ed.). Pearson Education, Inc.
• Kroah-Hartman, G. (2016, August 29). Understanding Linux Memory Management. Retrieved from https://lwn.net/Articles/615983/
• Microsoft Docs. (2021). VirtualAlloc function. Retrieved from https://docs.microsoft.com/en-us/windows/win32/api/memoryapi/nf-memoryapi-virtualalloc