Implements virtual memory, provides a core set of services such as memory mapped files, copy-on-write memory, large memory support and underlying support for the cache manager.
Each process on a 32-bit Windows OS has a virtual address space (PAS) of 4GB (It is 8TB for a process in 64-bit Windows). Threads cannot access memory that belongs to another process, which protects a process from being corrupted by another process.
VAS (PAS):
- VAS for a process is the set of virtual memory addresses that it can use. The address space of a process P1, cannot be accessed by other process(es) P2, *unless it is shared*.
- This PAS does not represent the actual physical location in memory; instead the system maintains a Page Table for each process (an internal data structure used to translate VA to PA). Each time a thread references an address, the system translates the VA to PA.
- VAS for 32-bit (Windows) process is divided into two partitions; one for process itself and the other for kernel. Default partition will be 2GB:2GB (0x00000000 - 0x7FFFFFFF: 0x80000000 - 0xFFFFFFFF). If 4GT (4GB Tuning) is enabled, then it will be 3GB:1GB (0x00000000 - 0xBFFFFFFF: 0xC0000000 - 0xFFFFFFFF).
- After 4GT, the process that has IMAGE_FILE_LARGE_ADDRESS_AWARE flag set in its image header will have access to the additional 1GB of memory above the low 2GB.
IMPORTANT NOTE:
We can adjust the VAS of 32-bit process, using the following command, which sets a boot entry option that configures the size of the partition that is available for use by the process to a value between 2048 MB (2GB) and 3072 MB(3GB).
BCDEdit /set increaseuserva <XYZ Megabytes>
after which, partition would be like (0x00000000 - XYZ: XYZ+1 - 0xFFFFFFFF).
Working Set:
- The subset of VAS of a process that resides in physical memory is known as "Working Set"
- If the thread of a process attemt to use more physical memory than is currently available, the kernel, "pages" (swaps) some of the memory contents to disk (the total amount of VAS available to a process is limited by physical memory and the free space on disk available for the paging file)
- Physical storage and the VAS of each process are organized into "pages", units of memory, whose size depends on the host computer (usually 4 KB for x86 computers - Use GetSystemInfo API to get the page size)
- When a thread needs space in RAM, the kernel moves the "least recently used" pages in RAM to paging file in disk.
- When a thread references a page that is not a part of Working Set (means not present in main memory, a "page fault"(called Page Exception) occurs, on which Kernel's Page Fault Handler attempts to resolve the page fault (by "Demand Paging") and if it succeeds, the page is added to the working set.
- Hard Page Fault: Occurs when the demanded page is not on Main Memory
- Soft Page Fault: Occurs when the demanded page is on Main memory but in the working set of some other process or if it is demanded for the first time (called "Demand-Zero" fault)
- Process can reduce or empty its working set by calling "SetProcessWorkingSetSize" or "EmptyWorkingSet" functions
PS: The details on the page state at http://spot-myblog.blogspot.com/2009/07/page-state.html
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