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MINOR: limits: explain a bit better what to do when fd limits are exceeded

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As shown in github issue #3191, the error message shown when FD limits
are exceeded is not very useful as-is, since the current hard limit is
not displayed, and no suggestion is made about what to change in the
config. Let's explain about maxconn/ulimit-n/fd-hard-limit, suggest
dropping them or setting them to a context-based value at roughly 49%
of the current limit minus the known used FDs for listeners and checks.
This allows common "large" hard limits to report mostly round maxconns.
Example:

  [ALERT]    (25330) : [haproxy.main()] Cannot raise FD limit to 4001020,
  current limit is 1024 and hard limit is 4096. You may prefer to let
  HAProxy adjust the limit by itself; for this, please just drop any
  'maxconn' and 'ulimit-n' from the global section, and possibly add
  'fd-hard-limit' lower than this hard limit. You may also force a new
  'maxconn' value that is a bit lower than half of the hard limit minus
  listeners and checks. This results in roughly 1500 here.
This commit is contained in:
Willy Tarreau 2025-11-19 16:38:29 +01:00
parent 91d4f4f618
commit 8438ca273f
1 changed files with 23 additions and 2 deletions
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25
src/limits.c
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@ -454,8 +454,19 @@ void apply_nofile_limit(void)
limit.rlim_cur = global.fd_hard_limit;
if (global.tune.options & GTUNE_STRICT_LIMITS) {
ha_alert("[%s.main()] Cannot raise FD limit to %d, limit is %d.\n",
progname, global.rlimit_nofile, (int)limit.rlim_cur);
/* suggest roughly half of the limit minus used FDs for listeners, checks
* etc. This will give roughly round numbers for 64k and above while
* reserving enough listeners for small values such as 1024.
*/
ha_alert("[%s.main()] Cannot raise FD limit to %d, current "
"limit is %d and hard limit is %d. You may prefer to let HAProxy "
"adjust the limit by itself; for this, please just drop any 'maxconn' and "
"'ulimit-n' from the global section, and possibly add 'fd-hard-limit' lower "
"than this hard limit. You may also force a new 'maxconn' value that is a bit "
"lower than half of the hard limit minus listeners and checks. This results in "
"roughly %u here.\n",
progname, global.rlimit_nofile, (int)limit.rlim_cur, (int)limit.rlim_max,
round_2dig((uint)MAX(limit.rlim_max - global.est_fd_usage, 3) * 49ULL / 100));
exit(1);
}
else {
@ -469,6 +480,16 @@ void apply_nofile_limit(void)
ha_warning("[%s.main()] Cannot raise FD limit to %d, limit is %d.\n",
progname, global.rlimit_nofile, (int)limit.rlim_cur);
ha_warning("[%s.main()] Cannot raise FD limit to %d, current "
"limit is %d and hard limit is %d. You may prefer to let HAProxy "
"adjust the limit by itself; for this, please just drop any 'maxconn' and "
"'ulimit-n' from the global section, and possibly add 'fd-hard-limit' lower "
"than this hard limit. You may also force a new 'maxconn' value that is a bit "
"lower than half of the hard limit minus listeners and checks. This results in "
"roughly %u here.\n",
progname, global.rlimit_nofile, (int)limit.rlim_cur, (int)limit.rlim_max,
round_2dig((uint)MAX(limit.rlim_max - global.est_fd_usage, 3) * 49ULL / 100));
global.rlimit_nofile = limit.rlim_cur;
}
}