Motivation

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Usually, database management systems and operating systems use modifications of LRU algorithms. These algorithms have higher maintenance costs (pages list should be modified on each page access), but often they are effective from a "page-fault rate" point of view and have O(1) complexity for a searching page to replace. Simple LRU is not sequential scan resistant, but modifications that utilize page access frequency are resistant to sequential scan. Some databases uses CLOCK as page replacement algorithm.

We can try one of the modifications of LRU as well (for example, "segmented LRU" seems suitable for Ignite) or CLOCK algorithm (or one of its modifications).

Random-LRU algorithm

Random-LRU algorithm. Every time page is accessed, its timestamp gets updated. When a page fault occurs and it's required to replace some pages, the algorithm randomly chooses 5 pages from the page memory and evicts a page with the latest timestamp.

Segmented-LRU algorithm

Segmented-LRU algorithm is a scan-resistant variation of the Least Recently Used (LRU) algorithm. Segmented-LRU pages list is divided into two segments, a probationary segment, and a protected segment. Pages in each segment are ordered from the least to the most recently accessed. New pages are added to the most recently accessed end (tail) of the probationary segment. Existing pages are removed from wherever they currently reside and added to the most recently accessed end of the protected segment. Pages in the protected segment have thus been accessed at least twice. The protected segment is finite, so migration of a page from the probationary segment to the protected segment may force the migration of the LRU page in the protected segment to the most recently used end of the probationary segment, giving this page another chance to be accessed before being replaced. Page to replace is polled from the least recently accessed end (head) of the probationary segment.

CLOCK algorithm

The clock algorithm keeps a circular list of pages in memory, with the "hand" pointing to the last examined page frame in the list. When a page fault occurs and no empty frames exist, then the hit flag of the page is inspected at the hand's location. If the hit flag is 0, the new page is put in place of the page the "hand" points to, and the hand is advanced one position. Otherwise, the hit flag is cleared, then the clock hand is incremented and the process is repeated until a page is replaced.

Risks and Assumptions

Ignite is a memory-centric product, so "low maintenance cost" is very critical. And there is a risk that page replacement algorithm can affect workloads, where page replacement is not used (enough RAM to store all data). Any page replacement solution should be carefully benchmarked with different workloads.

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