Emacs Tramp and Speedier SSH Access


I often find myself editing files in terminals on remote machines, and I really need to add Tramp to my muscle memory. Tramp is an Emacs package (installed by default?) for editing remote files via ssh. It allows you to open files with urls like this: /ssh:user@host#port:/path/to/file.

Note: I use ssh instead of scp (used in all of the examples) because the webpage says it’s faster and also because it supports the #port piece while scp does not. Also, the user@ and #port pieces are optional. So feel free to use the minimum: /ssh:host:/path/to/file.

sudo, too

Even better, have you ever opened a root-owned file in an editor, made some changes, only to have the editor tell you that the file is read-only because you didn’t use sudo? For example, if you opened /etc/root_owned_file using Emacs, write the buffer to a “new” file, ^X^W, using this tramp-enabled file path: /sudo::/etc/root_owned_file. Emacs doesn’t know that you’re writing to the same file. It will open the file using tramp and sudo, save your changes, and replace your non-sudo buffer with the new one.

OpenSSH ControlMaster

The biggest problem I’ve had using Tramp mode, though, is latency. If your connection isn’t local, and you have the good habit of constantly saving, the delays can be annoying or at least jarring. On the EmacsWiki Tramp page, there’s a hint about using the ControlMaster option in OpenSSH. With ControlMaster set, you simply open a regular ssh session to your remote host, and OpenSSH will use that already established connection for all of your future file access.

Enabling it

Just put the following in ~/.ssh/config:

Host *
ControlMaster auto
ControlPath ~/.ssh/master-%r@%h:%p

That’s it! Your welcome. Enjoy your better life. Here’s the effect in action on a local machine:

$ ./time_ssh_access.py
0.36 0.34 0.33 0.36 0.31 0.27 0.33 0.34 0.35 0.35
Average: 0.334
# ControlMaster added to ~/.ssh/config and a new terminal opened
$ ./time_ssh_access.py
0.02 0.02 0.04 0.02 0.02 0.04 0.01 0.02 0.02 0.02
Average: 0.023

That’s a 93% improvement. On a remote machine, times went from 0.861 down to 0.161 for an 81% improvement.


Here’s the script used for testing. It uses the sh python module which is so awesome it deserves its own blog post.