Buffer Management

1
Buffer Management

• The buffer manager reads disk pages into a
• main memory page as needed. The general
• idea is to minimize the amount of disk I/O by
• keeping likely-to-be-requested pages in
• memory.

2
Overview
Requests
Buffer Manager
memory
Disk Files
disk

• The collection of main memory pages used by the
• buffer manager to store needed data is called
  the
• buffer pool.

3
When a Page is Requested...

• If the requested page is not in the pool
• Choose a buffer block for replacement
• If it has been modified, write it back to the
  disk
• Read the requested page into the buffer block
• Pin the page and return its address
• A pin_count is used since a page can be requested
  multiple times.

4
When a Page is Released...

• Requestor of page must unpin it after using, and
  indicate if the page has been modified.
• Set the dirty flag accordingly
• Decrement the pin_count
• A page is a candidate for replacement iff its
  pin_count 0

5
Buffer Replacement Strategies(1)

• Least-Recently Used (LRU)
• Choose the block that has not been accessed for
  the longest time
• Intuitively, buffers that have not been used for
  a long time are less likely to be accessed sooner
  than those that have been accessed recently
• Maintain information after each access operation

6
Buffer Replacement Strategies(2)

• Most-Recently Used (MRU)
• Opposite to LRU
• First-In-First-Out (FIFO)
• Choose the block that has been occupied by the
  same page for the longest time
• Require less maintenance than LRU, but it can
  make more mistakes. A block used repeatedly will
  eventually become the oldest block in the pool.
  It will be written back only to be reread shortly
  thereafter into another block

7
Buffer Replacement Strategies(3)

• Clock
• Efficient approximation to LRU
• Think of the buffer blocks as arranged in a
  circle, each associated with a flag, which is
  either 0 or 1. Blocks with a 0 flag are
  vulnerable to be replaced, while with a 1 flag
  are not. When the block is accessed, its flag is
  set to 1

8
Buffer Replacement Strategies(4)

• The buffer manager maintain a hand pointing to
  one of the blocks. When choosing a block for
  replacement, it looks for the first 0 it can
  find, rotating clockwise, and sets all 1s it
  passed to 0

9
More on Buffer Management

• Replacement strategy can have big impact on the
  amount of disk I/O, which depends on the access
  pattern
• When buffer blocks (LRUrepeated sequential scans) means each page
  request causes an I/O, while MRU is much better
  in this situation.
• Adjust replacement policy and pre-fetch pages
  based on access patterns