man > proc_pid_clear_refs(5)

proc_pid_clear_refs(5)                   File Formats Manual                  proc_pid_clear_refs(5)

NAME
       /proc/pid/clear_refs - reset the PG_Referenced and ACCESSED/YOUNG bits

DESCRIPTION
       /proc/pid/clear_refs (since Linux 2.6.22)

              This is a write-only file, writable only by owner of the process.

              The following values may be written to the file:

              1 (since Linux 2.6.22)
                     Reset  the  PG_Referenced  and ACCESSED/YOUNG bits for all the pages associated
                     with the process.  (Before Linux 2.6.32, writing any nonzero value to this file
                     had this effect.)

              2 (since Linux 2.6.32)
                     Reset the PG_Referenced and ACCESSED/YOUNG bits for all anonymous pages associ‐
                     ated with the process.

              3 (since Linux 2.6.32)
                     Reset the PG_Referenced and ACCESSED/YOUNG bits for all file-mapped pages asso‐
                     ciated with the process.

              Clearing the PG_Referenced and ACCESSED/YOUNG bits provides a method  to  measure  ap‐
              proximately  how much memory a process is using.  One first inspects the values in the
              "Referenced" fields for the VMAs shown in /proc/pid/smaps to get an idea of the memory
              footprint of the process.  One then clears the PG_Referenced and  ACCESSED/YOUNG  bits
              and,  after some measured time interval, once again inspects the values in the "Refer‐
              enced" fields to get an idea of the change in memory footprint of the  process  during
              the  measured  interval.  If one is interested only in inspecting the selected mapping
              types, then the value 2 or 3 can be used instead of 1.

              Further values can be written to affect different properties:

              4 (since Linux 3.11)
                     Clear the soft-dirty bit for all the pages associated with the  process.   This
                     is used (in conjunction with /proc/pid/pagemap) by the check-point restore sys‐
                     tem  to  discover  which  pages  of  a process have been dirtied since the file
                     /proc/pid/clear_refs was written to.

              5 (since Linux 4.0)
                     Reset the peak resident set size ("high water mark") to the  process's  current
                     resident set size value.

              Writing any value to /proc/pid/clear_refs other than those listed above has no effect.

              The  /proc/pid/clear_refs  file is present only if the CONFIG_PROC_PAGE_MONITOR kernel
              configuration option is enabled.

SEE ALSO
       proc(5)

Linux man-pages 6.7                          2023-09-07                       proc_pid_clear_refs(5)