I know similar questions have been asked and answered here, but my issue is different: I want to get rid of screen tearing staying with the nouveau Xorg driver, not with NVIDIA proprietary driver.
The reason is that my notebook (Asus G75VW) is peculiar in having only Nvidia GPU (no Intel GPU on board). As a result, when using dual monitor setup (with external monitor attached via display port), the GPU temperature quickly rises to 50 degrees even in idle state (no movies/games), and after that point the GPU fan speeds up, yielding that annoying noise... Well, that was back a while ago when I had MS Windows installed, so I used a single external monitor. Recently I switched to Ubuntu 18.04 and - heavens! - my notebook became much less noisy in dual-monitor setup, as the GPU temperature hardly reaches 43 degrees even when an HD movie is run on external monitor!
One problem though: screen tearing while playing videos. I found the solution at SE: in nvidia-settings turn on the option Force composition pipeline. For this to work, I obviously had to switch to proprietary Nvidia driver from the nouveau driver which was used by default. And - voila! - the temperature-an-noise problem is back again. By the way, simply switching to proprietary driver does not solve screen tearing; additionally enabling force composition pipeline removes screen tearing indeed. The GPU temperature becomes high even without enabling it.
So I would like to stick with the nouveau driver but to get rid of screen tearing. Thus far, I tried compton instead of the xfce4 native window manager, both with xrender and glx backends, but that didn't solve screen-tearing issue.
[Alternatively, since I don't play games, I might consider using Nvidia driver but tweak GPU to minimize its load and hence temperature. I investigated options available in nvidia-settings and found only few that I could change; they made no difference; could there be something I have missed, though?]
So the solution that worked for me was to switch to Wayland instead of X11. Wayland uses
nouveau, and there is no screen-tearing when playing videos.