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 associated with the
                     process.

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

              Clearing the PG_Referenced and ACCESSED/YOUNG bits provides a method to measure approximately  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 "Referenced" 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 system 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)