|
|
| (41 intermediate revisions by the same user not shown) |
| Line 1: |
Line 1: |
| (currently under construction!)
| |
| ==Swapping==
| |
| A memory management technique where a process may periodically have its primary memory space deallocated. This forces the swapped-out process to compete for memory before it can once again compete for the processor
| |
|
| |
|
| Swapping is especially well suited to timesharing systems since such systems often have times when a user logs onto the machine and hence is using some resources, but is inactive for relatively long periods ( and hence not using the CPU). A swapping memory manager accommodates the timesharing scenario by allocating memory to a process while the user is requesting service at a relatively high rate but deallocating memory
| |
|
| |
|
| |
| ==Overlays==
| |
|
| |
|
| |
| Significant problem with overlays , when do you load x, y or z, we have to make sure they are there. Developers need to think carefully what they put in these modules.
| |
| Another problem with swapping is that x, y, and z can’t now talk to each other , they need some sort of cut / paste buffer.
| |
| Problem is that x, y and z are different sizes and we need to fit the biggest one in the overlay area.
| |
|
| |
| X,Y and Z are all desired to be loaded in the same area of RAM. In traditional overlay schemes this would be an index.
| |
|
| |
| Classic problem of memory management:
| |
| One problem: fragmentation, difference between size of overlay area and size of the overlays
| |
|
| |
| Instead of having one overlay area what if we had many?
| |
|
| |
|
| |
|
| |
| ==Fragmenatation==
| |
|
| |
| ;'''Data fragmentation'''
| |
| Reduces the amount of fragmentation in file systems. Occurs when a piece of data in memory is broken up into many pieces. Two types of fragmentation which exist are internal fragmentation and external fragmentation
| |
|
| |
| ;'''Internal Fragmentation'''
| |
| Space is wasted in return for increased efficiency or simplicity. Ex in many file systems files always start at the beginning of a sector, which simplifies organization and makes it easier to grow files. Any space that is left over between the last byte of the file and the first byte of the next sector is internal fragmentation. Internal fragmentation occurs when the index pages are not being used to their maximum volume
| |
|
| |
| ;'''External Fragmentation'''
| |
| In such a fragmentation, the free space which is available for storage is divided into many small pieces. The storage space is of many different sizes.
| |
|
| |
| ;'''Page'''
| |
| fixed-length block of main memory, that is contiguous in both physical addressing and virtual memory addressing.
| |