Memory ManagementDesign & Implementation

Segmentation

Chapter 4

Design & implementation issues

• Thrashing– A process is thrashing if it spending more time

paging than executing.

• Global vs. local page frame allocation• Page size• Separate instruction & data spaces• Page sharing• Paging daemon

Why thrashing occurs• When CPU utilization is low, the OS

increases the degree of multiprogramming.• If some process enters a new phase and

needs more frames, these frames may be taken away from other processes– These other processes then begin to create page

faults

• As the processes wait for paging devices, CPU utilization decreases, so more processes are added…

How to stop thrashing

• Decrease the degree of multiprogramming by suspending some processes and writing their pages out to storage, thus freeing up some frames.

• Use a local page-replacement algorithm. – Then, if one process starts thrashing, it cannot

take away pages from another process and cause that process to thrash also.

• Use a working set algorithm

Global vs Local Allocation

• Global replacement schemes allow a process to select a replacement frame from the set of all frames, even if that frame has been allocated to another process.– Thrashing is a common side effect

• Local replacement requires that each process select from its own set of allocated frames.– The number of frames allocated to a process is

fixed.

Page Size

• Advantages of small page size– less internal fragmentation – better fit for various data structures, code

sections– less unused program in memory

• Disadvantages– programs need many pages, larger page

tables

Separate Instruction and Data Spaces

One address space Separate I and D spaces

Separate page tables

Sharing Pages

• Two processes can share code– Best to have only one copy of the code in

main memory

• If instructions and data have separate page tables, it is easy to share only those pages with instructions

Sharing Pages

Two processes share a program bysharing its instruction page table

P1

P2

Paging daemon• Periodically inspects the state of memory

– If too few frames are free, it uses the page replacement algorithm to select pages to evict

• It may also maintain a list of modified pages.– Whenever the paging device is idle, the paging daemon

may select a modified page and writes it out to the disk. It then sets the modified bit back to 0.

– This increases the probability that a page will be clean when it is selected for replacement

Segmentation

• When we write a structured program, we create a main function and other functions.– Each of these functions is a code segment of

variable length

• Locations within a segment are offset from the beginning of the segment

• Segmentation is a memory-management scheme that supports this user’s view of memory

Segments

• The logical & physical address spaces store each segment as a unit. – Each segment has a name and a length– Addresses specify the segment and the offset

• The user specifies addresses in terms of segment name and offset– Compare this with paging, where and the

hardware divides the process into pages

Hardware requirements for segmentation

• To map the logical to the physical address we need a segment table

• Each entry in the segment table has a segment base and a segment limit– The segment base contains the starting

physical address of the segment– The segment limit specifies the length of the

segment

Advantages of segmentation• Protection can be assigned per segment

• Code segments can be defined as read-only or execute only

• Arrays can be placed in their own segment. This means that the hardware will check for attempts to stray outside array boundaries (by using illegal indexes)

Paging, segmentation & fragmentation

• Paging – eliminates external fragmentation

• pages & frames are fixed sizes / a page is 2k bytes

– a small amount of internal fragmentation is introduced

• Segmentation– segments are of variable length, so external

fragmentation is still a problem – swapping is more difficult

Comparing paging and segmentation