Virtual memory is a computer system technique which gives an application program the impression that it has contiguous working memory, while in fact it is physically fragmented and may even overflow on to disk storage. Systems which use this technique make programming of large applications easier and use real physical memory (e.g. RAM) more efficiently than those without virtual memory.
Virtual Memory Operating systems based on Microsoft Windows NT technologies have always provided applications with a flat 32-bit virtual address space that describes 4 gigabytes (GB) of virtual memory. The address space is usually split so that 2 GB of address space is directly accessible to the application and the other 2 GB is only accessible to the Windows executive software.The virtual address space of processes and applications is still limited to 2 GB unless the /3GB switch is used in the Boot.ini file. When the physical RAM in the system exceeds 16 GB and the /3GB switch is used, the operating system will ignore the additional RAM until the /3GB switch is removed. This is because of the increased size of the kernel required to support more Page Table Entries. The assumption is made that the administrator would rather not lose the /3GB functionality silently and automatically; therefore, this requires the administrator to explicitly change this setting.Virtual memory (VM) in UNIX operating system is just what it says -- "virtual" -- it really doesn't exist. The VM size is NOT consuming any disk space. Unless a user's X system is performing swapping there's absolutely no need to worry about the swap file size nor its location. Swapping activity is provided by observing the "0(0) pageouts" in the last header line of the Terminal top command. Another useful Terminal command is the vm_stat(1) command (see man vm_stat). This command also displays the number of Pageouts. The pageout value is an indication that physical memory is being paged(swapped) to the swap file. This i/o is done in page chunks. A page chunk is 4096 bytes in size. When physical memory is paged (swapped) to the swap file it is being done so because physical memory is being over-subscribed. The best solution for avoiding frequent over-subscription of physical memory is to have fewer Apps running at same time or install more physical memory. When physical memory becomes over-subscribed the OS will seek out inactive memory pages and copy them to the swap file in order to make room for the active memory pages -- which may have to be copied from the swap file back into physical memory. a page fault is a fixed length block of memory that is used as a unit of transfer between physical memory and external storage like a disk, and a page fault is an interrupt (or exception) to the software raised by the hardware, when a program accesses a page that is mapped in address space, but not loaded in physical memory.The hardware that detects this situation is the memory management unit in a processor. The exception handling software that handles the page fault is generally part of an operating system. The operating system tries to handle the page fault by making the required page accessible at a location in physical memory or kills the program in case it is an illegal access. working set of information W(t,τ) of a process at time t to be the collection of information referenced by the process during the process time interval (t − τ,t)”. Typically the units of information in question are considered to be memory pages. This is suggested to be an approximation of the set of pages that the process will access in the future (say during the next τ time units), and more specifically is suggested to be an indication of what pages ought to be kept in main memory to allow most progress to be made in the execution of that process.
The effect of choice of what pages to be kept in main memory (as distinct from being paged out to auxiliary storage) is important: if too many pages of a process are kept in main memory, then fewer other processes can be ready at any one time. If too few pages of a process are kept in main memory, then the page fault frequency is greatly increased and the number of active (non-suspended) processes currently executing in the system are set to zero.
The working set model states that a process can be in RAM if and only if all of the pages that it is currently using (the most recently used pages) can be in RAM. The model is an all or nothing model, meaning if the pages it needs to use increases, and there is no room in RAM, the process is kicked from memory to free the memory for other processes to use.
In other words, the working set strategy prevents thrashing while keeping the degree of multiprogramming as high as possible. Thus it optimizes CPU utilization and throughput.
The main hurdle in implementing the working set model is keeping track of the working set. The working set window is a moving window. At each memory reference a new reference appears at one end and the oldest reference drops off the other end. A page is in the working set if it is referenced in the working set window.Page size is usually determined by a processor architecture. Traditionally, pages in a system had uniform size, for example 4096 bytes. However, processor designs often allow two or more, sometimes simultaneous, page sizes due to the benefits and penalties. There are several points that can factor into choosing the best page size.Intel x86 supports 4MB pages (2MB pages if using PAE) in addition to its standard 4kB pages, and other architectures may often have similar feature. IA-64 supports as much as eight different page sizes, from 4kB up to 256MB. This support for huge pages (or, in Microsoft Windows terminology, large pages) allows "the best of both worlds", reducing the pressure on the TLB cache (sometimes increasing speed by as much as 15%, depending on the application and the allocation size) for large allocations and still keeping memory usage at a reasonable level for small allocations.
Huge pages, despite being implemented in most contemporary personal computers, are not in common use except in large servers and computational clusters. Commonly, their use requires elevated privileges, cooperation from the application making the large allocation (usually setting a flag to ask the operating system for huge pages) or manual administrator configuration; operating systems commonly, sometimes by design, cannot page them out to disk.
Linux has supported huge pages on several architectures since the 2.6 series. Windows Server 2003 (SP1 and newer) and Windows Server 2008 supports huge pages under the name of large pages, but Windows XP and Windows Vista do not.The read/write speed of a hard drive is much slower than RAM, and the technology of a hard drive is not geared toward accessing small pieces of data at a time. If your system has to rely too heavily on virtual memory, you will notice a significant performance drop. The key is to have enough RAM to handle everything you tend to work on simultaneously -- then, the only time you "feel" the slowness of virtual memory is is when there's a slight pause when you're changing tasks. When that's the case, virtual memory is perfect.
When it is not the case, the operating system has to constantly swap information back and forth between RAM and the hard disk. This is called thrashing, and it can make your computer feel incredibly slow.
The effect of choice of what pages to be kept in main memory (as distinct from being paged out to auxiliary storage) is important: if too many pages of a process are kept in main memory, then fewer other processes can be ready at any one time. If too few pages of a process are kept in main memory, then the page fault frequency is greatly increased and the number of active (non-suspended) processes currently executing in the system are set to zero.
The working set model states that a process can be in RAM if and only if all of the pages that it is currently using (the most recently used pages) can be in RAM. The model is an all or nothing model, meaning if the pages it needs to use increases, and there is no room in RAM, the process is kicked from memory to free the memory for other processes to use.
In other words, the working set strategy prevents thrashing while keeping the degree of multiprogramming as high as possible. Thus it optimizes CPU utilization and throughput.
The main hurdle in implementing the working set model is keeping track of the working set. The working set window is a moving window. At each memory reference a new reference appears at one end and the oldest reference drops off the other end. A page is in the working set if it is referenced in the working set window.Page size is usually determined by a processor architecture. Traditionally, pages in a system had uniform size, for example 4096 bytes. However, processor designs often allow two or more, sometimes simultaneous, page sizes due to the benefits and penalties. There are several points that can factor into choosing the best page size.Intel x86 supports 4MB pages (2MB pages if using PAE) in addition to its standard 4kB pages, and other architectures may often have similar feature. IA-64 supports as much as eight different page sizes, from 4kB up to 256MB. This support for huge pages (or, in Microsoft Windows terminology, large pages) allows "the best of both worlds", reducing the pressure on the TLB cache (sometimes increasing speed by as much as 15%, depending on the application and the allocation size) for large allocations and still keeping memory usage at a reasonable level for small allocations.
Huge pages, despite being implemented in most contemporary personal computers, are not in common use except in large servers and computational clusters. Commonly, their use requires elevated privileges, cooperation from the application making the large allocation (usually setting a flag to ask the operating system for huge pages) or manual administrator configuration; operating systems commonly, sometimes by design, cannot page them out to disk.
Linux has supported huge pages on several architectures since the 2.6 series. Windows Server 2003 (SP1 and newer) and Windows Server 2008 supports huge pages under the name of large pages, but Windows XP and Windows Vista do not.The read/write speed of a hard drive is much slower than RAM, and the technology of a hard drive is not geared toward accessing small pieces of data at a time. If your system has to rely too heavily on virtual memory, you will notice a significant performance drop. The key is to have enough RAM to handle everything you tend to work on simultaneously -- then, the only time you "feel" the slowness of virtual memory is is when there's a slight pause when you're changing tasks. When that's the case, virtual memory is perfect.
When it is not the case, the operating system has to constantly swap information back and forth between RAM and the hard disk. This is called thrashing, and it can make your computer feel incredibly slow.
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