This is why physical memory is preferred nowadays and VM is used when physical memory is not enough. VM uses the disk and I/O with the disk is much slower than I/O with the RAM. Somewhere it also says that virtual spaces are used only when the physical memory is filled, which confused me a lot. In this case, the resource used is the disk, because it has a lot of space, more than RAM, where the OS can run its VM stuff. In computing, virtual memory (also virtual storage) is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very large (main) memory."īecause virtual memory is an illusory memory (so, non-existent), some other computer resources rather than RAM is used. Not sure you mean by "When collecting memory for other process", so can't answer that. The contents of virtual memory are not normally shared, except when they are (when using shared memory for example). A normal process never handles actual physical memory addresses, all that it sees is virtual memory, which may be mapped to different physical locations.
Physical memory is shared between processes the same way as computer power supply is shared, or CPU is shared. They could be solved differently, but virtual memory is the way today. Then OS takes care of actually reading bytes (in page-sized chunks) from disk and writing changes back, the process can just access the memory like any memory.Įvery modern multi-tasking general purpose operating system uses virtual memory, because the CPUs they run support it, and because it solves a big bunch of problems, for example memory fragmentation, transparently using swapping to disk, memory protection. Same page of memory can also be mapped to several places at once, for example with shared memory, so same data can be accessed by several processes at once (virtual address is probably different, so can't share pointer variables).Īnother special case of virtual memory use is mapping a regular file on disk to virtual memory (same thing which happens with swap file, but now controlled by normal application process).
#Virtual memory free#
OS then needs to either map that virtual memory page to RAM (possibly needing to free some RAM first by swapping current data out to swap file, or terminating some application if out of all memory) and load the right data into it, or it can terminate the application (address was not in valid range, or is read-only but process tries to write). In other cases, if process accesses that page, there is a CPU exception, which transfers control to operating system. Only when virtual memory page is mapped to physical RAM page, is there something there. If the process accesses the page (read or write), it needs to be loaded to page in RAM first (see above).
not stored physically (assumed to be all 0).Each valid page can be in 3 different states:
This address space is divided into pages for easier management (example size 4 KB). In context of modern common operating systems, each process has its own virtual memory space, with overlapping virtual memory addresses. "Virtual memory" means there is a valid address space, which does not map to any particular physical memory or storage, hence virtual.
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