Section 4.9. Summary

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4.9. Summary

In this chapter we have examined memory management, both in general and in MINIX 3. We saw that the simplest systems do not swap or page at all. Once a program is loaded into memory, it remains there until it finishes. Embedded systems usually work like this, possibly with the code even in ROM. Some operating systems allow only one process at a time in memory, while others support multiprogramming.

The next step up is swapping. When swapping is used, the system can handle more processes than it has room for in memory. Processes for which there is no room are swapped out to the disk. Free space in memory and on disk can be kept track of with a bitmap or a hole list.

More advanced computers often have some form of virtual memory. In the simplest form, each process' address space is divided up into uniformly sized blocks called pages, which can be placed into any available page frame in memory. Many page replacement algorithms have been proposed. Two of the better known ones are second chance and aging. To make paging systems work well, choosing an algorithm is not enough; attention to issues such as determining the working set, memory allocation policy, and page size are required.

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Segmentation helps in handling data structures that change size during execution and simplifies linking and sharing. It also facilitates providing different protection for different segments. Sometimes segmentation and paging are combined to provide a two-dimensional virtual memory. The Intel Pentium supports segmentation and paging.

Memory management in MINIX 3 is simple. Memory is allocated when a process executes a fork or exec system call. The memory so allocated is never increased or decreased as long as the process lives. On Intel processors there are two memory models used by MINIX 3. Small programs can have instructions and data in the same memory segment. Larger programs use separate instruction and data space (separate I and D). Processes with separate I and D space can share the text portion of their memory, so only data and stack memory must be allocated during a fork. This may also be true during an exec if another process already is using the text needed by the new program.

Most of the work of the PM is concerned not with keeping track of free memory,-which it does using a hole list and the first fit algorithm, but rather with carrying out the system calls relating to process management. A number of system calls support POSIX-style signals, and since the default action of most signals is to terminate the signaled process, it is appropriate to handle them in the PM, which initiates termination of all processes. Several system calls not directly related to memory are also handled by the PM, mainly because it is smaller than the file system, and thus it was most convenient to put them here.