When diagnosing Apache2 startup failures, we need to examine multiple indicators:

# System service status shows Apache isn't running despite apparent successful start
$ service --status-all
 [ - ]  apache2

# Error logs reveal critical thread creation failures
[Sat May 01 14:45:18 2010] [alert] (11)Resource temporarily unavailable: apr_thread_create: unable to create worker thread

The key error apr_thread_create: unable to create worker thread typically indicates one of:

• System resource exhaustion (ulimit restrictions)
• Process limit reached (max user processes)
• Memory constraints
• Zombie Apache processes holding resources

First, check current system limits and resource usage:

# Check process limits
$ ulimit -a

# Count existing Apache processes
$ ps -ef | grep apache | grep -v grep | wc -l

# Check system-wide process limit
$ cat /proc/sys/kernel/threads-max

# Verify available memory
$ free -h

1. Cleaning Up Stale Processes

# Force kill all Apache processes
$ sudo pkill -9 apache2

# Remove stale PID file
$ sudo rm -f /var/run/apache2.pid

2. Adjusting System Limits

Edit /etc/security/limits.conf:

www-data soft nproc 1024
www-data hard nproc 2048
* soft nproc 4096
* hard nproc 8192

3. Optimizing Apache Configuration

Modify /etc/apache2/apache2.conf:

# Reduce worker threads if system resources are limited
StartServers 2
MinSpareThreads 4
MaxSpareThreads 8
ThreadLimit 16
ThreadsPerChild 8
MaxRequestWorkers 16

For persistent issues, examine kernel messages:

$ dmesg | grep -i 'thread'
$ cat /var/log/kern.log | grep -i 'limit'

• Implement proper Apache graceful restart procedures
• Monitor system resources with tools like Nagios or Datadog
• Consider using mod_event instead of worker MPM for high-traffic servers

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When your Apache2 service appears to start successfully but fails to maintain operation, you're likely dealing with deeper system resource issues. The key indicators in your logs are:

[alert] (11)Resource temporarily unavailable: apr_thread_create: unable to create worker thread
[alert] No active workers found... Apache is exiting!

First, verify the actual process status:

ps aux | grep apache
systemctl status apache2
ulimit -a

Check for competing processes that might be consuming resources:

lsof -i :80
lsof -i :443
netstat -tulnp | grep -E '80|443'

The "Resource temporarily unavailable" error typically indicates one of:

• System-wide process/thread limit reached
• Insufficient memory for thread creation
• Zombie Apache processes holding resources
• Improper MaxClients/MaxRequestWorkers configuration

Increase system limits by editing /etc/security/limits.conf:

* soft nproc 10000
* hard nproc 10000
* soft nofile 10000
* hard nofile 10000
www-data soft nproc 10000
www-data hard nproc 10000

Adjust Apache's MPM configuration (/etc/apache2/mods-available/mpm_prefork.conf or equivalent):

    StartServers            5
    MinSpareServers         5
    MaxSpareServers        10
    MaxRequestWorkers     150
    MaxConnectionsPerChild  0

Force clean any lingering processes:

pkill -9 apache2
rm -f /var/run/apache2/apache2.pid
service apache2 start

For systemd-based systems, add proper resource limits:

# Create/edit /etc/systemd/system/apache2.service.d/limits.conf
[Service]
LimitNOFILE=10000
LimitNPROC=10000

After changes, verify operation:

systemctl daemon-reload
service apache2 restart
apache2ctl status
journalctl -u apache2 --since "1 hour ago"

Monitor thread creation with:

watch -n 1 "ps -eLf | grep apache | wc -l"