Debugging 'Maximum number of clients reached ... unable to open display ":0"'

Question

I am running an openSUSE Tumbleweed installation with KDE Plasma 5.4.3 . I tend to use a fair number of programs ( Eclipse, Firefox, Slack, Evolution, Konsole, various java applications ) and after some time I can't open new programs anymore due to

Maximum number of clients reachedxlsclients:  unable to open display ":0"

I am unable to determine which program causes the error. When xlsclients works it displays around 20-30 programs so I suspect that it does not display the right information.

How can I debug this issue and find out which program causes the error?

Answer 1

xlsclients and xrestop didn't tell me much, either. Most writeups of people researching this problem focus on finding the total open Unix sockets. But the lsofc -c Xorg advice is much better.

However, you can use ss to get at that info more directly these days. This command shows me the unix (local) sockets connecting to Xorg:

sudo ss -x src "*/tmp/.X11-unix/*" 

You might need to experiment with the filter part to get it to work on your environment. Experiment with sudo ss -x alone to see everything and try to filter from there.

I can't find a way to have ss identify the source app (like lsofc does) but with a few pipes I can get to this:

sudo ss -x src "*/tmp/.X11-unix/*" | grep -Eo "[0-9]+\s*$" | while read port
do sudo ss -p -x | grep -w $port | grep -v X11-unix 
done | grep -Eo '".+"' | sort | uniq -c | sort -rn

Output looks like this:

  8 "slack"
  8 "code"
  7 "brave"
  5 "kded5"
  2 "kwin_x11"
  2 "ksmserver"
  2 "kscreen_backend"
  1 "zoom"
  1 "zim"
  :

When I ran this while getting the "Maximum number of clients reached" problem, this search turned up a list of 256 clients. So I'm pretty confident that this is the right tool to find the culprit.

You can find out more about ss at these pages:

• https://man7.org/linux/man-pages/man8/ss.8.html
• https://www.linode.com/docs/guides/ss/

Here's a commented version of that gnarly script line:

sudo ss -x src "*/tmp/.X11-unix/*" |  # List X11 sessions
  grep -Eo "[0-9]+\s*$" |             # extract the port number
  while read port ; do                # For every connected port
    sudo ss -p -x | grep -w $port |   #   Find the connecting process
    grep -v X11-unix                  #   but ignore the x11 side
  done | grep -Eo '".+"' |            # extract process names
  sort | uniq -c |                    # Count the number of repeats
  sort -rn                            # And sort them descending by count