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| <h4 class="subsection">3.4.1 Why Lock Pages</h4> |
| |
| <p>Because page faults cause paged out pages to be paged in transparently, |
| a process rarely needs to be concerned about locking pages. However, |
| there are two reasons people sometimes are: |
| |
| <ul> |
| <li>Speed. A page fault is transparent only insofar as the process is not |
| sensitive to how long it takes to do a simple memory access. Time-critical |
| processes, especially realtime processes, may not be able to wait or |
| may not be able to tolerate variance in execution speed. |
| <a name="index-realtime-processing-355"></a><a name="index-speed-of-execution-356"></a> |
| A process that needs to lock pages for this reason probably also needs |
| priority among other processes for use of the CPU. See <a href="Priority.html#Priority">Priority</a>. |
| |
| <p>In some cases, the programmer knows better than the system's demand |
| paging allocator which pages should remain in real memory to optimize |
| system performance. In this case, locking pages can help. |
| |
| <li>Privacy. If you keep secrets in virtual memory and that virtual memory |
| gets paged out, that increases the chance that the secrets will get out. |
| If a password gets written out to disk swap space, for example, it might |
| still be there long after virtual and real memory have been wiped clean. |
| |
| </ul> |
| |
| <p>Be aware that when you lock a page, that's one fewer page frame that can |
| be used to back other virtual memory (by the same or other processes), |
| which can mean more page faults, which means the system runs more |
| slowly. In fact, if you lock enough memory, some programs may not be |
| able to run at all for lack of real memory. |
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