Translation Lookaside Buffer (TLB): Speeding Up Address Translation in Paging Systems

Understand the Translation Lookaside Buffer (TLB) and its crucial role in optimizing address translation in paging systems. This guide explains how the TLB caches page table entries, reduces memory access for address translation, and improves the performance of virtual memory systems.

Translation Lookaside Buffer (TLB) in Paging

Drawbacks of Traditional Paging

While paging is an effective memory management technique, it has some drawbacks:

  • Large Page Tables: Page tables can consume significant amounts of main memory, reducing space for processes.
  • Slow Address Translation: Accessing the page table to translate virtual addresses to physical addresses adds overhead, slowing down memory access.

Improving Page Table Access: Multilevel Paging and Other Approaches

Several methods aim to mitigate the performance and memory limitations of traditional paging:

  • Increasing the page size can reduce the page table size but increases internal fragmentation.
  • Multilevel paging creates a hierarchy of page tables, reducing the size of each individual table but increasing the number of memory accesses needed for address translation.
  • Storing the page table in CPU registers would speed up access, but registers are limited and expensive.

The Concept of Locality of Reference

The principle of locality of reference states that processes tend to access a subset of their pages more frequently than others. This observation forms the basis of using a Translation Lookaside Buffer (TLB).

Translation Lookaside Buffer (TLB)

A TLB is a small, fast cache that stores recent page table entries. It's closer to the CPU than main memory, leading to faster address translation. Because the TLB only stores frequently accessed entries, it significantly reduces the number of times the CPU needs to access the main page table in memory. A TLB hit is when the requested entry is found in the TLB; a TLB miss is when it's not, requiring accessing the main page table.

TLBs use tags and keys for fast lookups. A TLB hit results in faster memory access; a TLB miss adds the overhead of accessing the main page table.

Effective Access Time (EAT) Calculation

The effective access time (EAT) considers the TLB hit rate (probability of finding the entry in the TLB). The formula for calculating EAT is:

EAT = P(t + m) + (1 - P)(t + k*m + m)

Where:

  • P = TLB hit rate
  • t = time to access TLB
  • m = time to access main memory
  • k = number of levels in the page table (1 for single-level paging)

This formula shows that increasing the TLB hit rate reduces EAT, but multilevel paging can increase EAT.