From 8438ca273f4b174b51b5ff2a504ff5d2278134af Mon Sep 17 00:00:00 2001 From: Willy Tarreau Date: Wed, 19 Nov 2025 16:38:29 +0100 Subject: [PATCH] MINOR: limits: explain a bit better what to do when fd limits are exceeded 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. --- src/limits.c | 25 +++++++++++++++++++++++-- 1 file changed, 23 insertions(+), 2 deletions(-) diff --git a/src/limits.c b/src/limits.c index d2aa3cffc..b00cb8cc8 100644 --- a/src/limits.c +++ b/src/limits.c @@ -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; } }