time_namespaces(7) — Linux manual page


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

NAME         top

       time_namespaces - overview of Linux time namespaces

DESCRIPTION         top

       Time namespaces virtualize the values of two system clocks:

         CLOCK_MONOTONIC_RAW), a nonsettable clock that represents monotonic
         time  since—as described  by  POSIX—"some  unspecified  point in
         the past".

       · CLOCK_BOOTTIME (and likewise CLOCK_BOOTTIME_ALARM), a nonsettable
         clock that is identical to CLOCK_MONOTONIC, except that it also
         includes 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
       unshare(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 namespace.  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

       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

           <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 off‐
       set (seconds plus nanoseconds) for the clock in this time namespace.
       These offsets are expressed relative to the clock values in the ini‐
       tial time namespace.  The offset-secs value can be negative, subject
       to restrictions 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
       multiple times, but after the first process has been created in or
       has entered 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 namespace.

       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;

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

              · 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

       In a new time namespace created by unshare(2), the contents of the
       timens_offsets file are inherited from the time namespace of the cre‐
       ating process.

NOTES         top

       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
       redability, the use of the symbolic names over the numbers is

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

EXAMPLES         top

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

           $ readlink /proc/$$/ns/time

       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_BOOTTIME 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 pro‐
       cesses 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 namespace, 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_chil‐
       dren 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/$$/ns/time
           ns2# readlink /proc/$$/ns/time_for_children
           ns2# readlink /proc/self/ns/time   # Creates a child process

       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         top

       nsenter(1), unshare(1), clock_settime(2), setns(2), unshare(2),
       namespaces(7), time(7)

COLOPHON         top

       This page is part of release 5.08 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

Linux                            2020-06-09               TIME_NAMESPACES(7)

Pages that refer to this page: nsenter(1)unshare(1)clock_getres(2)clock_gettime(2)clock_settime(2)unshare(2)namespaces(7)time(7)