Question about limit_req_zone and max shared memory

[Christos Chatzaras]

I want to rate limit PHP requests with "client ip + vhost + same url" but on same servers sometimes I see:

[alert] 78841#0: could not allocate node in limit_req zone "req_limit_per_ip_per_uri" 

which causes 429 errors in all domains.

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FreeBSD/amd64

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# Max connections

events {
worker_connections 16384;
}

# Rate limit

limit_req_zone "$binary_remote_addr$host$request_uri" zone=req_limit_per_ip_per_uri:10m rate=2r/s;

# Vhosts

server {

   server_name www.example1.com;
   ...
   location ~ [^/]\.php(/|$) {
      ...
      limit_req zone=req_limit_per_ip_per_uri burst=20 nodelay;
      ...
   }
...
}

server {

   server_name www.example2.com;
   ...
   location ~ [^/]\.php(/|$) {
      ...
      limit_req zone=req_limit_per_ip_per_uri burst=20 nodelay;
      ...
   }
...
}

----

 Ιs it better to use different variables for key? Which is the max shared memory needed?