Why does setting SESSION_MANAGER='' make Vim faster to load without breaking the clipboard?

Question

I recently noticed that my Vim was loading slower than instantaneously, so I decided to benchmark it and noticed it was taking a full second:

time ex +q -u NONE

real    0m1.031s
user    0m0.020s
sys     0m0.009s

After Googling around, I came across this question and found that both of the suggested approaches makes Vim load fast again:

Disabling connection to X

This gives us 20 ms load time but the clipboard does not work anymore.

time ex +q -u NONE  -X

real    0m0.020s
user    0m0.004s
sys     0m0.016s

Setting SESSION_MANAGER to empty string

This give us 35 ms load time and the clipboard works.

time env SESSION_MANAGER="" ex +q -i NONE

real    0m0.035s
user    0m0.023s
sys     0m0.009s

For reference, my default SESSION_MANAGER is the following:

echo $SESSION_MANAGER
local/x1-7thGen:@/tmp/.ICE-unix/2463,unix/x1-7thGen:/tmp/.ICE-unix/2463

My system OS and kernel are as follows:

cat /etc/issue
Ubuntu 20.04.6 LTS \n \l

uname -a
Linux x1-7thGen 5.4.0-204-generic #224-Ubuntu SMP Thu Dec 5 13:38:28 UTC 2024 x86_64 x86_64 x86_64 GNU/Linux

What specific behavior does setting SESSION_MANAGER="" change in Vim, and is there any reason I should not always clear this when running Vim?

Answer 1

From vim's source code, you can see that when no X connection or SESSION_MANAGER is not set the there's no initialization of XSMP

    if (!x_no_connect)
    {
    char *p = getenv("SESSION_MANAGER");

    if (p != NULL && *p != NUL)
    {
        xsmp_init();
        TIME_MSG("xsmp init");
    }
    }

from here.

That's why you may see a similar improvement in both cases.

See XSMP 's purpose.