Memory Management with respect to Page replacement in the Linux Kernel

1
Memory Management with respect to Page
replacement in the Linux Kernel

• Kernel reference tree-2.4.20
• By
• A.R.Karthick
• (karthick_r_at_infosys.com)

2
Memory Hierarchies
L1 cache
T I M E
L2 cache
RAM
Hard Disk
3
Page Tables

• Define the virtual to physical mapping
• Page directory,page mid level directory,page
  table entry define the course of translation
• Example
• PGD 10 bits PTE 10 BITS (PMD folded in 32
  bit)
• 0000 0000 00 00 1000 0000 1100 0000 1111 -gt
  (0x00080c0f)
• pgd index(0) pte_index(1 ltlt 7) , pmd is folded to
  pgd

4
Page Table Entry Status Bits (PTE Entry)
11 10 9 8 7 6 5 4 3 2 1 0
PAGE_PRESENT PAGE_RW PAGE_USER PAGE_RESERVED PAGE_
ACCESSED PAGE_DIRTY INTERNAL_STATUS
5
Page Fault

• Processor Exception raised when there is a
  problem mapping the virtual address to physical
  address.
• Handled by do_page_fault in arch/i386/mm/fault.c.
• Write protection faults or COW faults map to
  do_wp_page.
• For pages in swap, do_swap_page is called.
• For pages not found, do_no_page is called that
  either faults in an anonymous zero page or an
  existing page.
• Page faults populate the LRU cache.

6
Page Replacement Algorithms

• Optimal Replacement ? Not possible
• Not Recently Used (NRU) ? Crude hack
• FIFO ? Inefficient
• Second Chance ? Better than above
• Clock Replacement ? Efficient than above

7
Page Replacement Algorithms

• LRU Least Recently used replacement
• NFU Not Frequently Used replacement
• Page Ageing based replacement
• Working Set algorithm based on locality of
  references per process
• Working Set based clock algorithms
• LRU with Ageing and Working Set algorithms are
  efficient to use and are commonly used

8
Page replacement handling in Linux Kernel

• Page Cache
• Pages are added to the Page cache for fast
  lookup.
• Page cache pages are hashed based on their
  address space and page index
• Inode or disk block pages, shared pages and
  anonymous pages form the page cache.
• Swap cached pages also part of the page cache
  represent the swapped pages.
• Anonymous pages enter the swap cache at swap-out
  time and shared pages enter when they become
  dirty.

9
LRU CACHE

• LRU cache is made up of active lists and inactive
  lists.
• These lists are populated during page faults and
  when page cached pages are accessed or
  referenced.
• kswapd is the page out kernel thread that
  balances the LRU cache and trickles out pages
  based on an approximation to LRU algorithm.
• Active lists contains referenced pages. This list
  is monitored for Page references through
  refill_inactive
• Referenced pages are given a chance to age
  through Move To Front and unreferenced pages are
  moved to the inactive list
• The inactive lists contains the set of Inactive
  clean and inactive dirty pages.
• This set is monitored on a timely basis when
  pages_high threshold is reached for free pages on
  a per zone basis is crossed.

10
Kswapd or Kernel Page out daemon

• Kswapd performs zone balancing based on
  pages_high, pages_low and pages_min
• The page replacement policy is an LRU
  approximation that is empirical in nature
• It doesnt follow strict page ageing based on
  page reference _times_
• The shrink cache routine is the principal page
  replacement routine
• It launders the dirty pages inode or disk block
  pages by scheduling write backs
• It swaps out pages based on the anonymous pages
  in the inactive list

11
Page replacement code snippet
if (unlikely(TryLockPage(page))) if
(PageLaunder(page) (gfp_mask __GFP_FS))
page_cache_get(page) spin_unlock(pagem
ap_lru_lock) wait_on_page(page) page_cac
he_release(page) spin_lock(pagemap_lru_lock)
continue if (PageDirty(page)
is_page_cache_freeable(page) page-gtmapping)
int (writepage)(struct page
) writepage page-gtmapping-gta_ops-gtwritepage
if ((gfp_mask __GFP_FS) writepage)
ClearPageDirty(page) SetPageLaunder(pag
e) page_cache_get(page) spin_unlock(pag
emap_lru_lock) writepage(page) page_cach
e_release(page) spin_lock(pagemap_lru_lock)
continue
12
Page replacement code snippet

• if (!page-gtmapping !is_page_cache_freeable(page
  ))
• spin_unlock(pagecache_lock)
• UnlockPage(page)
• page_mapped
• if (--max_mapped gt 0)
• continue
• /
• Alert! We've found too many mapped pages on
  the
• inactive list, so we start swapping out
  now!
• /
• spin_unlock(pagemap_lru_lock)
• swap_out(priority, gfp_mask, classzone)
• return nr_pages

13
MM for MIR Kernel

• MIR An Open Sourced Kernel by Indians
• Support for two level page tables
• Bitmap allocator for PAGE_SIZEd allocations
• Slab cache allocator for less than PAGE_SIZEd
  granularity
• vmalloc allocations for virtually contiguous,
  physically discontiguous allocations
• Basic support for per process virtual address
  space management
• Todo
• Support COW, mmap functionality and full fledged
  virtual address space management
• Implement a LRU based page replacement policy

14
References

• Linux Kernel Source Code
• www.surriel.com
• www.google.com