I have a load balanced application on AWS, on two c4.2xlarge (8 vCPUs, 15GB ram) servers and they serve a very simplistic site but with a system call to ffmpeg encode a <5s mp4. Individually, the script takes < 1s to execute, but under stress, the response time goes over the roof.

500 concurrent users over a period of 20s


Testing with 1000 concurrent users over the span of 1 minute gets a lot worse, regardless of the load balance setup.

enter image description here

I cannot figure out why the response time of the request increases quite significantly over the duration of my stress tests.

fpm pool configuration

pm.max_children = 100
pm.start_servers = 25
pm.min_spare_servers = 25
pm.max_spare_servers = 50
;pm.process_idle_timeout = 10s;
pm.max_requests = 500

request_terminate_timeout = 180s

php_admin_value[memory_limit] = 128M
php_admin_value[upload_max_filesize] = 3M
php_admin_value[post_max_size] = 3M
php_admin_value[max_execution_time] = 180

nginx configuration

events {
    worker_connections 66536;
    use epoll;
    multi_accept on;

worker_rlimit_nofile 30000;


sendfile on;
tcp_nopush on;
tcp_nodelay on;
keepalive_timeout 65;
types_hash_max_size 2048;
server_tokens off;
access_log off;
log_not_found off;

location ~ \.php$ {
    try_files $uri /index.php =404;

    include fastcgi_params;

    fastcgi_split_path_info ^(.+\.php)(/.+)$;
    fastcgi_pass unix:/var/run/php/php7.0-fpm.sock;
    fastcgi_index index.php;
    fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
    fastcgi_read_timeout 180; 

The duration of the request to be completed is not a particular issue in my case, as long as is more or less consistent and < 15s.

So, can't help but wonder, where am I going wrong about my approach and what should I look into, in order to solve this increasing response time?


I have now, changed to io1 with 2000 iops for a 40gb root block device (don't really need EBS)

Some server stats, with peeks of when I ran the tests

CPU 1 minute interval cpu-utilization-1minute

CPU 5 minutes interval cpu-utilization-5minutes

Really strange how it peeks to 100% in the 1 minute interval


Command and its arguments

ffmpeg -r 7 -s 470x264 -i upload/1494515054/%01d.jpg -vcodec libx264 -crf 25 -pix_fmt yuv420p -preset ultrafast test/`date +%s`.mp4

Execution times: ranging from 0m0.148s to 0m0.163s

Howeve, the command is executed via php's shell_exec, and in that script it rounds around 350ms 500ms

  • You didn't show any information about your host resource usage - CPU, Disk, memory utilization – AlexD May 23 '17 at 15:44
  • Just updated with some stats – w0rldart May 23 '17 at 19:12
  • Could you provide some benchmark for encoding script? But in general encoding is CPU bound and if the script takes 0.5 seconds to complete you won't be able to serve more than 2 *8 / 0.5 = 32 requests per second and you are hitting servers with hundreds of requests so obviously your response time skyrockets. – AlexD May 23 '17 at 20:03
  • Added some more stats... (where does the 2 come from?) – w0rldart May 23 '17 at 21:06
  • You said that you have 2 load balanced servers. – AlexD May 23 '17 at 21:19

Sounds like it might just be the disk I/O; 1000 concurrent users over the span of 1 minute ~ 16.7/second, presumably each accessing a different mp4 file that you have to encode. How long does it take you to encode 1000 different mp4 files simultaneously (or if you can schedule it, spawning 17 new encoding tasks every second), and what is the disk I/O like?

  • interesting idea. Will try and run that encoding for 1000 mp4s – w0rldart May 23 '17 at 19:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.