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Faculty of Mathematics

Energy Saving, Shutdowns and Reboots

Historical Introduction

In the past, Maths Linux computers were mostly kept switched on, and you may have been told not to turn them off. There were several reasons for this, but they no longer apply and there is now more concern about saving energy. Before we discovered Wake-on-LAN we often had to go and switch on computers which had been turned off. Users did not always check that nobody else was running a job on their machine before turning it off, and DAMTP computers were mostly running jobs constantly so there was no point in configuring them to power-save, and were also exporting their /scratch spaces over the network. Some desktops still export data spaces and these have been configured not to power-save.

Power Saving

Most of the Maths Linux computers will shut down automatically when they have been idle and unused for half an hour. (To be precise, the machine checks for user processes every 30 minutes. If it finds nothing on two successive checks, it will shut itself down.) There are exceptions for servers including machines which NFS-export data spaces, so you won't find yourself cut off from your data because the computer on which it is stored is down.

Windows computers will go into hibernation when they have been idle for a period of time, whether anyone is logged in or not.

Monthly Reboots

Linux computers will reboot every month to install security updates including a new kernel. Generally this will happen on the last Wednesday of every month in the small hours of the morning - however the exact timing may vary, especially if the computer has been off for a while. The best way to minimise the disruption this causes is to get into the habit of logging off when you have finished using your computer for the day. This gives it a chance to reboot when you are not using it, so that it doesn't need to reboot when you are using it.

Note: if you are leaving your computer idle and it is failing to switch itself off, a process has probably got stuck. You may be able to shut it down manually with safeshutdown.

If there is a critical security update it may be necessary to reboot the Linux computers at short notice.

No attempt is made to protect long running jobs from being killed by the monthly reboots, and you must ensure that your jobs checkpoint regularly.

Advice and policies on long jobs

The monthly reboots are advertised on the Maths Computing News page. You can also sign up to receive them by email:

Shutting Down and Rebooting Your PC

You can shut down a Maths Linux PC with the command safeshutdown or reboot it with safereboot. These commands will check that no-one else is using the computer.

Most other ways to shut down / restart the computer (e.g. the Shut Down menu option) have been disabled so that you cannot accidentally shut down a machine which someone else is using. If the safeshutdown command fails, please do not just turn the machine off with the power button. Please email help@maths explaining why you wanted to switch off the machine and we will investigate.

If you have a reasonably new Linux PC (less than 5 years old) in your office, be aware that other people may want to use it for number-crunching jobs.

  • It's fine to disconnect the monitor, keyboard or mouse for use with your laptop, but please leave newer Linux PCs connected to power and network.
  • Unplugging PCs from the network so you can plug in your laptop won't work because laptops are connected to a separate network. Please email help@maths to request a laptop network connection in your office if the wifi is not sufficient.
  • If you want your PC removed from your office email help@maths. We may be able to find it a good home.
  • If your PC is making a lot of noise email help@maths.
  • If your PC is overloaded with someone else's jobs please contact them directly. Our policy on running long jobs

Windows computers can be rebooted via the "Sleep, Shutdown or Reboot" desktop icon. If you need to shut down a Windows machine please email help@maths.

Waking up PCs Remotely

Alternatively, you can use ssh.maths.cam.ac.uk which is always switched on.

Energy Saving In General

  • Please leave monitors on. They will automatically go into a power-saving mode when left idle, using less than 1 Watt, and the PC will be able to ask the monitor for the screen resolution when it is rebooted.
  • Office printers, but not public printers, can safely be turned off.
  • Tips on saving energy at work (external link)