Linux Kernel Architecture

Chapter 18: Page Reclaim and Swapping

Four values, whose meanings are given below, are calculated^12 :

❑ distressis the key indicator as to how urgently the kernel needs fresh memory. It is calculated

using theprev_priorityvalue for right-shifting the fixed value 100.prev_priorityspecifies

the priority with which the zone had to be scanned during the lasttry_to_free_pagesrun

until the required number of pages was freed. Notice that the lowerprev_priorityis, the higher

the priority. The shift operations produce the followingdistressvalues for various priorities:

Priority Distress

70

61

53

46

312

225

150

0 100

Allpriorityvalues greater than 7 yield adistressfactor of 0. While 0 ensures the kernel that

there is no problem at all, 100 indicates massive trouble.

❑ mapped_ratioindicates the ratio of mapped memory pages (not only used to cache data but also

explicitly requested by processes to store data) to the total available memory. The ratio is calcu-

lated by dividing the current number of mapped pages by the total number of pages available at

system start. The result is scaled to a percentage value by multiplying by 100.

❑ mapped_ratiois used only to calculate a further value that is calledswap_tendencyand — as its

name suggests — indicates the swap tendency of the system. You are already familiar with the

first two calculation variables.sc_swappinessis an additional kernel parameter that is usually

based on the setting in/proc/sys/vm/swappiness.

❑ If there is a large imbalance between the lengths of the active and inactive lists, the kernel allows

swapping and page reclaim to happen more easily than usual to balance the situation. However,

some effort is made that large imbalances do not have much influence at lowswappinessvalues.

❑ The kernel now reduces all the information calculated so far to a truth value that answers the

following question: Are mapped pages to be swapped out or not?

Ifswap_tendencyis greater than or equal to 100, mapped pages are also swapped out, and

reclaim_mappedis set to 1. Otherwise, the variable retains its default value of 0 so that pages are

only reclaimed from the page cache.

Asvm_swappinessis added toswap_tendency, the administrator can enable the swapping of

mapped pages at any time regardless of the other system parameters by assigning the value of

100 to the variable.

Thelru_add_drainprocedure, which is invoked after the parameters have been calculated, distributes

the data currently held in the LRU cache to the system’s LRU lists. In contrast tolru_cache_add,touched

upon in Section 18.6.2, copying is performed when the temporary caches contain at least one element, not

only when they are completely filled.

(^12) The individual formulas were derived heuristically and are designed to guarantee good performance in many different situations.