Update.

Updated to 2.7.0beta2 since I saw it had some caching improvements but It only helped a little.
At this moment I stopped using Piwigo, but I REALLY want to use it, it's such an amazing script that it's a steal to get for free.

I will need some of your Piwigo Team members to help me throubleshoot this problem. And you guys can use my website to tweak your system for high traffic websites like mine.

If any of the team members read this, please send me an e-mail or reply here. I'm eager to have this script working.

--
Applepiee

Hello!

You're right about the plugins, but I disabled them the first 10 minutes, that was my first try (Specially the language changer is a real killer)

About the first time, I used galleries and I synced it, I can't see any process, and I trying 20 random images and they had all sizes.

You should see when it goes online on my website.

some words from my hosting:
There are hundreds of PHP processes running on the server.  MySQL CPU usage is
through the roof.  I have gotten your Apache values as high as I can without the
server blowing up, your CPU usage hasn't dropped below 100% for some time and
your memory is almost full as well.

They have a feeling that somewhere this is missing: http://php.net/manual/en/function.mysql-close.php

My website works perfectly but when changing to your gallery script it works for 2 minutes but when I see the active users going over around 250 in google analytics then the cpu just blows up some how.

--
Applepiee

I'm discussing with Applepiee to understand what can be wrong.

I am interested in knowing what the resolution was. Please post if you've solved the issue.

As described by Applepiee when visitors were sent to Piwigo, the server load was rising up 300 and more. In just a few seconds. Sometimes the server was also going very high in load with the old "specific" PHP script, but not for a long time.

Considering the number of "php <defunct>" processes visible on htop, I wondered how exactly the PHP scripts were handled: nginx (cache for images, reverse-proxy on Apache for PHP) => Apache => suPHP. I think suPHP is awfully slow on high traffic.

After many hours of tests, we moved the website on a new clean server, with the same kind of CPU but with another way to handle PHP execution: nginx => PHP-FPM. It is now "crazy fast". The load doesn't go higher than 7 (which is fine on a 8 threads CPU) and Piwigo page generation time never go higher than 100ms.

So now, my recommendation for high traffic on Piwigo is to use nginx + PHP-FPM. Here is the nginx conf I use:

server {
  server_name www.piwigonginx.com piwigonginx.com;
  root /var/www/piwigonginx.com;

  # with this you can remove ".php?" in Piwigo URLs
  location @rewrite {
    rewrite ^/picture((/|$).*)$ /picture.php$1 last;
    rewrite ^/index((/|$).*)$ /index.php$1 last;
    rewrite ^/i((/|$).*)$ /i.php$1 last;
  }

  location / {
    index index.php;
    try_files $uri $uri/ @rewrite;
  }

  location ~ ^(?<script_name>.+?\.php)(?<path_info>/.*)?$ {
    fastcgi_pass unix:/var/run/php5-fpm.sock;
    include fastcgi_params;
  }
}