gdb Thread Dumps

When Apache is stuck, a thread dump tells you exactly where each worker is blocked. You do not need to restart the service or attach a debugger permanently. Three commands in gdb give you a complete backtrace of every thread, then you detach cleanly.

Prerequisites

• You have root or sudo access on the target host.
• gdb is installed (yum install gdb or apt install gdb).
• You know the PID of the httpd process (check with ps aux | grep httpd).

Steps

1. Attach gdb to the running httpd process:

   gdb /usr/sbin/httpd <pid>

   Replace <pid> with the actual process ID. gdb will pause the process while attached.

2. Print a backtrace for every thread:

   (gdb) thread apply all bt

   This outputs a stack trace for each thread. Look for threads stuck in futex_wait, poll, or a specific handler function to identify the bottleneck.

3. Detach gdb so httpd resumes normal operation:

   (gdb) detach

   The process continues running as if gdb was never attached.

Tips

• If the output is long, redirect it: thread apply all bt > /tmp/httpd_threads.txt (run inside gdb).
• Repeat the dump a few seconds apart if you suspect a race condition or intermittent hang.
• Detach promptly. Leaving gdb attached freezes the process and will cause clients to time out.

• SysAdmin

• Bash
• Troubleshooting