Consider a paging system where the page table is stored in translation lookaside

Question

Consider a paging system where the page table is stored in translation lookaside buffer(TLB). The hit ratio is 99% meaning the page table entry will be found in TLB 99% of the time. The normal memory access time is t = 1.7 microseconds whereas TLB access is 0.1 microseconds.

a) When calcuating the effective paged memory access time, why is the cost of a TLB miss the sum of TLB access time plus normal memory access time?

b) If we consider only swap-in/swap-out time and if swap-in time = swap-out time = 18milliseconds, and, on the average, 85% of the pages are dirty, what is the effective page fault service time?

Explanation / Answer

a) The TLB is can be implemented as content-addressable memory (CAM). The CAM search key is the virtual address and the search result is a physical address. If the requested address is present in the TLB, the CAM search yields a match quickly and the retrieved physical address can be used to access memory. This is called a TLB hit. If the requested address is not in the TLB, it is a miss, and the translation proceeds by looking up the page table in a process called a page walk. The page walk requires a lot of time when compared to the processor speed, as it involves reading the contents of multiple memory locations and using them to compute the physical address. After the physical address is determined by the page walk, the virtual address to physical address mapping is entered into the TLB.

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