Implementation of lru, Operating System

1. On every access, mark the page with a timestamp. Whenever we need to evict a page, we search through memory for the oldest page, the least-recently used page. But we need memory accesses to be fast. With this approach, on every memory access we would also need to read(load) a clock (or counter) and perform a store.

2. Maintain a queue of pages. Every time we touch a page, wemove that page to the beginning of the queue. When we need to evict a page, we evict the last element of the queue. Again, the pointer manipulation to do this would slow down the memory accesses, which we need to be fast on average.

3. Use a hash table rather than a queue. In this case, we have to compute a hash address on every memory access. Bad idea.

4. Approximate LRU.We can do this by maintaining reference bits for every page. On each memory access, the MMU hardware sets the page's reference bit to 1. Periodically, the OS tells the MMU to reset all the reference bits. Whenever we need to evict a page, we select one that has a reference bit not set. (So eviction may require an O(n) search through page tables to ?nd a page with reference bit not set, but the common case, cache hits, are very fast, done in hardware.) This algorithm considers any page which is zeroed to be "old enough". Then, although we can't know exactly what is the least-recently used, we do know that whatever is marked is a least more recent than something not marked. In practice, this approach works pretty well.

LRU is already only an approximation to OPT. In practice, we also approximate LRU itself. Our LRU approximation optimizes for the common case, which is cache hits, rather than cache misses. (If the common case is not cache hits, then your cache is not helping you.)

Posted Date: 3/12/2013 6:00:04 AM | Location : United States







Related Discussions:- Implementation of lru, Assignment Help, Ask Question on Implementation of lru, Get Answer, Expert's Help, Implementation of lru Discussions

Write discussion on Implementation of lru
Your posts are moderated
Related Questions
Write a short note on disk scheduling   One of the responsibities of operating system O/S is to use the hardware efficiently. Whenever a process requires I/O to or from the

Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4


Define UnixWare UnixWare manages resources at the process level. Each resource allocated to the application is actually allocated to the process representing the application. C

FIXED PARTITIONING Using fixed partitioning we are able to allocate the memory Here we are dividing the memory into a few fixed partitions.Every partition may not be of the si

Q. Definition of Operating System? Definition of Operating System: • An Operating system is a program which controls execution of application programs and acts as an interfa

Refer a program with a linked origin of 5000. Suppose the memory area allocated to it have the start address of 70000. Determine the value to be loaded in relocation register?

i need to complete project on lan architceture RAIDs etc

Recall that condition variables are synchronization primitives that enable threads to wait until a particular condition occurs. Generalizing, the combination of locks and condit

What is Multitasking? Multitasking provides power to users by offering them to run multiple applications at once. The applications are loaded into memory and appear to the user