Hi,

On an up to date CentOS 7.8.2003 Xen domU host with plenty of RAM just had a site coming up suddenly with "503 Service Unavailable" and, on closer inspection, I found these entries in the main php-fpm service error_log:
[12-Jul-2020 12:29:09] NOTICE: using inherited socket fd=392, "/var/www/vhosts/system/domain.com.au/php-fpm.sock"
[12-Jul-2020 12:29:09] NOTICE: using inherited socket fd=392, "/var/www/vhosts/system/domain.com.au/php-fpm.sock"
[12-Jul-2020 12:29:09] NOTICE: using inherited socket fd=392, "/var/www/vhosts/system/domain.com.au/php-fpm.sock"
[12-Jul-2020 12:29:09] NOTICE: using inherited socket fd=392, "/var/www/vhosts/system/domain.com.au/php-fpm.sock"
[12-Jul-2020 12:29:09] ERROR: epoll: unable to add fd 392
After the first "ERROR: epoll: unable to add fd" the sitestarted to return 503, it was fine before that.
I saw a report with a similar error message at https://forum.remirepo.net/viewtopic.php?id=2944 but that's for inet, so they're likely 2 different issues. In case it helps, here's the local sysctl configuration with the customized tunables I'm using:
net.core.rmem_max = 67108864
net.core.wmem_max = 67108864
net.ipv4.tcp_rmem = 4096 87380 67108864
net.ipv4.tcp_wmem = 4096 65536 67108864
net.ipv4.tcp_mem = 67108864 67108864 67108864
net.ipv4.route.flush = 1
net.core.netdev_max_backlog = 120000
net.ipv4.tcp_max_syn_backlog = 30000
net.ipv4.tcp_max_tw_buckets = 2000000
net.ipv4.tcp_congestion_control = htcp
net.ipv4.tcp_mtu_probing = 1
net.ipv4.tcp_tw_reuse = 1
net.ipv4.tcp_fin_timeout = 10
net.ipv4.tcp_slow_start_after_idle = 0
net.ipv4.tcp_reordering = 127
net.netfilter.nf_conntrack_tcp_timeout_syn_recv = 30
fs.inotify.max_user_instances = 2048
vm.legacy_va_layout = 1
kernel.sem = 250 256000 32 1024
kernel.sysrq = 1

Happy to provide additional information if needed. Thanks.