1TIME_NAMESPACES(7)         Linux Programmer's Manual        TIME_NAMESPACES(7)
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NAME

6       time_namespaces - overview of Linux time namespaces
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DESCRIPTION

9       Time namespaces virtualize the values of two system clocks:
10
11CLOCK_MONOTONIC  (and likewise CLOCK_MONOTONIC_COARSE and CLOCK_MONO‐
12         TONIC_RAW),  a  nonsettable  clock  that  represents  monotonic  time
13         since—as described  by  POSIX—"some  unspecified  point in the past".
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15CLOCK_BOOTTIME  (and  likewise  CLOCK_BOOTTIME_ALARM),  a nonsettable
16         clock that is identical to CLOCK_MONOTONIC, except that it  also  in‐
17         cludes any time that the system is suspended.
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19       Thus,  the processes in a time namespace share per-namespace values for
20       these clocks.  This affects various APIs  that  measure  against  these
21       clocks,  including: clock_gettime(2), clock_nanosleep(2), nanosleep(2),
22       timer_settime(2), timerfd_settime(2), and /proc/uptime.
23
24       Currently, the only way to create a time namespace is  by  calling  un‐
25       share(2)  with  the  CLONE_NEWTIME  flag.  This call creates a new time
26       namespace but does not place the calling process in the new  namespace.
27       Instead, the calling process's subsequently created children are placed
28       in the new namespace.  This allows clock offsets (see  below)  for  the
29       new namespace to be set before the first process is placed in the name‐
30       space.  The /proc/[pid]/ns/time_for_children symbolic  link  shows  the
31       time  namespace in which the children of a process will be created.  (A
32       process can use a file descriptor opened on this  symbolic  link  in  a
33       call to setns(2) in order to move into the namespace.)
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35   /proc/PID/timens_offsets
36       Associated with each time namespace are offsets, expressed with respect
37       to the initial time namespace, that define the values of the  monotonic
38       and  boot-time clocks in that namespace.  These offsets are exposed via
39       the file /proc/PID/timens_offsets.  Within this file, the  offsets  are
40       expressed as lines consisting of three space-delimited fields:
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42           <clock-id> <offset-secs> <offset-nanosecs>
43
44       The  clock-id  is  a string that identifies the clock whose offsets are
45       being shown.  This field is either monotonic, for  CLOCK_MONOTONIC,  or
46       boottime,  for CLOCK_BOOTTIME.  The remaining fields express the offset
47       (seconds plus nanoseconds) for the clock in this time namespace.  These
48       offsets  are expressed relative to the clock values in the initial time
49       namespace.  The offset-secs value can be negative, subject to  restric‐
50       tions noted below; offset-nanosecs is an unsigned value.
51
52       In  the initial time namespace, the contents of the timens_offsets file
53       are as follows:
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55           $ cat /proc/self/timens_offsets
56           monotonic           0         0
57           boottime            0         0
58
59       In a new time namespace that has had no  member  processes,  the  clock
60       offsets  can  be  modified by writing newline-terminated records of the
61       same form to the timens_offsets file.  The file can be written to  mul‐
62       tiple times, but after the first process has been created in or has en‐
63       tered the namespace, write(2)s on this file fail with the error EACCES.
64       In  order  to write to the timens_offsets file, a process must have the
65       CAP_SYS_TIME capability in the user namespace that owns the time  name‐
66       space.
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68       Writes to the timens_offsets file can fail with the following errors:
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70       EINVAL An offset-nanosecs value is greater than 999,999,999.
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72       EINVAL A clock-id value is not valid.
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74       EPERM  The caller does not have the CAP_SYS_TIME capability.
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76       ERANGE An offset-secs value is out of range.  In particular;
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78offset-secs  can't be set to a value which would make the cur‐
79                rent time on the corresponding clock inside  the  namespace  a
80                negative value; and
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82offset-secs  can't be set to a value such that the time on the
83                corresponding clock inside the namespace would exceed half  of
84                the  value  of  the kernel constant KTIME_SEC_MAX (this limits
85                the clock value to a maximum of approximately 146 years).
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87       In a new time namespace created by  unshare(2),  the  contents  of  the
88       timens_offsets file are inherited from the time namespace of the creat‐
89       ing process.
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NOTES

92       Use of time namespaces requires a kernel that is  configured  with  the
93       CONFIG_TIME_NS option.
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95       Note  that  time namespaces do not virtualize the CLOCK_REALTIME clock.
96       Virtualization of this clock was avoided for reasons of complexity  and
97       overhead within the kernel.
98
99       For  compatibility  with  the  initial  implementation,  when writing a
100       clock-id to the /proc/[pid]/timens_offsets file, the  numerical  values
101       of  the  IDs  can  be written instead of the symbolic names show above;
102       i.e., 1 instead of monotonic, and 7 instead of boottime.  For readabil‐
103       ity, the use of the symbolic names over the numbers is preferred.
104
105       The  motivation  for  adding time namespaces was to allow the monotonic
106       and boot-time clocks to maintain consistent values during container mi‐
107       gration and checkpoint/restore.
108

EXAMPLES

110       The  following  shell  session demonstrates the operation of time name‐
111       spaces.  We begin by displaying the inode number of the time  namespace
112       of a shell in the initial time namespace:
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114           $ readlink /proc/$$/ns/time
115           time:[4026531834]
116
117       Continuing  in the initial time namespace, we display the system uptime
118       using uptime(1) and  use  the  clock_times  example  program  shown  in
119       clock_getres(2) to display the values of various clocks:
120
121           $ uptime --pretty
122           up 21 hours, 17 minutes
123           $ ./clock_times
124           CLOCK_REALTIME : 1585989401.971 (18356 days +  8h 36m 41s)
125           CLOCK_TAI      : 1585989438.972 (18356 days +  8h 37m 18s)
126           CLOCK_MONOTONIC:      56338.247 (15h 38m 58s)
127           CLOCK_BOOTTIME :      76633.544 (21h 17m 13s)
128
129       We then use unshare(1) to create a time namespace and execute a bash(1)
130       shell.  From the new shell, we use the built-in echo command  to  write
131       records  to  the  timens_offsets  file  adjusting  the  offset  for the
132       CLOCK_MONOTONIC clock forward 2 days and the offset for the CLOCK_BOOT‐
133       TIME clock forward 7 days:
134
135           $ PS1="ns2# " sudo unshare -T -- bash --norc
136           ns2# echo "monotonic $((2*24*60*60)) 0" > /proc/$$/timens_offsets
137           ns2# echo "boottime  $((7*24*60*60)) 0" > /proc/$$/timens_offsets
138
139       Above,  we started the bash(1) shell with the --norc options so that no
140       start-up scripts were executed.  This ensures that no  child  processes
141       are  created  from  the  shell  before  we  have a chance to update the
142       timens_offsets file.
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144       We then use cat(1) to display the contents of the timens_offsets  file.
145       The execution of cat(1) creates the first process in the new time name‐
146       space, after which further attempts to update the  timens_offsets  file
147       produce an error.
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149           ns2# cat /proc/$$/timens_offsets
150           monotonic      172800         0
151           boottime       604800         0
152           ns2# echo "boottime $((9*24*60*60)) 0" > /proc/$$/timens_offsets
153           bash: echo: write error: Permission denied
154
155       Continuing   in  the  new  namespace,  we  execute  uptime(1)  and  the
156       clock_times example program:
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158           ns2# uptime --pretty
159           up 1 week, 21 hours, 18 minutes
160           ns2# ./clock_times
161           CLOCK_REALTIME : 1585989457.056 (18356 days +  8h 37m 37s)
162           CLOCK_TAI      : 1585989494.057 (18356 days +  8h 38m 14s)
163           CLOCK_MONOTONIC:     229193.332 (2 days + 15h 39m 53s)
164           CLOCK_BOOTTIME :     681488.629 (7 days + 21h 18m  8s)
165
166       From the above output, we can see  that  the  monotonic  and  boot-time
167       clocks have different values in the new time namespace.
168
169       Examining  the /proc/[pid]/ns/time and /proc/[pid]/ns/time_for_children
170       symbolic links, we see that the shell is a member of the  initial  time
171       namespace, but its children are created in the new namespace.
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173           ns2# readlink /proc/$$/ns/time
174           time:[4026531834]
175           ns2# readlink /proc/$$/ns/time_for_children
176           time:[4026532900]
177           ns2# readlink /proc/self/ns/time   # Creates a child process
178           time:[4026532900]
179
180       Returning  to  the shell in the initial time namespace, we see that the
181       monotonic and boot-time clocks are  unaffected  by  the  timens_offsets
182       changes that were made in the other time namespace:
183
184           $ uptime --pretty
185           up 21 hours, 19 minutes
186           $ ./clock_times
187           CLOCK_REALTIME : 1585989401.971 (18356 days +  8h 38m 51s)
188           CLOCK_TAI      : 1585989438.972 (18356 days +  8h 39m 28s)
189           CLOCK_MONOTONIC:      56338.247 (15h 41m  8s)
190           CLOCK_BOOTTIME :      76633.544 (21h 19m 23s)
191

SEE ALSO

193       nsenter(1),  unshare(1),  clock_settime(2), setns(2), unshare(2), name‐
194       spaces(7), time(7)
195

COLOPHON

197       This page is part of release 5.12 of the Linux  man-pages  project.   A
198       description  of  the project, information about reporting bugs, and the
199       latest    version    of    this    page,    can     be     found     at
200       https://www.kernel.org/doc/man-pages/.
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204Linux                             2021-03-22                TIME_NAMESPACES(7)
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