ENVS 1800 Lecture 5: ENVS 1800 Tutorial 5 Notes
14 May 2018
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ENVS 1800 Tutorial 5 Notes – Thrashing
Introduction
• Both second chance algorithms reduce the number of disk swaps by keeping what
would otherwise be swapped-out pages in memory.
• However, the first of these algorithms has the potential of keeping a page beyond its
usefulness
• The second decreases the number of possible pages in memory by using some of those
pages for the free pool.
• A condition that can arise when a system is heavily loaded is called thrashing.
• Thrashig is every syste adiistrator’s ightmare.
• Thrashing occurs when every frame of memory is in use, and programs are allocated just
enough pages to meet their minimum requirement.
• A page fault occurs in a program, and the page is replaced by another page that will
itself be needed for replacement almost immediately.
• Thrashing is most serious when global page replacement is used.
• In this case, the stolen page may come from another program.
• When the second program tries to execute, it is immediately faced with its own page
fault.
• Unfortunately, the time required to swap a page from the disk is long compared to CPU
execution time
• As the page fault is passed around from program to program, no program is able to
execute, and the system as a whole slows to a crawl or crashes.
• The programs simply continue to steal pages from each other.
• With local page replacement, the number of thrashing programs is more limited, but
thrashing can still have a serious effect on system performance.
• As we mentioned previously, the data in the page table must be stored in memory.
• The first of these algorithms has the potential of keeping a page beyond its usefulness
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