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)