fix: Normalize `RLIMIT_NOFILE` to sensible defaults

During review, it was decided to remove `LimitNOFILE` from `docker.service` to rely on the systemd v240 implicit default of `1024:524288`. On supported platforms with systemd prior to v240, packagers will patch the service with an explicit `LimitNOFILE=1024:524288`.

- `1024` soft limit is an implicit default, avoiding unexpected breakage. Software that needs a higher limit should request to raise the soft limit for its process.
- `524288` hard limit is an implicit default since systemd v240 and is adequate for most processes (_half of the historical limit from `fs.nr_open` of `1048576`_), while 4096 is the implicit default from the kernel (often too low). Individual containers can be started with `--ulimit` when a larger hard limit is required.
- The hard limit may not exceed `fs.nr_open` (_which a value of `infinity` will resolve to_). On most systems with systemd v240 or newer, this will resolve to an excessive size of 2^30 (over 1 billion).
- When set to `infinity` (usually as the soft limit) software may experience significantly increased resource usage, resulting in a performance regression or runtime failures that are difficult to troubleshoot.
- OpenRC current config approach lacks support for different soft/hard limits being set as it adjusts additional limits and `ulimit` does not support mixed usage of `-H` + `-S`. A soft limit of `524288` is not ideal, but 2^19 is much less overhead than 2^30, whilst a hard limit of 4096 would be problematic for Docker.

Signed-off-by: Brennan Kinney <5098581+polarathene@users.noreply.github.com>

contrib/init/openrc/docker.confd

@@ -17,7 +17,7 @@
 #DOCKER_PIDFILE="/run/docker.pid"
 
 # Settings for process limits (ulimit)
-#DOCKER_ULIMIT="-c unlimited -n 1048576 -u unlimited"
+#DOCKER_ULIMIT="-c unlimited -n 524288 -u unlimited"
 
 # seconds to wait for sending SIGTERM and SIGKILL signals when stopping docker
 #DOCKER_RETRY="TERM/60/KILL/10"

contrib/init/openrc/docker.initd

@@ -13,7 +13,7 @@ start_stop_daemon_args="--background \
 
 extra_started_commands="reload"
 
-rc_ulimit="${DOCKER_ULIMIT:--c unlimited -n 1048576 -u unlimited}"
+rc_ulimit="${DOCKER_ULIMIT:--c unlimited -n 524288 -u unlimited}"
 
 retry="${DOCKER_RETRY:-TERM/60/KILL/10}"
 

contrib/init/systemd/docker.service

@@ -28,7 +28,6 @@ StartLimitInterval=60s
 
 # Having non-zero Limit*s causes performance problems due to accounting overhead
 # in the kernel. We recommend using cgroups to do container-local accounting.
-LimitNOFILE=infinity
 LimitNPROC=infinity
 LimitCORE=infinity
 

contrib/init/sysvinit-debian/docker

@@ -58,7 +58,8 @@ case "$1" in
 		touch "$DOCKER_LOGFILE"
 		chgrp docker "$DOCKER_LOGFILE"
 
-		ulimit -n 1048576
+		# Only set the hard limit (soft limit should remain as the system default of 1024):
+		ulimit -Hn 524288
 
 		# Having non-zero limits causes performance problems due to accounting overhead
 		# in the kernel. We recommend using cgroups to do container-local accounting.