nginx在访问量稍大的情况下，出现503 Service Temporarily Unavaila

当前环境是LNMP（云环境是SLB+ECS+Ploardb），使用jmeter测试时，300的并发，循环5次，当中有20%的请求会出现503错误，看了网上的资料是说，做了ip访问次数限制（limit_conn），但是我再配置文件中没有设置，比较头大，请各位帮下忙，十分感谢 下面是nginx的配置文件： /etc/nginx/nginx.conf：

# For more information on configuration, see:
#   * Official English Documentation: http://nginx.org/en/docs/
#   * Official Russian Documentation: http://nginx.org/ru/docs/

user nginx;
worker_processes auto;
worker_rlimit_nofile 65535;
error_log /var/log/nginx/error.log;
pid /run/nginx.pid;

# Load dynamic modules. See /usr/share/doc/nginx/README.dynamic.
include /usr/share/nginx/modules/*.conf;

events {
    worker_connections 20480;
    use epoll;
}

http {
    log_format  main  '$remote_addr - $remote_user [$time_local] "$request" '
                      '$status $body_bytes_sent "$http_referer" '
                      '"$http_user_agent" "$http_x_forwarded_for"';

    access_log  /var/log/nginx/access.log  main;

    sendfile            on;
    tcp_nopush          on;
    tcp_nodelay         on;
    keepalive_timeout   200;
    types_hash_max_size 2048;

    fastcgi_connect_timeout 300;
    fastcgi_send_timeout 300;
    fastcgi_read_timeout 300;

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

    # Load modular configuration files from the /etc/nginx/conf.d directory.
    # See http://nginx.org/en/docs/ngx_core_module.html#include
    # for more information.
    include /etc/nginx/conf.d/*.conf;

    server {
        listen       80 default_server;
        listen       [::]:80 default_server;
        server_name  _;
        root         /usr/share/nginx/html;

        # Load configuration files for the default server block.
        include /etc/nginx/default.d/*.conf;

        location / {
        }

        error_page 404 /404.html;
            location = /40x.html {
        }

        error_page 500 502 503 504 /50x.html;
            location = /50x.html {
        }
    }

# Settings for a TLS enabled server.
#
#    server {
#        listen       443 ssl http2 default_server;
#        listen       [::]:443 ssl http2 default_server;
#        server_name  _;
#        root         /usr/share/nginx/html;
#
#        ssl_certificate "/etc/pki/nginx/server.crt";
#        ssl_certificate_key "/etc/pki/nginx/private/server.key";
#        ssl_session_cache shared:SSL:1m;
#        ssl_session_timeout  10m;
#        ssl_ciphers HIGH:!aNULL:!MD5;
#        ssl_prefer_server_ciphers on;
#
#        # Load configuration files for the default server block.
#        include /etc/nginx/default.d/*.conf;
#
#        location / {
#        }
#
#        error_page 404 /404.html;
#            location = /40x.html {
#        }
#
#        error_page 500 502 503 504 /50x.html;
#            location = /50x.html {
#        }
#    }

}

/etc/nginx/conf.d/default.conf：

server {
    listen 80;
    server_name 127.0.0.1;
    error_page 404 /404.html;
    error_page 500 502 503 504 /50x.html;
   #   \.php 只处理动态请求，对于静态资源请求由下面的 location匹配和处理
    location ~ \.php {
            root /www/html;
            fastcgi_pass 127.0.0.1:9000;
            #包含nginx服务器传递给fastcgi程序的参数，php中通过$_SERVER['参数名']可获取
            include   fastcgi.conf;
            #定义变量$fastcgi_script_name_new赋值为$fastcgi_script_name变量
            set $path_info "";
            set $fastcgi_script_name_new $fastcgi_script_name;
            #判断url是否是pathinfo形式的，如果是则把这个url分割成两部分，index.php入口文件之后的pathinfo部分存入$path_info变量中，剩下的部分和$document_root根目录定位index.php入口文件在文件系统中的绝对路径 .
 
          if ($fastcgi_script_name ~*   "^(.+\.php)(/.+)$"  ) {
                    set $fastcgi_script_name_new $1;
                    set $path_info $2;
            }
 
                    #对fastcgi.conf中的SCRIPT_FILENAME和SCRIPT_NAME fastcgi参数进行重写,目的是指定入口文件在文件系统中的绝对路径给script_filename参数，让fastcgi知道index.php文件位置。
                    fastcgi_param   SCRIPT_FILENAME   $document_root$fastcgi_script_name_new;
                    fastcgi_param   SCRIPT_NAME   $fastcgi_script_name_new;
                    #定义一个新的nginx服务器传递给fastcgi的参数PATH_INFO,thinkphp需要这个入口文件index.php后的pathinfo信息
                    fastcgi_param   PATH_INFO $path_info;
    }
# 用来匹配静态资源，如果不是静态资源就重写，然后重新轮训所有的location块，由上面的location块匹配后动态处理这个请求
    location / {
        root /www/html;
        index index.php index.html index.htm;
 
        if (!-e  $request_filename){
                rewrite ^(.*)$ /index.php$1 last;
        }
    }
}