time_namespaces(7) - Linux manual page (original) (raw)

timenamespaces(7) Miscellaneous Information Manual timenamespaces(7)

NAME top

   time_namespaces - overview of Linux time namespaces

DESCRIPTION top

   Time namespaces virtualize the values of two system clocks:

   •  **CLOCK_MONOTONIC** (and likewise **CLOCK_MONOTONIC_COARSE** and
      **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)](../man2/clock%5Fgettime.2.html), [clock_nanosleep(2)](../man2/clock%5Fnanosleep.2.html),
   [nanosleep(2)](../man2/nanosleep.2.html), [timer_settime(2)](../man2/timer%5Fsettime.2.html), [timerfd_settime(2)](../man2/timerfd%5Fsettime.2.html), and
   _/proc/uptime_.

   Currently, the only way to create a time namespace is by calling
   [unshare(2)](../man2/unshare.2.html) 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/timeforchildren_ 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)](../man2/setns.2.html) in order to move into the namespace.)

[1m/proc/[24mpid_/timensoffsets_ 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/timensoffsets. 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 restrictions noted below;
   _offset-nanosecs_ is an unsigned value.

   In the initial time namespace, the contents of the _timensoffsets_
   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 _timensoffsets_ 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)](../man2/write.2.html)s on this file
   fail with the error **EACCES**.  In order to write to the
   _timensoffsets_ file, a process must have the **CAP_SYS_TIME**
   capability in the user namespace that owns the time namespace.

   Writes to the _timensoffsets_ 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 **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 years).

   In a new time namespace created by [unshare(2)](../man2/unshare.2.html), the contents of the
   _timensoffsets_ file are inherited from the time namespace of the
   creating 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_/timensoffsets_ 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 readability, 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 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**
       time:[4026531834]

   Continuing in the initial time namespace, we display the system
   uptime using [uptime(1)](../man1/uptime.1.html) and use the _clocktimes_ example program
   shown in [clock_getres(2)](../man2/clock%5Fgetres.2.html) 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)](../man1/unshare.1.html) to create a time namespace and execute a
   [bash(1)](../man1/bash.1.html) shell.  From the new shell, we use the built-in **echo**
   command to write records to the _timensoffsets_ 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 <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo stretchy="false">(</mo><mo stretchy="false">(</mo><mn>2</mn><mo>∗</mo><mn>24</mn><mo>∗</mo><mn>60</mn><mo>∗</mo><mn>60</mn><mo stretchy="false">)</mo><mo stretchy="false">)</mo><mn>0</mn><mi mathvariant="normal">&quot;</mi><mo>&gt;</mo><mi mathvariant="normal">/</mi><mi>p</mi><mi>r</mi><mi>o</mi><mi>c</mi><mi mathvariant="normal">/</mi></mrow><annotation encoding="application/x-tex">((2*24*60*60)) 0&quot; &gt; /proc/</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mopen">((</span><span class="mord">2</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.6444em;"></span><span class="mord">24</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.6444em;"></span><span class="mord">60</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">60</span><span class="mclose">))</span><span class="mord">0&quot;</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">&gt;</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">/</span><span class="mord mathnormal">p</span><span class="mord mathnormal">roc</span><span class="mord">/</span></span></span></span>$/timens_offsets**
       ns2# **echo "boottime  <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo stretchy="false">(</mo><mo stretchy="false">(</mo><mn>7</mn><mo>∗</mo><mn>24</mn><mo>∗</mo><mn>60</mn><mo>∗</mo><mn>60</mn><mo stretchy="false">)</mo><mo stretchy="false">)</mo><mn>0</mn><mi mathvariant="normal">&quot;</mi><mo>&gt;</mo><mi mathvariant="normal">/</mi><mi>p</mi><mi>r</mi><mi>o</mi><mi>c</mi><mi mathvariant="normal">/</mi></mrow><annotation encoding="application/x-tex">((7*24*60*60)) 0&quot; &gt; /proc/</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mopen">((</span><span class="mord">7</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.6444em;"></span><span class="mord">24</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.6444em;"></span><span class="mord">60</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">60</span><span class="mclose">))</span><span class="mord">0&quot;</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">&gt;</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">/</span><span class="mord mathnormal">p</span><span class="mord mathnormal">roc</span><span class="mord">/</span></span></span></span>$/timens_offsets**

   Above, we started the [bash(1)](../man1/bash.1.html) shell with the **--norc** option 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 _timensoffsets_ file.

   We then use [cat(1)](../man1/cat.1.html) to display the contents of the _timensoffsets_
   file.  The execution of [cat(1)](../man1/cat.1.html) creates the first process in the
   new time namespace, after which further attempts to update the
   _timensoffsets_ file produce an error.

       ns2# **cat /proc/$$/timens_offsets**
       monotonic      172800         0
       boottime       604800         0
       ns2# **echo "boottime <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mo stretchy="false">(</mo><mo stretchy="false">(</mo><mn>9</mn><mo>∗</mo><mn>24</mn><mo>∗</mo><mn>60</mn><mo>∗</mo><mn>60</mn><mo stretchy="false">)</mo><mo stretchy="false">)</mo><mn>0</mn><mi mathvariant="normal">&quot;</mi><mo>&gt;</mo><mi mathvariant="normal">/</mi><mi>p</mi><mi>r</mi><mi>o</mi><mi>c</mi><mi mathvariant="normal">/</mi></mrow><annotation encoding="application/x-tex">((9*24*60*60)) 0&quot; &gt; /proc/</annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mopen">((</span><span class="mord">9</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.6444em;"></span><span class="mord">24</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:0.6444em;"></span><span class="mord">60</span><span class="mspace" style="margin-right:0.2222em;"></span><span class="mbin">∗</span><span class="mspace" style="margin-right:0.2222em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">60</span><span class="mclose">))</span><span class="mord">0&quot;</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">&gt;</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:1em;vertical-align:-0.25em;"></span><span class="mord">/</span><span class="mord mathnormal">p</span><span class="mord mathnormal">roc</span><span class="mord">/</span></span></span></span>$/timens_offsets**
       bash: echo: write error: Permission denied

   Continuing in the new namespace, we execute [uptime(1)](../man1/uptime.1.html) and the
   _clocktimes_ 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/timeforchildren_
   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
   _timensoffsets_ 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)

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Linux man-pages 6.10 2024-05-02 timenamespaces(7)