Describe paging technique used in UNIX systems.
When a process begin in UNIX, not every its memory pages are read in from the disk at once. In its place, the kernel loads into RAM only a few pages at a time. Subsequent to the CPU digests these, the next page is demanded. If it isn't found in RAM, a page fault take place, signaling the kernel to load the next few pages from disk into RAM. This is called as demand paging and is a perfectly normal system activity in UNIX. Just thus you know, it is probable for you, like a programmer, to read in complete processes if there is sufficient memory available to do so. The Unix SVR4 daemon which carry out the paging out operation is called as pageout. It is a long running daemon as well as is created at boot time. The pageout process can't be killed and there are three kernel variables which control the paging operation (Unix SVR4):
- minfree - the absolute least of free RAM required, If free memory drops below this limit, a memory management system does its best to get back above it. It does consequently by page stealing from running processes, other if practical.
- desfree - the amount of RAM the kernel needs to have free at all times. If free memory is fewer than the pageout, the desfree and syscall is called every clock cycle.
- lotsfree - the amount of memory essential before the kernel stops calling pageout. Between lotsfree and desfree, pageout is called 4 times a second.