self_doubt

Question

what is page fault service time ? what are its factor and how it is  related/not related with the memory access time??
plss help with suitable example

Answer 1

Page fault means we cant access the page for any reason whatsoever...it's a kind of an interrupt....page fault every time doesnt means that the page is not present in main memory...some time it may happen that u want to access a page which dont have any read permission etc..in these cases also we will face page fault...

now simply put to access a word from memory we need to find its physical address then fetch it from memory...
to convert VA->PA(virtual address to physical address) we go into tlb(say)..here also page fault can happen...though tlb updates with recently used pages only but as i said page fault doesnt means always that page is not present....if we get a page fault here then we have to get the page from memory(m nt taking cache into consideration here)......

other case may be suppose we got PA from tlb and entered the main memory to fetch the word...here also is a chance that page fault may occur and then u have to bring the page from secondary memory...and this time u spent to service the page from  secondary memory to main memory and then load it into main memory is page fault service time generally...
the more page faults u will have more will be the service time and more will be the effective memory access time...because time to fetch the word will be increasing with no of page faults..

Answer 2

CPU check page table for page and then take associated frame .If that frame is not present in main memory then page fault occur. When we are looking for something[frame]  in main memory and we did not not found that then at this time OS that Frame [we assume frame we referenced present in main memory] will be brought from secondary memory to main memory by using page replacement algorithm . then according to that page table is updated .

SO page fault = page taken from secondary memory to main memory[page replacement algorithm time include] + page table update time