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 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)

Linux man-pages 6.9.1             2024-05-02            proc_pid_clear_refs(5)