man5/proc_pid_clear_refs.5

   1 .\" Copyright (C) 1994, 1995, Daniel Quinlan <quinlan@yggdrasil.com>
   2 .\" Copyright (C) 2002-2008, 2017, Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\" Copyright (C) 2023, Alejandro Colomar <alx@kernel.org>
   4 .\"
   5 .\" SPDX-License-Identifier: GPL-3.0-or-later
   6 .\"
   7 .TH proc_pid_clear_refs 5 (date) "Linux man-pages (unreleased)"
   8 .SH NAME
   9 /proc/pid/clear_refs \- reset the PG_Referenced and ACCESSED/YOUNG bits
  10 .SH DESCRIPTION
  11 .TP
  12 .IR /proc/ pid /clear_refs " (since Linux 2.6.22)"
  13 .\" commit b813e931b4c8235bb42e301096ea97dbdee3e8fe (2.6.22)
  14 .\" commit 398499d5f3613c47f2143b8c54a04efb5d7a6da9 (2.6.32)
  15 .\" commit 040fa02077de01c7e08fa75be6125e4ca5636011 (3.11)
  16 .\"
  17 .\"       "Clears page referenced bits shown in smaps output"
  18 .\"       write-only, writable only by the owner of the process
  19 .IP
  20 This is a write-only file, writable only by owner of the process.
  21 .IP
  22 The following values may be written to the file:
  23 .RS
  24 .TP
  25 1 (since Linux 2.6.22)
  26 .\" Internally: CLEAR_REFS_ALL
  27 Reset the PG_Referenced and ACCESSED/YOUNG
  28 bits for all the pages associated with the process.
  29 (Before Linux 2.6.32, writing any nonzero value to this file
  30 had this effect.)
  31 .TP
  32 2 (since Linux 2.6.32)
  33 .\" Internally: CLEAR_REFS_ANON
  34 Reset the PG_Referenced and ACCESSED/YOUNG
  35 bits for all anonymous pages associated with the process.
  36 .TP
  37 3 (since Linux 2.6.32)
  38 .\" Internally: CLEAR_REFS_MAPPED
  39 Reset the PG_Referenced and ACCESSED/YOUNG
  40 bits for all file-mapped pages associated with the process.
  41 .RE
  42 .IP
  43 Clearing the PG_Referenced and ACCESSED/YOUNG bits provides a method
  44 to measure approximately how much memory a process is using.
  45 One first inspects the values in the "Referenced" fields
  46 for the VMAs shown in
  47 .IR /proc/ pid /smaps
  48 to get an idea of the memory footprint of the
  49 process.
  50 One then clears the PG_Referenced and ACCESSED/YOUNG bits
  51 and, after some measured time interval,
  52 once again inspects the values in the "Referenced" fields
  53 to get an idea of the change in memory footprint of the
  54 process during the measured interval.
  55 If one is interested only in inspecting the selected mapping types,
  56 then the value 2 or 3 can be used instead of 1.
  57 .IP
  58 Further values can be written to affect different properties:
  59 .RS
  60 .TP
  61 4 (since Linux 3.11)
  62 Clear the soft-dirty bit for all the pages associated with the process.
  63 .\" Internally: CLEAR_REFS_SOFT_DIRTY
  64 This is used (in conjunction with
  65 .IR /proc/ pid /pagemap )
  66 by the check-point restore system to discover which pages of a process
  67 have been dirtied since the file
  68 .IR /proc/ pid /clear_refs
  69 was written to.
  70 .TP
  71 5 (since Linux 4.0)
  72 .\" Internally: CLEAR_REFS_MM_HIWATER_RSS
  73 Reset the peak resident set size ("high water mark") to the process's
  74 current resident set size value.
  75 .RE
  76 .IP
  77 Writing any value to
  78 .IR /proc/ pid /clear_refs
  79 other than those listed above has no effect.
  80 .IP
  81 The
  82 .IR /proc/ pid /clear_refs
  83 file is present only if the
  84 .B CONFIG_PROC_PAGE_MONITOR
  85 kernel configuration option is enabled.
  86 .SH SEE ALSO
  87 .BR proc (5)