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TLB Issues

• Updating TLB tricky if it is full: need to discard something.

• Context switch may requires TLB flush so that next process doesn’t use wrong page table entries. 

– Today many TLBs support process tags (sometimes called address space numbers) to improve performance. 

• Hit ratio is the percentage of time a page entry is found in TLB

• e.g. consider TLB search time of 20ns, memory access time of 100ns, and a hit ratio of 80% 

⇒ assuming one memory reference required for page table lookup, the effective memory access time is 

0.8 × 120 + 0.2 × 220 = 140ns. 

• Increase hit ratio to 98% gives effective access time of 122ns — only a 13% improvement.


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