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.

11.1 Pageout Daemon (kswapd)

At system start, a kernel thread called kswapd is started from kswapd_init() which continuously executes the function kswapd() in mm/vmscan.c which usually sleeps. This daemon is responsible for reclaiming pages when memory is running low. Historically, kswapd used to wake up every 10 seconds but now it is only woken by the physical page allocator when the pages_low number of free pages in a zone is reached (see Section 3.2.1).

Figure 11.1: Call Graph: kswapd()

It is this daemon that performs most of the tasks needed to maintain the page cache correctly, shrink slab caches and swap out processes if necessary. Unlike swapout daemons such as Solaris [#!mauro01!#] which is woken up with increasing frequency as there is memory pressure, kswapd keeps freeing pages until the pages_high watermark is reached. Under extreme memory pressure, processes will do the work of kswapd synchronously by calling balance_classzone() which calls try_to_free_pages_zone(). The physical page allocator will also call try_to_free_pages_zone() when the zone it is allocating from is under heavy pressure.

When kswapd is woken up, it performs the following:

• Calls kswapd_can_sleep() which cycles through all zones checking the need_balance field in the struct zone_t. If any of them are set, it can not sleep;

• If it cannot sleep, it is removed from the kswapd_wait wait queue;

• kswapd_balance() is called which cycles through all zones. It will free pages in a zone with try_to_free_pages_zone() if need_balance is set and will keep freeing until the pages_high watermark is reached;

• The task queue for tq_disk is run so that pages queued will be written out;

• Add kswapd back to the kswapd_wait queue and go back to the first step.