Error de nginx, php-fpm mientras que con Apache no hay errores

Question

Tengo una aplicación opencart implementada con nginx, fastcgi. Estoy usando RDS como servidor Mysql.

Últimamente he tenido estos errores en mi registro y ejecuté una prueba de carga con asedio para 400 usuarios simultáneos. Recibo este error.

FastCGI sent in stderr: "PHP message: PHP Warning: mysqli::mysqli

mientras ejecuta la misma prueba de carga con Siege usando Apache como servidor. La prueba arroja resultados positivos.

Estos son los resultados del asedio con nginx

Transactions:               3246 hits
Availability:              77.80 %
Elapsed time:             146.43 secs
Data transferred:          35.27 MB
Response time:              6.86 secs
Transaction rate:          22.17 trans/sec
Throughput:             0.24 MB/sec
Concurrency:              152.01
Successful transactions:        1049
Failed transactions:             926
Longest transaction:           36.83
Shortest transaction:           0.65

y con apache

Transactions:               7921 hits
Availability:              99.01 %
Elapsed time:             462.60 secs
Data transferred:         218.75 MB
Response time:              9.94 secs
Transaction rate:          17.12 trans/sec
Throughput:             0.47 MB/sec
Concurrency:              170.12
Successful transactions:        4925
Failed transactions:              79
Longest transaction:          114.25
Shortest transaction:           0.59

Estos son nginx y fpm conf:

user www-data;
worker_processes 1;
pid /run/nginx.pid;

events {
        worker_connections 1024;
        # multi_accept on;
}

http {

        ##
        # Basic Settings
        ##

        sendfile on;
        tcp_nopush on;
        tcp_nodelay on;
        keepalive_timeout 65;
        types_hash_max_size 2048;
        # server_tokens off;

        # server_names_hash_bucket_size 64;
        # server_name_in_redirect off;

        include /etc/nginx/mime.types;
        default_type application/octet-stream;

        ##
        # Logging Settings
        ##

        access_log /var/log/nginx/access.log;
        error_log /var/log/nginx/error.log;

        ##
        # Gzip Settings
        ##

        # Compression

        # Enable Gzip compressed.
        gzip on;
        # Enable compression both for HTTP/1.0 and HTTP/1.1.
        #gzip_http_version  1.1;

        # Compression level (1-9).
        # 5 is a perfect compromise between size and cpu usage, offering about
        # 75% reduction for most ascii files (almost identical to level 9).
        gzip_comp_level    4;

        # Don't compress anything that's already small and unlikely to shrink much
        # if at all (the default is 20 bytes, which is bad as that usually leads to
        # larger files after gzipping).
        gzip_min_length    256;

        # Compress data even for clients that are connecting to us via proxies,
        # identified by the "Via" header (required for CloudFront).
        gzip_proxied       any;

        # Tell proxies to cache both the gzipped and regular version of a resource
        # whenever the client's Accept-Encoding capabilities header varies;
        # Avoids the issue where a non-gzip capable client (which is extremely rare
        # today) would display gibberish if their proxy gave them the gzipped version.
        gzip_vary          on;

        # Compress all output labeled with one of the following MIME-types.
        gzip_types
          application/atom+xml
          application/javascript
          application/x-javascript
          application/json
          application/rss+xml
          application/vnd.ms-fontobject
          application/x-font-ttf
          application/x-web-app-manifest+json
          application/xhtml+xml
          application/xml
          font/opentype
          image/svg+xml
          image/x-icon
          text/css
          text/plain
          text/x-component
          text/xml
          text/javascript
          text/x-javascript
        # text/html is always compressed by HttpGzipModule


        ##
        # nginx-naxsi config
        ##
        # Uncomment it if you installed nginx-naxsi
        ##
        #include /etc/nginx/naxsi_core.rules;

        ##
        # nginx-passenger config
        ##
        # Uncomment it if you installed nginx-passenger
        ##

        #passenger_root /usr;
        #passenger_ruby /usr/bin/ruby;

        ##
        # Virtual Host Configs
        ##

        include /etc/nginx/conf.d/*.conf;
        include /etc/nginx/sites-enabled/*;
}