vim and that weird one-second startup delay

Are you using vim, tmux, a graphical Linux desktop and are you experiencing random sluggishness when starting your editor? If not, you can skip this one.

This is something that had been bugging me for ages, first at work on my workstation, then at home: Long-running tmux sessions would sporadically induce startup delays of the vim editor of exactly one second. Reattaching tmux didn't solve the problem; logging out and back into my desktop always did.

First I thought I was just being impatient, but after some profiling with time, I was getting curious. strace revealed that the delay was exactly one second: Something in vims startup process was calling the nanosleep(2) system call with one second as an argument!

To make a long story short: This is caused by some X library that is mislead by a broken environment variable SESSION_MANAGER from a former X session. tmux tends to get rather attached to environment variables, which, in this instance, causes the sluggishness.

If the problem goes away after executing


or something similar for your shell, you can fix it permanently by appending the following line to your .tmux.conf:

set-option -g -a update-environment " SESSION_MANAGER"

If you are now wondering why vim would need access to some X-related variables in the first place (as I was): It lets vim access your X clipboard! (Strangely, the variable SESSION_MANAGER is not actually needed for that, but you can verify it by overwriting some more critical X variable like DISPLAY or XAUTHORITY and subsequently trying to use the X clipboard from within vim.)

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