502 Gateway Errors under High Load (nginx/php-fpm)
I work for a rather busy internet site that is often gets very large spikes of traffic. During these spikes hundreds of pages per second are requested and this produces random 502 gateway errors.
Now we run Nginx (1.0.10) and PHP-FPM on a machine with 4x SAS 15k drives (raid10) with a 16 core CPU and 24GB of DDR3 ram. Also we make use of the latest Xcache version. The DB is located on another machine, but this machine's load is very low, and has no issues.
Under normal load everything runs perfect, system load is below 1, and PHP-FPM status report never really shows more than 10 active processes at one time. There is always about 10GB of ram still available. Under normal load the machine handles about 100 pageviews per second.
The problem arises when huge spikes of traffic arrive, and hundreds of page-views per second are requested from the machine. I notice that FPM's status report then shows up to 50 active processes, but that is still way below the 300 max connections that we have configured. During these spikes Nginx status reports up to 5000 active connections instead of the normal average of 1000.
OS Info: CentOS release 5.7 (Final)
CPU: Intel(R) Xeon(R) CPU E5620 @ 2.40GH (16 cores)
php-fpm.conf
daemonize = yes
listen = /tmp/fpm.sock
pm = static
pm.max_children = 300
pm.max_requests = 1000
I have not setup rlimit_files, because as far as I know it should use the system default if you don't.
fastcgi_params (only added values to standard file)
fastcgi_connect_timeout 60;
fastcgi_send_timeout 180;
fastcgi_read_timeout 180;
fastcgi_buffer_size 128k;
fastcgi_buffers 4 256k;
fastcgi_busy_buffers_size 256k;
fastcgi_temp_file_write_size 256k;
fastcgi_intercept_errors on;
fastcgi_pass unix:/tmp/fpm.sock;
nginx.conf
worker_processes 8;
worker_connections 16384;
sendfile on;
tcp_nopush on;
keepalive_timeout 4;
Nginx connects to FPM via Unix Socket.
sysctl.conf
net.ipv4.ip_forward = 0
net.ipv4.conf.default.rp_filter = 1
net.ipv4.conf.default.accept_source_route = 0
kernel.sysrq = 1
kernel.core_uses_pid = 1
net.ipv4.tcp_syncookies = 1
kernel.msgmnb = 65536
kernel.msgmax = 65536
kernel.shmmax = 68719476736
kernel.shmall = 4294967296
net.ipv4.conf.all.send_redirects = 0
net.ipv4.conf.default.send_redirects = 0
net.ipv4.tcp_max_syn_backlog = 2048
net.ipv4.icmp_echo_ignore_broadcasts = 1
net.ipv4.conf.all.accept_source_route = 0
net.ipv4.conf.all.accept_redirects = 0
net.ipv4.conf.all.secure_redirects = 0
net.ipv4.conf.all.log_martians = 1
net.ipv4.conf.default.accept_redirects = 0
net.ipv4.conf.default.secure_redirects = 0
net.ipv4.icmp_echo_ignore_broadcasts = 1
net.ipv4.icmp_ignore_bogus_error_responses = 1
net.ipv4.conf.default.rp_filter = 1
net.ipv4.tcp_timestamps = 0
net.ipv4.conf.all.rp_filter=1
net.ipv4.conf.default.rp_filter=1
net.ipv4.conf.eth0.rp_filter=1
net.ipv4.conf.lo.rp_filter=1
net.ipv4.ip_conntrack_max = 100000
limits.conf
* soft nofile 65536
* hard nofile 65536
These are the results for the following commands:
ulimit -n
65536
ulimit -Sn
65536
ulimit -Hn
65536
cat /proc/sys/fs/file-max
2390143
Question: If PHP-FPM is not running out of connections, the load is still low, and there is plenty of RAM available, what bottleneck could be causing these random 502 gateway errors during high traffic?
Note: by default this machine's ulimit's were 1024, since I changed it to 65536 I have not fully rebooted the machine, as it's a production machine and it would mean too much downtime.
Answer
This should fix it...
You have: fastcgi_buffers 4 256k;
Change it to: fastcgi_buffers 256 16k; // 4096k total
Also set fastcgi_max_temp_file_size 0, that will disable buffering to disk if replies start to exceeed your fastcgi buffers.