Unevictable: 4 kB 0.1

Unevictable:           4 kB
The Unevictable LRU infrastructure addresses the following classes of
unevictable pages:

+ page owned by ram disks or ramfs
+ page mapped into SHM_LOCKed shared memory regions
+ page mapped into VM_LOCKED [mlock()ed] vmas
Least  Recently Used  (LRU): discards  the least  recently  used items
first. This  algorithm requires keeping  track of what was  used when,
which  is expensive if  one wants  to make  sure the  algorithm always
discards the least recently used item. General implementations of this
technique require  to keep  "age bits" for  cache-lines and  track the
"Least  Recently   Used"  cache-line   based  on  age-bits.   In  such
implementation, every time a cache-line  is used, the age of all other
cache-lines changes.  LRU is actually  a family of  caching algorithms
with members including:  2Q by Theodore Johnson and  Dennis Shasha and
LRU/K by Pat O'Neil, Betty O'Neil and Gerhard Weikum.

Hugetlb pages are also  unevictable. Hugepages are already implemented
in a way  that these pages don't  reside on the LRU and  hence are not
iterated over during  the vmscan.  So there is no  need to move around
these pages  across different  LRU's. We just  account these  pages as
unevictable for correct statistics.


The Unevictable LRU  adds an additional LRU list  to track unevictable
pages and to hide these pages from vmscan.  This mechanism is based on
a patch  by Larry  Woodman of Red  Hat to address  several scalability
problems with page reclaim in  Linux.  The problems have been observed
at  customer sites  on large  memory x86_64  systems.  For  example, a
non-numal x86_64 platform with 128GB  of main memory will have over 32
million 4k  pages in a  single zone.  When  a large fraction  of these
pages are not evictable for  any reason [see below], vmscan will spend
a lot of time scanning the LRU lists looking for the small fraction of
pages that  are evictable.  This can  result in a  situation where all
cpus are  spending 100% of their time  in vmscan for hours  or days on
end, with the system completely unresponsive.



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