Definition
Cleaning Policy
A cleaning policy determines when modified (dirty) pages are written back to secondary storage. It is the complement of the fetch policy.
Strategies
Demand Cleaning
A page is written to disk only when selected for replacement by the replacement policy.
Benefit: Minimises unnecessary writes; pages modified multiple times incur only one write.
Cost: Page fault latency increases when a dirty page must be written before replacement.
Pre-cleaning
Modified pages are written to disk proactively, before they are selected for replacement.
Benefit: Replacement can proceed immediately (page is already clean).
Cost: Redundant I/O if a page is written then modified again.
Hybrid Approach
Page Buffering
Modern systems combine both strategies. Replaced pages enter a buffer queue. Dirty pages are batched to disk when the disk is idle or when clean page reserves fall below a threshold.