Three PHP-FPM failure modes and how to actually diagnose them

Tuning guides talk about throughput. Nobody pages you about throughput. They page you about symptoms, and the useful skill is mapping a symptom back to a cause before you spend money on hardware. Three failure modes account for most of what I find on inherited servers. Each has a distinct signature. The 502 nobody can reproduce Server has 8GB. PHP-FPM is set to 100 workers. Each worker uses 60MB under load. That's 6GB of PHP, plus MariaDB, plus Nginx, plus the OS. Under normal traffic you never approach 100 workers, so it looks fine for months. Then a marketing email goes out, concurrency spikes, and the kernel runs out of memory. The OOM killer picks a process and terminates it, usually the biggest one, which is a PHP-FPM worker holding an in-flight request. User gets a 502. The application log has nothing, because the process died before it could write anything. Nginx logs recv() failed (104: Connection reset by peer) . Ten minutes later everything looks normal. sudo dmesg -T | grep -i "killed process" sudo journalctl -k | grep -i oom Hits there mean you don't have a mystery. You have a pm.max_children value nobody checked against real memory. The site that degrades all day and resets overnight TTFB is 180ms at 8am. By 4pm it's 900ms. Nobody deployed. Overnight it's fast again because something restarted PHP-FPM. That's OPcache running out of room. When the cache fills, it stops caching new scripts or wipes and rebuilds, and every miss pays full parse-and-compile again. It degrades gradually, which is why it goes unnoticed for months. The counters are oom_restarts and hash_restarts from opcache_get_status() . Here's the part that trips people up. OPcache state is per SAPI. Run that function from the CLI and you're reading the CLI cache, which is empty, separate, and tells you nothing about your site. You have to ask through PHP-FPM. <?php // drop in webroot, lock to your IP, delete when done $allowed = [ '203.0.113.42' ]; if ( ! in_array ( $_SERVER [ 'REMOTE_ADDR'