TIME_NAMESPACES(7)



TIME_NAMESPACES(7)         Linux Programmer's Manual        TIME_NAMESPACES(7)

NAME
       time_namespaces - overview of Linux time namespaces

DESCRIPTION
       Time namespaces virtualize the values of two system clocks:

       o CLOCK_MONOTONIC  (and likewise CLOCK_MONOTONIC_COARSE and CLOCK_MONO-
         TONIC_RAW),  a  nonsettable  clock  that  represents  monotonic  time
         since--as  described   by   POSIX--"some   unspecified   point in the
         past".

       o CLOCK_BOOTTIME (and  likewise  CLOCK_BOOTTIME_ALARM),  a  nonsettable
         clock  that  is identical to CLOCK_MONOTONIC, except that it also in-
         cludes any time that the system is suspended.

       Thus, the processes in a time namespace share per-namespace values  for
       these  clocks.   This  affects  various APIs that measure against these
       clocks, including: clock_gettime(2), clock_nanosleep(2),  nanosleep(2),
       timer_settime(2), timerfd_settime(2), and /proc/uptime.

       Currently,  the  only  way to create a time namespace is by calling un-
       share(2) with the CLONE_NEWTIME flag.  This call  creates  a  new  time
       namespace  but does not place the calling process in the new namespace.
       Instead, the calling process's subsequently created children are placed
       in  the  new  namespace.  This allows clock offsets (see below) for the
       new namespace to be set before the first process is placed in the name-
       space.   The  /proc/[pid]/ns/time_for_children  symbolic link shows the
       time namespace in which the children of a process will be created.   (A
       process  can  use  a  file descriptor opened on this symbolic link in a
       call to setns(2) in order to move into the namespace.)

   /proc/PID/timens_offsets
       Associated with each time namespace are offsets, expressed with respect
       to  the initial time namespace, that define the values of the monotonic
       and boot-time clocks in that namespace.  These offsets are exposed  via
       the  file  /proc/PID/timens_offsets.  Within this file, the offsets are
       expressed as lines consisting of three space-delimited fields:

           <clock-id> <offset-secs> <offset-nanosecs>

       The clock-id is a string that identifies the clock  whose  offsets  are
       being  shown.   This field is either monotonic, for CLOCK_MONOTONIC, or
       boottime, for CLOCK_BOOTTIME.  The remaining fields express the  offset
       (seconds plus nanoseconds) for the clock in this time namespace.  These
       offsets are expressed relative to the clock values in the initial  time
       namespace.   The offset-secs value can be negative, subject to restric-
       tions noted below; offset-nanosecs is an unsigned value.

       In the initial time namespace, the contents of the timens_offsets  file
       are as follows:

           $ cat /proc/self/timens_offsets
           monotonic           0         0
           boottime            0         0

       In  a  new  time  namespace that has had no member processes, the clock
       offsets can be modified by writing newline-terminated  records  of  the
       same  form to the timens_offsets file.  The file can be written to mul-
       tiple times, but after the first process has been created in or has en-
       tered the namespace, write(2)s on this file fail with the error EACCES.
       In order to write to the timens_offsets file, a process must  have  the
       CAP_SYS_TIME  capability in the user namespace that owns the time name-
       space.

       Writes to the timens_offsets file can fail with the following errors:

       EINVAL An offset-nanosecs value is greater than 999,999,999.

       EINVAL A clock-id value is not valid.

       EPERM  The caller does not have the the CAP_SYS_TIME capability.

       ERANGE An offset-secs value is out of range.  In particular;

              o offset-secs can't be set to a value which would make the  cur-
                rent  time  on  the corresponding clock inside the namespace a
                negative value; and

              o offset-secs can't be set to a value such that the time on  the
                corresponding  clock inside the namespace would exceed half of
                the value of the kernel constant  KTIME_SEC_MAX  (this  limits
                the clock value to a maximum of approximately 146 years).

       In  a  new  time  namespace  created by unshare(2), the contents of the
       timens_offsets file are inherited from the time namespace of the creat-
       ing process.

NOTES
       Use  of  time  namespaces requires a kernel that is configured with the
       CONFIG_TIME_NS option.

       Note that time namespaces do not virtualize the  CLOCK_REALTIME  clock.
       Virtualization  of this clock was avoided for reasons of complexity and
       overhead within the kernel.

       For compatibility with  the  initial  implementation,  when  writing  a
       clock-id  to  the /proc/[pid]/timens_offsets file, the numerical values
       of the IDs can be written instead of the  symbolic  names  show  above;
       i.e.,  1 instead of monotonic, and 7 instead of boottime.  For redabil-
       ity, the use of the symbolic names over the numbers is preferred.

       The motivation for adding time namespaces was to  allow  the  monotonic
       and boot-time clocks to maintain consistent values during container mi-
       gration and checkpoint/restore.

EXAMPLES
       The following shell session demonstrates the operation  of  time  name-
       spaces.   We begin by displaying the inode number of the time namespace
       of a shell in the initial time namespace:

           $ readlink /proc/$$/ns/time
           time:[4026531834]

       Continuing in the initial time namespace, we display the system  uptime
       using  uptime(1)  and  use  the  clock_times  example  program shown in
       clock_getres(2) to display the values of various clocks:

           $ uptime --pretty
           up 21 hours, 17 minutes
           $ ./clock_times
           CLOCK_REALTIME : 1585989401.971 (18356 days +  8h 36m 41s)
           CLOCK_TAI      : 1585989438.972 (18356 days +  8h 37m 18s)
           CLOCK_MONOTONIC:      56338.247 (15h 38m 58s)
           CLOCK_BOOTTIME :      76633.544 (21h 17m 13s)

       We then use unshare(1) to create a time namespace and execute a bash(1)
       shell.   From  the new shell, we use the built-in echo command to write
       records to  the  timens_offsets  file  adjusting  the  offset  for  the
       CLOCK_MONOTONIC clock forward 2 days and the offset for the CLOCK_BOOT-
       TIME clock forward 7 days:

           $ PS1="ns2# " sudo unshare -T -- bash --norc
           ns2# echo "monotonic $((2*24*60*60)) 0" > /proc/$$/timens_offsets
           ns2# echo "boottime  $((7*24*60*60)) 0" > /proc/$$/timens_offsets

       Above, we started the bash(1) shell with the --norc options so that  no
       start-up  scripts  were executed.  This ensures that no child processes
       are created from the shell before  we  have  a  chance  to  update  the
       timens_offsets file.

       We  then use cat(1) to display the contents of the timens_offsets file.
       The execution of cat(1) creates the first process in the new time name-
       space,  after  which further attempts to update the timens_offsets file
       produce an error.

           ns2# cat /proc/$$/timens_offsets
           monotonic      172800         0
           boottime       604800         0
           ns2# echo "boottime $((9*24*60*60)) 0" > /proc/$$/timens_offsets
           bash: echo: write error: Permission denied

       Continuing  in  the  new  namespace,  we  execute  uptime(1)  and   the
       clock_times example program:

           ns2# uptime --pretty
           up 1 week, 21 hours, 18 minutes
           ns2# ./clock_times
           CLOCK_REALTIME : 1585989457.056 (18356 days +  8h 37m 37s)
           CLOCK_TAI      : 1585989494.057 (18356 days +  8h 38m 14s)
           CLOCK_MONOTONIC:     229193.332 (2 days + 15h 39m 53s)
           CLOCK_BOOTTIME :     681488.629 (7 days + 21h 18m  8s)

       From  the  above  output,  we  can see that the monotonic and boot-time
       clocks have different values in the new time namespace.

       Examining the /proc/[pid]/ns/time and  /proc/[pid]/ns/time_for_children
       symbolic  links,  we see that the shell is a member of the initial time
       namespace, but its children are created in the new namespace.

           ns2# readlink /proc/$$/ns/time
           time:[4026531834]
           ns2# readlink /proc/$$/ns/time_for_children
           time:[4026532900]
           ns2# readlink /proc/self/ns/time   # Creates a child process
           time:[4026532900]

       Returning to the shell in the initial time namespace, we see  that  the
       monotonic  and  boot-time  clocks  are unaffected by the timens_offsets
       changes that were made in the other time namespace:

           $ uptime --pretty
           up 21 hours, 19 minutes
           $ ./clock_times
           CLOCK_REALTIME : 1585989401.971 (18356 days +  8h 38m 51s)
           CLOCK_TAI      : 1585989438.972 (18356 days +  8h 39m 28s)
           CLOCK_MONOTONIC:      56338.247 (15h 41m  8s)
           CLOCK_BOOTTIME :      76633.544 (21h 19m 23s)

SEE ALSO
       nsenter(1), unshare(1), clock_settime(2), setns(2),  unshare(2),  name-
       spaces(7), time(7)

COLOPHON
       This  page  is  part of release 5.07 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2020-06-09                TIME_NAMESPACES(7)

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