I/O Devices and disk scheduling - Csbdu.in

that block that has been in the cache longest with no reference to it. Logically, the cache
consists of a stack of blocks, with the most recently reference block on the top of the
stack.
Another possibility is least frequently used (LFU): Replace the block that is not
frequently used. The LFU can be implemented by associating a counter with each block.
When a block is brought in, it is assigned a count of 1; with each reference to the block,
its count is incremented by 1. When replacement required, the block with smallest count
is selected. One problem of LFU is the effect of locality may actually cause the LFU
algorithm to make poor replacement choices.
is proposed.
To overcome this difficulty, a technique known as frequency-based replacement
New section Old section
MRU LRU
Re-reference;
count unchanged Re-reference
Miss(new block brought in)
count :=1
Count:=count+1
(a) FIIFO
New section Middle section Old section
… … …
(b)Use of three sections
Figure 4.8 Frequency-based Replacement
MRU
First consider a simplified version, given figure 4.8 a. The blocks are organized in a
stack, as with the LRU algorithm. A certain portion of the top part of the stack is set aside
as a new section. When there is cache hit, the referenced block is moved to the top of the
LRU