Uptime

Key takeaways

Uptime belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Uptime to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Uptime from memory before moving on to harder problems.

Reference excerpt

Uptime is a measure of system reliability, expressed as the period of time a machine, typically a computer, has been continuously working and available. Uptime is the opposite of downtime.

It is often used as a measure of computer operating system reliability or stability, in that this time represents the time a computer can be left unattended without crashing or needing to be rebooted for administrative or maintenance purposes. Conversely, long uptime may indicate negligence, because some critical updates can require reboots on some platforms.

Records In 2005, Novell reported a server with a 6-year uptime. This level of uptime is common when servers are maintained under an industrial context and host critical applications such as banking systems. Netcraft maintains the uptime records for many thousands of web hosting computers. A server running Novell NetWare has been reported to have been shut down after 16 years of uptime due to a failing hard disk.

Determining system uptime

Unix-like systems (Linux, FreeBSD)

Using uptime Most Unix-like systems including Linux, FreeBSD, Mac OS X, and SySVr4 have the uptime command. It also displays the system load averages for the past 1, 5, and 15-minute intervals:

This command originated in 3.0BSD and has become ubiquitous among Unix systems ever since. Despite almost always providing the same collection of information and being so ubiquitous, it is not part of any Unix-related standard, neither SUS nor POSIX. On Linux, an uptime command is part of GNU coreutils, BusyBox and Toybox. On BSD, the uptime command is hard link to the w program. The w program is based on the RSTS/E, TOPS-10, and TOPS-20 SYSTAT program.

Using system-specific interfaces Each Unix-like system also has its own private interfaces for obtaining the system uptime.

Linux On Linux, several procfs files provide information on the system uptime. The main file is /proc/uptime, but /proc/stat provides related information too.

The first number is the total number of seconds the system has been up. The second number is how much of that time the machine has spent idle, in seconds. On multi-core systems (and some Linux versions) the second number is the sum of the idle time accumulated by each CPU. The C API for uptime information is sysinfo().

BSD On BSD systems and macOS (a combination of BSD and XNU parts), the uptime information is available from the sysctl system, both in the form of the sysctl command and the sysctl() C function. The sysctl entry is called kern.boottime, which provides boot time that can be converted to an uptime by subtracting it from the current time:

macOS also provides the uptime as clock_gettime(CLOCK_MONOTONIC_RAW), a POSIX interface with implementation-defined starting point; macOS sets the starting point at system boot. mach_timebase_info() and CACurrentMediaTime() are additional ways to access the same raw monotonic clock. On FreeBSD (but not macOS), the kernel exports three symbols that can be used as extern variables in C to obtain the boot time and uptime. They are called boottime, time_second, and time_uptime. These require no function call at all to use, unlike sysctl(). Higher-precision uptime is provided from functions such as microuptime().

Microsoft Windows

Windows Task Manager

Some versions of Microsoft Windows include an uptime field in Windows Task Manager, under the "Performance" tab. The format is D:HH:MM:SS (days, hours, minutes, seconds).

systeminfo The output of the systeminfo command includes a "System Up Time" or "System Boot Time" field.

The exact text and format are dependent on the language and locale. The time given by systeminfo is not reliable. It does not take into account time spent in sleep or hibernation. Thus, the boot time will drift forward every time the computer sleeps or hibernates.

NET command The NET command with its STATISTICS sub-command provides the date and time the computer started, for both the NET STATISTICS WORKSTATION and NET STATISTICS SERVER variants. The command NET STATS SRV is shorthand for NET STATISTICS SERVER. The exact text and date format is dependent on the configured language and locale.

Windows Management Instrumentation (WMI) Uptime can be determined via Windows Management Instrumentation (WMI), by querying the LastBootUpTime property of the Win32_OperatingSystem class. At the command prompt, this can be done using the wmic command:

The timestamp uses the format yyyymmddhhmmss.nnn, so in the above example, the computer last booted up on 8 May 2011 at 16:17:51.822. The text "LastBootUpTime" and the timestamp format do not vary with language or locale. WMI can also be queried using a variety of application programming interfaces, including VBScript or PowerShell.

Uptime.exe Microsoft formerly provided a downloadable utility called Uptime.exe, which reports elapsed time in days, hours, minutes, and seconds.

The time given by Uptime.exe is not reliable. It does not take into account time spent in sleep or hibernation. Thus, the boot time will drift forward every time the computer sleeps or hibernates.

FreeDOS The uptime command is also available for FreeDOS. The version was developed by M. Aitchison.

OpenVMS On OpenVMS systems, the show system command can be used at the DCL command prompt to obtain the system uptime. The first line of the resulting display includes the system's uptime, displayed as days followed by hours:minutes:seconds. In the following example, the command qualifier /noprocess suppresses the display of per-process detail lines of information.

The command output above shows that node JACK on 29 January 2008 at 16:32:04.67 has an uptime of 894 days 22 hours 28 minutes and 52 seconds.

Illustrations

Worked examples

Example 1 — a first encounter with Uptime

Start with the simplest possible case. Write down what Uptime claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Uptime before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Uptime ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Uptime

In research: Uptime appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Uptime in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Uptime is common in secondary-school and first-year university syllabi. It links to neighbouring topics Fault-tolerant computer systems, Real-time computing, Unix user management and support-related utilities, so understanding it makes those chapters shorter.
In everyday life: Look for Uptime outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Uptime in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Uptime means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Uptime out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Uptime in simple terms?

Uptime is a measure of system reliability, expressed as the period of time a machine, typically a computer, has been continuously working and available. Uptime is the opposite of downtime.

Why does Uptime matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Uptime?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Uptime.