• 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.