Randomly on my laptop screen this appears and debian just freezes. Sometimes these vertical lines don't appear and system freezes anyway. Its just random. How do i identify if this is hardware or software issue? and then how to identify exact piece of hardware or software causing this problem.
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That kind of looks like how a machine would look if the RAM is loose in the socket or otherwise have a bad contact. And since you mention that it's random and sometimes there are no vertical lines, that'd be my guess rather than anything to do with graphical software. Is the RAM on your machine soldered on or is it detachable? If it's the latter, I recommend taking the RAM out and reseating it to see if things improve (try this a couple of times, just to be sure the RAM is properly seated). If the RAM is soldered on then it might have something to do with out of memory. Does the system slow down a couple of seconds before freezing entirely? If it does then it's likely it's a OOM thing.
Oh, and for good measure, can you get to the tty (ctrl + alt + f2) when the system freezes? If you can then it's probably a graphical thing and you can try restarting those processes. If the system is in a complete freeze then I'd say that's another point for the OOM hypothesis.
yeah, i think you might be right about RAM. Because one day i was holding my latop in my one hand and i saw this screen flickering thing. that was the first time this issue happened. And yeah i can not login to tty when the freeze happens this mean system is completely blocked and is not a graphical problem. I dont think its OOM issue as i have enough RAM and no system doesnt slow down a couple of seconds before freezing entirely. The freeze is random and instant and it happens sometimes when i slide laptop so i think its due to RAM loose connection. Not sure if the RAM is soldered one or is detachable. I will try to replug it and see if it fixes the problem. btw the machine in question is macbook pro 2012 and thanks for replying kind stranger!
Edit: the ram is detachable.