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Operating system 33 swapping
- 1. Operating System 33 Swapping Prof Neeraj Bhargava Vaibhav Khanna Department of Computer Science School of Engineering and Systems Sciences Maharshi Dayanand Saraswati University Ajmer
- 2. Swapping • A process needs to be in the memory to be executed. • A process can be swapped temporarily out of memory to a backing store, and then brought back into memory for continued execution • Total physical memory space of processes can exceed physical memory • Backing store – fast disk large enough to accommodate copies of all memory images for all users; must provide direct access to these memory images
- 3. Swapping • Roll out, roll in – swapping variant used for priority-based scheduling algorithms; lower- priority process is swapped out so higher- priority process can be loaded and executed • Major part of swap time is transfer time; total transfer time is directly proportional to the amount of memory swapped • System maintains a ready queue of ready-to- run processes which have memory images on disk
- 4. Swapping (Cont.) • Does the swapped out process need to swap back in to same physical addresses? • Depends on address binding method – Plus consider pending I/O to / from process memory space • Modified versions of swapping are found on many systems (i.e., UNIX, Linux, and Windows) – Swapping normally disabled – Started if more than threshold amount of memory allocated – Disabled again once memory demand reduced below threshold
- 5. Schematic View of Swapping
- 6. Context Switch Time including Swapping • If next processes to be put on CPU is not in memory, need to swap out a process and swap in target process • Context switch time can then be very high • 100MB process swapping to hard disk with transfer rate of 50MB/sec – Swap out time of 2000 ms – Plus swap in of same sized process – Total context switch swapping component time of 4000ms (4 seconds) • Can reduce if reduce size of memory swapped – by knowing how much memory really being used – System calls to inform OS of memory use via request_memory() and release_memory()
- 7. Context Switch Time and Swapping (Cont.) • Other constraints as well on swapping – Pending I/O – can’t swap out as I/O would occur to wrong process – Or always transfer I/O to kernel space, then to I/O device • Known as double buffering, adds overhead • Standard swapping not used in modern operating systems – But modified version common • Swap only when free memory extremely low
- 8. Binding to physical memory • Normally, a process that is swapped out will be swapped back into the same memory space that it occupied previously. • This restriction is dictated by the method of address binding. • If binding is done at assembly time or load time, then the process cannot be moved to different locations. • If execution-time binding is being used, then it is possible to swap a process into a different memory space, because physical addresses are computed during execution time
- 9. Swapping Considerations • If we want to swap a process, we must be sure that it is completely idle. • Of particular concern is any pending I/O. If a process is waiting for an I/O operation, we may want to swap that process to free up its memory. • However, if the I/O is asynchronously accessing the user memory for I/O buffers, then the process cannot be swapped. • Assume the I/O operation was queued because the device was busy. Then, if were to swap out process P1 and swap in process P2, the I/O operation might attempt to use memory that now belongs to process P1. • The two main solutions to this problem are – (1) never to swap a process with pending I/O, or – (2) to execute I/O operations only into operating system buffers.
- 10. Swapping on Mobile Systems • Not typically supported – Flash memory based • Small amount of space • Limited number of write cycles • Poor throughput between flash memory and CPU on mobile platform • Instead use other methods to free memory if low – iOS asks apps to voluntarily relinquish allocated memory • Read-only data thrown out and reloaded from flash if needed • Failure to free can result in termination – Android terminates apps if low free memory, but first writes application state to flash for fast restart – Both OSes support paging as discussed below
- 11. Assignment • Explain the concept of Swapping