diff --git a/man/udev.conf.xml b/man/udev.conf.xml
index 0f51a1aaef..f4d573ddc6 100644
--- a/man/udev.conf.xml
+++ b/man/udev.conf.xml
@@ -53,7 +53,8 @@
         <term><varname>children_max=</varname></term>
 
         <listitem>
-          <para>An integer. The maximum number of events executed in parallel.</para>
+          <para>An integer. The maximum number of events executed in parallel. When unspecified or 0 is
+          specified, the maximum is determined based on the system resources.</para>
 
           <para>This is the same as the <option>--children-max=</option> option.</para>
         </listitem>
diff --git a/man/udevadm.xml b/man/udevadm.xml
index 900bdac4b3..1c339a6b6d 100644
--- a/man/udevadm.xml
+++ b/man/udevadm.xml
@@ -607,8 +607,8 @@
           <term><option>-m</option></term>
           <term><option>--children-max=</option><replaceable>value</replaceable></term>
           <listitem>
-            <para>Set the maximum number of events, systemd-udevd will handle at the
-            same time.</para>
+            <para>Set the maximum number of events, systemd-udevd will handle at the same time. When 0 is
+            specified, then the maximum is determined based on the system resources.</para>
           </listitem>
         </varlistentry>
         <varlistentry>
diff --git a/src/udev/udev-manager.c b/src/udev/udev-manager.c
index b97ccab3b0..6653de6b45 100644
--- a/src/udev/udev-manager.c
+++ b/src/udev/udev-manager.c
@@ -3,6 +3,7 @@
 #include "blockdev-util.h"
 #include "cgroup-util.h"
 #include "common-signal.h"
+#include "cpu-set-util.h"
 #include "daemon-util.h"
 #include "device-monitor-private.h"
 #include "device-private.h"
@@ -14,6 +15,7 @@
 #include "hashmap.h"
 #include "inotify-util.h"
 #include "io-util.h"
+#include "limits-util.h"
 #include "list.h"
 #include "mkdir.h"
 #include "process-util.h"
@@ -33,6 +35,8 @@
 #include "udev-watch.h"
 #include "udev-worker.h"
 
+#define WORKER_NUM_MAX UINT64_C(2048)
+
 #define EVENT_RETRY_INTERVAL_USEC (200 * USEC_PER_MSEC)
 #define EVENT_RETRY_TIMEOUT_USEC  (3 * USEC_PER_MINUTE)
 
@@ -829,6 +833,28 @@ static int on_worker(sd_event_source *s, int fd, uint32_t revents, void *userdat
         return 1;
 }
 
+static void manager_set_default_children_max(Manager *manager) {
+        uint64_t cpu_limit, mem_limit, cpu_count = 1;
+        int r;
+
+        assert(manager);
+
+        if (manager->children_max != 0)
+                return;
+
+        r = cpus_in_affinity_mask();
+        if (r < 0)
+                log_warning_errno(r, "Failed to determine number of local CPUs, ignoring: %m");
+        else
+                cpu_count = r;
+
+        cpu_limit = cpu_count * 2 + 16;
+        mem_limit = MAX(physical_memory() / (128*1024*1024), UINT64_C(10));
+
+        manager->children_max = MIN3(cpu_limit, mem_limit, WORKER_NUM_MAX);
+        log_debug("Set children_max to %u", manager->children_max);
+}
+
 /* receive the udevd message from userspace */
 static int on_ctrl_msg(UdevCtrl *uctrl, UdevCtrlMessageType type, const UdevCtrlMessageValue *value, void *userdata) {
         Manager *manager = ASSERT_PTR(userdata);
@@ -921,7 +947,7 @@ static int on_ctrl_msg(UdevCtrl *uctrl, UdevCtrlMessageType type, const UdevCtrl
                 break;
         }
         case UDEV_CTRL_SET_CHILDREN_MAX:
-                if (value->intval <= 0) {
+                if (value->intval < 0) {
                         log_debug("Received invalid udev control message (SET_MAX_CHILDREN, %i), ignoring.", value->intval);
                         return 0;
                 }
@@ -929,6 +955,9 @@ static int on_ctrl_msg(UdevCtrl *uctrl, UdevCtrlMessageType type, const UdevCtrl
                 log_debug("Received udev control message (SET_MAX_CHILDREN), setting children_max=%i", value->intval);
                 manager->children_max = value->intval;
 
+                /* When 0 is specified, determine the maximum based on the system resources. */
+                manager_set_default_children_max(manager);
+
                 notify_ready(manager);
                 break;
         case UDEV_CTRL_PING:
@@ -1227,6 +1256,8 @@ int manager_init(Manager *manager, int fd_ctrl, int fd_uevent) {
 int manager_main(Manager *manager) {
         int fd_worker, r;
 
+        manager_set_default_children_max(manager);
+
         /* unnamed socket from workers to the main daemon */
         r = socketpair(AF_UNIX, SOCK_DGRAM|SOCK_CLOEXEC, 0, manager->worker_watch);
         if (r < 0)
diff --git a/src/udev/udevd.c b/src/udev/udevd.c
index d0856b4a55..257336aec6 100644
--- a/src/udev/udevd.c
+++ b/src/udev/udevd.c
@@ -6,14 +6,13 @@
  */
 
 #include <getopt.h>
+#include <unistd.h>
 
 #include "sd-daemon.h"
 
-#include "cpu-set-util.h"
 #include "env-file.h"
 #include "errno-util.h"
 #include "fd-util.h"
-#include "limits-util.h"
 #include "mkdir.h"
 #include "parse-util.h"
 #include "pretty-print.h"
@@ -27,8 +26,6 @@
 #include "udevd.h"
 #include "version.h"
 
-#define WORKER_NUM_MAX 2048U
-
 static bool arg_debug = false;
 static int arg_daemonize = false;
 
@@ -361,22 +358,6 @@ int run_udevd(int argc, char *argv[]) {
         if (r < 0)
                 return r;
 
-        if (manager->children_max == 0) {
-                unsigned long cpu_limit, mem_limit, cpu_count = 1;
-
-                r = cpus_in_affinity_mask();
-                if (r < 0)
-                        log_warning_errno(r, "Failed to determine number of local CPUs, ignoring: %m");
-                else
-                        cpu_count = r;
-
-                cpu_limit = cpu_count * 2 + 16;
-                mem_limit = MAX(physical_memory() / (128UL*1024*1024), 10U);
-
-                manager->children_max = MIN3(cpu_limit, mem_limit, WORKER_NUM_MAX);
-                log_debug("Set children_max to %u", manager->children_max);
-        }
-
         /* set umask before creating any file/directory */
         umask(022);