I am implementing some numerical simulations so after some hours or a day, the ubuntu system gets slower so I need to reboot. Is there any way to keep the system fast without reboot? It's sometimes annoying to re-open many terminals and the programs as well as to set the directory again after reboot.
Yes, that would be annoying to re-open many terminals. I suggest you install the CPU Frequency Scaling Indicator (An indicator for monitoring and switching CPU frequency scaling) using the command:
sudo apt-get install indicator-cpufreq
Once installed (and after you reboot), the app icon should appear in the status bar. You may also make sure that the app is in the list of the startup applications (usually by default). Then you would be able to switch to the
to prevent your system from going into the so called power-save mode and to stay on the performance mode.