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So I am using ssh -X to access a server. I am at a Xubuntu desktop accessing a Ubuntu server that is in the next room. Usually everything works fine, but when the system load gets high, any graphical applications I have freeze and fail to be restarted.

This happens even if the process that is causing the high load has been niced to a low priority with "nice -n 19". And even though the system load is high, the command line works fine with no delay, and other applications I have running on the server (e.g. virtual machines) run fine. But any graphical application running through X dies.

When the graphical applications fail they usually give out an error message that suggests a time-out.

It seems that something connected to X has a low priority and times out. But what is it, and how does one fix it?

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As I became interested I decided to conduct several experiments.

Firstly without ssh I launched on my computer three identical programs written using MPI. Each copy alone occupied all 8 cores of my computer on 100 percent. When working together with equal priorities they consumed in average the same amount of CPU time. Then I changed the priority of one of the programs to 19. But they continued to consume in average the same amount of CPU time. I was even more surprised when nothing changed even when one of three copies had priority -20 and two others 19.

Then I decided to use ssh. I connected to my computer from one staying in the other room in X mode and launched several heavy-going applications. Not depending on any priorities (priorities of Xorg, ssh, 3 copies of program) they shared CPU time in a way nothing seemed to stop though X-applications were little bit slower than usual.

The experiment of course can't be treated as 100%-correct but in fact it led me to the thought that the problem is not in CPU but in either memory consumption (as process priorities do not influence memory usage) or in speed of your network. Try to check these two issues. Best wishes!

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This does not answer the question at all. –  pauska Jan 17 '13 at 19:00
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