After this documentation was released in July 2003, I was approached by Prentice Hall and asked to write a book on the Linux VM under the Bruce Peren's Open Book Series.

The book is available and called simply "Understanding The Linux Virtual Memory Manager". There is a lot of additional material in the book that is not available here, including details on later 2.4 kernels, introductions to 2.6, a whole new chapter on the shared memory filesystem, coverage of TLB management, a lot more code commentary, countless other additions and clarifications and a CD with lots of cool stuff on it. This material (although now dated and lacking in comparison to the book) will remain available although I obviously encourge you to buy the book from your favourite book store :-) . As the book is under the Bruce Perens Open Book Series, it will be available 90 days after appearing on the book shelves which means it is not available right now. When it is available, it will be downloadable from http://www.phptr.com/perens so check there for more information.

To be fully clear, this webpage is not the actual book.

11.4 Shrinking all caches

The function responsible for shrinking the various caches is shrink_caches() which takes a few simple steps to free up some memory. The maximum number of pages that will be written to disk in any given pass is nr_pages which is initialised by try_to_free_pages_zone() to be SWAP_CLUSTER_MAX^11.1. The limitation is there so that if kswapd schedules a large number of pages to be swapped to disk, it will sleep occasionally to allow the IO to take place. As pages are freed, nr_pages is decremented to keep count.

The amount of work that will be performed also depends on the priority initialised by try_to_free_pages_zone() to be DEF_PRIORITY^11.2. For each pass that does not free up enough pages, the priority is decremented for the highest priority been 1.

The function first calls kmem_cache_reap() (see Section 9.1.7) which selects a slab cache to shrink. If nr_pages number of pages are freed, the work is complete and the function returns otherwise it will try to free nr_pages from other caches.

If other caches are to be affected, refill_inactive() will move pages from the active_list to the inactive_list before shrinking the page cache by reclaiming pages at the end of the inactive_list with shrink_cache().

Finally, it shrinks three special caches, the dcache (shrink_dcache_memory()), the icache (shrink_icache_memory()) and the dqcache (shrink_dqcache_memory()). These objects are quite small in themselves but a cascading effect allows a lot more pages to be freed in the form of buffer and disk caches.

Figure 11.4: Call Graph: shrink_caches()

Footnotes

...SWAP_CLUSTER_MAX^11.1

Currently statically defined as 32 in mm/vmscan.c.

...DEF_PRIORITY^11.2

DEF_PRIORITY is currently statically defined as 6 in mm/vmscan.c.