Problèmes de montée en charge

Limites de FileSystem

En faisant des tests avec la création de nombreux fichiers dans un seul dossier, on obtient des baisses de performance progressives de 100.000 à 1.000.000 de fichiers. Au delà de 1.000.000 de fichiers, le FileSystem finit par ne plus répondre et les requêtes finissent en time_out.

Si on reporte ces résultats sur WordPress MU, on a autour de 10 tables par blog et sous MySQL, on a 3 fichiers par table, soit autour de 30 fichiers créés par blog. (Il vaut mieux éviter d'utiliser des plugins qui ajoutent des tables supplémentaires…)

Avec 10.000 blogs, on aurait autour de 300.000 fichiers dans le seul répertoire de la database
Avec 20.000 blogs, on aurait autour de 600.000 fichiers dans le seul répertoire de la database
Avec 30.000 blogs, on aurait autour de 900.000 fichiers dans le seul répertoire de la database
Avec 40.000 blogs, on aurait autour de 1.200.000 fichiers dans le seul répertoire de la database

WordpressMU devrait pouvoir supporter 10.000 blogs et surement 20.000 blogs. Mais aller à 30.000 blogs et plus causera surement des problèmes de temps d'accès aux fichiers. Et quand la limite sera atteinte, le système refusera de créer de nouveaux blogs…

Mémoire Applicative PHP

Version 1.2.1

Avec une limite PHP de 32 Mo, lorsqu'une centaine de blogs a été créée, on voit apparaître des problèmes de limite de mémoire.

[21-Jun-2007 11:05:16] PHP Fatal error:  Allowed memory size of 31457280 bytes exhausted (tried to allocate 233994 bytes) in 
... /wp-includes/cache.php on line 48

Dans le fichier wp-config.php, j'ai alors changé les paramètres du cache:

define( "CACHE_EXPIRATION_TIME", 100 );
define( 'DISABLE_CACHE', true);

Mais les erreurs continuent de se produire… Donc j'ai changé le .htaccess pour augmenter la limite de mémoire à 40M

php_value memory_limit 40M

Cette erreur de mémoire se produit dans mon cas pour la page de création de nouveaux blogs: pour un utilisateur, la liste de tous les blogs créés est affichée. Et dans mon cas, l'admin a créé plus de 100 blogs. Quand on arrive autour de 150 blogs, la mémoire consommée peut dépasser les 40 Mo, il a donc fallu pousser la limite à 50 Mo.

C'est une situation assez rare dans la plupart des cas, mais il ne faut pas perdre de vue ce que représente 30Mo de mémoire.

Version 1.2.2

En mettant un error_log quand la mémoire utilisée dépasse 10Mo, on voit que cette version améliore l'utilisation de la mémoire. Peut-être que le bug du cache a été corrigé ?

Référence

Une page de 100×100 caractères donne 10.000 Octets soit 10Ko. 30Mo = 30.000 Ko, soit l'équivalent d'un livre de 3.000 pages! C'est complètement disproportionné! Jamais un programme PHP ne doit utiliser autant de mémoire pour créer une page HTML!

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