How to recover from a Frozen System

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How to recover from a Frozen System

Postby hugh on Sun Sep 23, 2007 5:03 pm

Came across this when looking through the PCLinuxOS forums -

http://fosswire.com/2007/09/08/fix-a-frozen-system-with-the-magic-sysrq-keys/

Quote from the site:-

You finally got your Linux environment to crash. Ctrl+Alt+Backspace does nothing, nor do the F-keys. You know you shouldn't have installed that bad driver, but you did it anyway.

So you reach for the power button.

Stop.

Mashing in the power button to reboot could cause a problem if your hard drive is still being written to, and usually causes more problems than it solves. The Linux kernel includes a secret method of restarting your PC should it ever stop doing its job.

1. Hold down the Alt and SysRq (Print Screen) keys.
2. While holding those down, type the following in order. Nothing will appear to happen until the last letter is pressed: REISUB
3. Watch your computer reboot magically.

What the individual keys do in that sequence are not as important as what it does as a whole: stops all programs, unmounts all drives, and reboots. A lot safer than just cutting the power.

Here it is again: REISUB. Remember that, as it will save you a lot of time when you are configuring a system and something gets messed up. Need a mnemonic? Try Raising Elephants Is So Utterly Boring.

As an aside, don't try this if you just want to reboot. A normal reboot, if it can be done, should always be used instead of the REISUB keys.

R-E-I-S-U-B.


I know it says you shouldn't, but I tried it anyway - it works :D

You have to use your little fingers for the Alt and SysRq keys, otherwise you can't type the rest :shock:

Edit by Husse: If your keyboard does not have a SysRq key the PrintScreen key probably works

Cheers,

Hugh
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Postby Husse on Mon Sep 24, 2007 5:33 am

Id does not happen often but when it does it's bloody annoying
Good tip!
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Postby scorp123 on Mon Sep 24, 2007 7:13 am

Husse wrote:Good tip!
Good tip yes, ..... but not really new. It's a very old trick in fact. But it probably can't harm to repeat those things from time to time so others get a chance to see this too.

And just to make sure everyone understands this:
This is for emergencies only, e.g. when the system absolutely locks up + freezes. This is not to be used on potentially still busy machines (e.g. which are still writing stuff to disk) or on machines that could still do a proper shutdown if you only were patient enough to wait. This is only for systems which have already died a horrible death, e.g. power-cycling them is the only way to get them back to life ..... *Then* and only then you may try your luck and use this.

Besides, I am not really sure this works on Ubuntu-based distros ... I read somewhere that they disabled this per default :D
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Postby hugh on Mon Sep 24, 2007 9:00 am

Works on this Cassandra based laptop, I just need to try it on my wife's PClinux laptop - now that does freeze up regularly, and I can't figure out why.

Cheers,

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Postby Husse on Mon Sep 24, 2007 6:03 pm

Just tested after making sure there was no disk activity
It worked so it's not disabled...
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Postby clem on Tue Sep 25, 2007 5:15 pm

There could be a lot of different reasons why the computer freezes. It's usually either hardware or drivers related. In the second case if you think your 3D support is dodgy, just go and edit /etc/X11/xorg.conf, comment out "load dri" and "load glx" and replace whatever driver it's using with "vesa". That basically means no 3D at all... but it also could mean no more freezes.

I noticed this on a Compaq Presario 1715AP.

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REISUB

Postby sonofmoog on Wed Sep 26, 2007 5:09 pm

This is a way-cool tip that is needed all too often, sadly. I reply to this thread to ask how this works? What is happening when I type REISUB?

TIA
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Re: REISUB

Postby scorp123 on Wed Sep 26, 2007 6:05 pm

sonofmoog wrote:I reply to this thread to ask how this works? What is happening when I type REISUB?
Ahemm ... how about reading the first posting in this thread? There is a link to a web site and there is an article and that article explains everything ....

And you don't type 'REISUB' just like that as you would e.g. type text into an editor .... you have to enter this as a command sequence for the 'Magic SysRequest' key, e.g. press the 'magic key' and while pressing it hit the 'R-E-I-S-U-B' keys one after the other ... See above. Wikipedia has an article too, maybe you could read that one as well:
http://en.wikipedia.org/wiki/Magic_SysRq_key
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Re: REISUB

Postby sonofmoog on Wed Sep 26, 2007 6:38 pm

scorp123 wrote:
sonofmoog wrote:I reply to this thread to ask how this works? What is happening when I type REISUB?
Ahemm ... how about reading the first posting in this thread? There is a link to a web site and there is an article and that article explains everything ....


Okay. Bad on me for failing to click the link. But, I'll still take a moment to wish that hugh had included the first comment from the forum that details what each of the letters do. It was short, terse, and IMO necessary. Nevertheless, lest I be considered an ingrate, let me say thanks for the tip ..

Here is the text I would have liked to see added:

I heard of this trick before at Linuxquestions.org but the poster of the tip there used the order: “Raising Skinny Elephants Is Utterly Boringâ€￾

R gives back control of the keyboard
S issues a sync
E sends all processes but init the term singal
I sends all processes but init the kill signal
U mounts all filesystem ro to prevent a fsck at reboot
B reboots the system
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Re: How to Fix a Frozen System

Postby hugh on Thu Sep 27, 2007 6:10 am

Sorry but I don't understand the problem; the results for the typing of the letters are already in the original post:-

What the individual keys do in that sequence are not as important as what it does as a whole: stops all programs, unmounts all drives, and reboots. A lot safer than just cutting the power.

I simply did not have the info. from that site on what each actual key does.

Although the contents of the web page were reproduced, the link to the actual page was put in to stimulate people to look for themselves.

The Wikipedia page is far more in depth, probably more useful for those with better knowledge than I have :) :)
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Re: How to Fix a Frozen System

Postby scorp123 on Thu Sep 27, 2007 3:43 pm

hugh wrote: The Wikipedia page is far more in depth
It's almost overkill :lol: ... I use Linux since 1996 but believe me, I didn't know half of the stuff Wikipedia wrote there :lol:

You learn something new every day :D
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Postby Lolo Uila on Sun Sep 30, 2007 5:06 pm

I used that quite a lot lately when trying to set up an ATI Radeon X1950 card on Celena beta. Saved a lot of time and headache since Celena was installed on an intel fakeraid volume (dual-boot with XP) and if it had gotten hosed, reinstalling would have been a pain.

Nice, clean reboots each time with no errors or fsck needed.

Aloha, Tim
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Re: How to Fix a Frozen System

Postby ptr_v on Sat Dec 29, 2007 1:08 pm

disable compiz and it doesnt freeze anymore :D
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Re: How to Fix a Frozen System

Postby peko on Mon Jan 21, 2008 7:44 pm

hi

I am using linumint daryna when I insert DVD into my DVD-RW it works perfect but when I insert CD system slows down. It works very very slow. moving mouse from one side of screen to another take 15 sec, my PC P4,1GBramDDR2, 2x SATA HDD , ATI radeon X600, MB MSI. same happen on ASUS notebook - CPU AMD some X2 and N vidia graphick

have anyone same problem? do You know how to fix it?

thanks

I hope you will understand my poor english
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Re: How to Fix a Frozen System

Postby fleamailman on Sun Feb 24, 2008 2:08 pm

a dell laptop(inpiron 2650), the buttons become "Insert/print screen" and "Delete/SysRq" so which one do I press with alt then, and can I use alt Gr instead since that would free the other hand to type the letters, also I hear that REISUO shuts down the computer if that helps B is reboot and O is shutdown.

thanks
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Re: How to Fix a Frozen System

Postby Nibes on Wed Feb 27, 2008 12:00 pm

Ha this is good news!

Thank you for the tip.
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Re: How to Fix a Frozen System

Postby hallandnash on Fri Jul 04, 2008 8:53 am

I've been freezing lately and I'm trying to debug why.

Is there a log that can help me understand why it's locking up? I looked in messages,syslog but there isn't anything in there at the time of the crash.
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Re: How to Fix a Frozen System

Postby telemekus on Thu Jul 24, 2008 4:20 pm

hey it worked when i didnt have a freeze and then when i did have one not a kick out of it!
This did not work for me when mint froze.
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Re: How to Fix a Frozen System

Postby Linux n00b on Fri Aug 01, 2008 10:57 am

Great tip. I sort of find out the Ctrl+Alt+Backspace thing myself when I was "searching" for the key combo to reboot Linux machine (like Ctrl+Alt+Del under windows). So far the only few freeze I had, I could use it to reboot.

Here is my question, I read that Ctrl+Alt+Backspace close x-session, should it be used as the first preferred option?
Am I right to assume that only Ctrl+Alt+Backspace doesn't work, then try this REISUB option?
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Re: How to Fix a Frozen System

Postby Husse on Sat Aug 02, 2008 4:37 pm

Good thinking Linux n00b :)
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