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| <h4 class="subsection">22.4.1 Overview about traditional Unix memory handling</h4> |
| |
| <p><a name="index-address-space-2772"></a><a name="index-physical-memory-2773"></a><a name="index-physical-address-2774"></a>Unix systems normally provide processes virtual address spaces. This |
| means that the addresses of the memory regions do not have to correspond |
| directly to the addresses of the actual physical memory which stores the |
| data. An extra level of indirection is introduced which translates |
| virtual addresses into physical addresses. This is normally done by the |
| hardware of the processor. |
| |
| <p><a name="index-shared-memory-2775"></a>Using a virtual address space has several advantage. The most important |
| is process isolation. The different processes running on the system |
| cannot interfere directly with each other. No process can write into |
| the address space of another process (except when shared memory is used |
| but then it is wanted and controlled). |
| |
| <p>Another advantage of virtual memory is that the address space the |
| processes see can actually be larger than the physical memory available. |
| The physical memory can be extended by storage on an external media |
| where the content of currently unused memory regions is stored. The |
| address translation can then intercept accesses to these memory regions |
| and make memory content available again by loading the data back into |
| memory. This concept makes it necessary that programs which have to use |
| lots of memory know the difference between available virtual address |
| space and available physical memory. If the working set of virtual |
| memory of all the processes is larger than the available physical memory |
| the system will slow down dramatically due to constant swapping of |
| memory content from the memory to the storage media and back. This is |
| called “thrashing”. |
| <a name="index-thrashing-2776"></a> |
| <a name="index-memory-page-2777"></a><a name="index-page_002c-memory-2778"></a>A final aspect of virtual memory which is important and follows from |
| what is said in the last paragraph is the granularity of the virtual |
| address space handling. When we said that the virtual address handling |
| stores memory content externally it cannot do this on a byte-by-byte |
| basis. The administrative overhead does not allow this (leaving alone |
| the processor hardware). Instead several thousand bytes are handled |
| together and form a <dfn>page</dfn>. The size of each page is always a power |
| of two byte. The smallest page size in use today is 4096, with 8192, |
| 16384, and 65536 being other popular sizes. |
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