frame allocation
Summary
Local and global replacement
| Method | Core Idea | Advantages | Disadvantages |
|---|---|---|---|
| local replacement | replace with page in saem process | - frames allocated to process is constant - stable performace | - insufficient allocation -> hinder process |
| global replacement | replace any page | - self-adjustment between processes | - malicious processes can affect others - different allocation between runs |
Concept
- how to allocated phycical memory frames across processes
- simple approaches:
- equal allocation
- proportional allocation - by the size of the process in relation to all the processes
Local replacement
- select victim page within the same process, using the process’s page table
- pros:
- frames allocated to a process remain constant -> stable performance between runs
- cons:
- less flexible, if insufficient frames are allocated -> process will be hindered
Global replacement
- page replacement across processes
- victim page can be chosen across all physical frames
- pros:
- self-adjustment, processes that need more can get more
- cons:
- malicious processes can get more frames
- inconsistent frame allocation across runs
Thrasing with frame allocation
- IO needed to bring data from secondary storage into RAM
- with global replacement
- thrashing process can steal pages from other processes
- causing other processes to thrash also, cascading thrashing
- with local replacement
- limited to one process
- but that process can block the IO
Working Set model
- a locality, a region of code that accesses the same set of pages
- transient - working set changing size
- stable - working set is about the same for a period of time
represents back in time, we only have past info
- working set window -> time interval
- if
- if