nginx container uses only one out of 4 cores

Question

created an Nginx container with 4 static CPUs

docker run -tid --cpus 4 --cpuset-cpus 6,7,8,9 --net=host --name mynginx nginx docker exec -ti mynginx cat /etc/nginx/nginx.conf|grep worker_processes worker_processes auto; 

Assigned CPUs

taskset -acp 1426038 pid 1426038's current affinity list: 6-9 

Sending Traffic:

ab -n 10000000 -c 1000 http://172.16.91.5:80/ 

On top i can see that core 6 is used where as all the remaining cores are idle.

from top-Nginx Cores

 %Cpu6 : 14.6 us, 57.3 sy, 0.0 ni, 28.2 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st %Cpu7 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st %Cpu8 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st %Cpu9 : 0.0 us, 0.0 sy, 0.0 ni,100.0 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st 

OS Cores

 %Cpu0 : 0.0 us, 2.8 sy, 0.0 ni, 32.1 id, 0.0 wa, 0.0 hi, 65.1 si, 0.0 st %Cpu1 : 0.0 us, 5.6 sy, 0.0 ni, 40.7 id, 0.0 wa, 0.0 hi, 53.7 si, 0.0 st %Cpu2 : 0.0 us, 0.9 sy, 0.0 ni, 29.9 id, 0.0 wa, 0.0 hi, 69.2 si, 0.0 st 

load average:

 34.84, 19.62, 8.36 

Any explanation to this behavior?

Answer 1

My guess is that nginx uses a hash of the client IP to select a worker from the pool. Since with ab all connections come from the same IP address, they all end up processed by single worker, therefore loading only one CPU core.