Graphics speed depends on user profile

Glynn Clements glynn at
Thu Nov 20 12:44:33 PST 2014

Juan Pablo de la Cruz wrote:

> We have an library which directly uses the Xlib API to do graphics.
> We are observing the following effect: when two different users log
> in the same machine, use the same window manager, and run the very same
> program, the graphics are slower (much) for one of the users.
> Concretely, the user for which the drawing is slower, logs himself in,
> and the other user executes su $USER on the terminal.
> Both use the exact same libraries (same PATH and LD_LIBRARY_PATH).
> Both use the same window manager.
> What other settings/factors could explain this behaviour?

What are their $DISPLAY settings?

If $DISPLAY is ":0" then the client will connect to the X server via
the Unix-domain socket at /tmp/.X11-unix/X0. Whereas if $DISPLAY is
"localhost:0", it will connect to the X server via TCP port 6000.

The latter will result in the data stream being chopped up into
packets and passed through the kernel's TCP/IP stack: routing,
firewall rules, etc.

It may also prevent the use of features such as direct rendering, as
the client and server are likely to be deemed to reside on different

Glynn Clements <glynn at>

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