CST Week 4 Post

 CST Week 4 Summary

This week we learned a ton about how the operating system manages virtual memory, especially paging. Paging is basically an alternative to the segmentation approach when it comes to managing memory. Instead of splitting up a process' address space into some number of variable sized segments, we divide it into fixed-size units, each of which is called a page. There are numerous advantages of paging, from avoiding external fragmentation to being very flexible and enabling the spare use of virtual address spaces. 

To be blunt, we cannot simply implement paging into our system willy nilly - we have to take numerous factors into account when doing so in order to ensure good memory management. One is ensuring that we have a tranlsation-lookaside buffer, or TLB in order to cache frequently used virtual-to-physical address mappings. The main purpose of the TLB is to keep our system running quickly - we wont have to perform a full page table search for an address mapping if it is in the TLB. Another important technique for ensuring good paging function is to implement a hybrid of small and large table sizes: instead of having a single page table for the entire address space of the process, we can have one per logical segment. We may thus have three page tables, one for the code, heap and stack parts of the address space. In addition, the OS actually packs away infrequently used portions of address spaces into hard disk drives, in order to maintain a single and large address space overall. When memory is near full, the OS will also page out one or more pages to make room for the new page about to be used. The process of picking a page to kick out or replace is known as the page replacement policy, and there are several. Notably, the optimal policy, developed by Belady, decrees that it is most optimal to replace the page which will be accessed furthest in the future. The optimal is an ideal template which developers can approach through implementing their own policies, which include FIFO, random, and LRU among others.