TIME_NAMESPACES(7) Linux Programmer's Manual TIME_NAMESPACES(7)
time_namespaces - overview of Linux time namespaces
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), 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
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 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
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 cre‐
ating process.
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
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.
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/$$/ns/time
time:[4026531834]
Continuing in the initial time namespace, we display the system
uptime using uptime(1) and use the clock_times example program shown
in clock_getres(2) to display the values of various clocks:
$ uptime --pretty
up 21 hours, 17 minutes
$ ./clock_times
CLOCK_REALTIME : 1585989401.971 (18356 days + 8h 36m 41s)
CLOCK_TAI : 1585989438.972 (18356 days + 8h 37m 18s)
CLOCK_MONOTONIC: 56338.247 (15h 38m 58s)
CLOCK_BOOTTIME : 76633.544 (21h 17m 13s)
We then use unshare(1) to create a time namespace and execute a
bash(1) shell. From the new shell, we use the built-in echo command
to write records to the timens_offsets file adjusting the offset for
the CLOCK_MONOTONIC clock forward 2 days and the offset for the
CLOCK_BOOTTIME clock forward 7 days:
$ PS1="ns2# " sudo unshare -T -- bash --norc
ns2# echo "monotonic $((2*24*60*60)) 0" > /proc/$$/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
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)
nsenter(1), unshare(1), clock_settime(2), setns(2), unshare(2),
namespaces(7), time(7)
This page is part of release 5.08 of the Linux man-pages project. A
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latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
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)
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