Centos – How to tune apache with mpm_event

regarding apache tuning, there are lots of good documents and posts on the web
eg. How To Tune Apache on Ubuntu 14.04 Server

unfortunately almost all of them describe how to tune apache with mpm_prefork.

as I understand though, prefork method is a bit old and lacks efficiency in multi threading and …
anyway my client insists on using mpm_event, and their main problem is with a few number of visitors their VPS become unresponsive, and I can see that the problem is with memory as their ram gets full, it gets to use the swap, swap gets full and the only way to restore the server would be restarting httpd service, or god forbid, restarting server itself!

The VPS has 2GB of ram while it servers as webserver, mailserver and MySql is also installed on it.
the php.ini file is allowing max_memory=140M (which i think is a bit high?!) but for now they really need this amount.
with a quick calculation about other tasks that VPS is doing, I think it is safe to assign 1GB of memory to apache.

But I can not find any method on the web to calculate the settings for mpm_event to limit it's usage to this 1GB max.

any help would be appreciated on this calculation

needed info from comments:

ps -ef | grep php => nothing

phpinfo() from Apache => Server Api : CGI/FastCGI

apachectl -M =>

Loaded Modules:
 core_module (static)
 authn_file_module (static)
 authn_dbm_module (static)
 authn_anon_module (static)
 authn_dbd_module (static)
 authn_socache_module (static)
 authn_core_module (static)
 authz_host_module (static)
 authz_groupfile_module (static)
 authz_user_module (static)
 authz_dbm_module (static)
 authz_owner_module (static)
 authz_dbd_module (static)
 authz_core_module (static)
 access_compat_module (static)
 auth_basic_module (static)
 auth_form_module (static)
 auth_digest_module (static)
 allowmethods_module (static)
 file_cache_module (static)
 cache_module (static)
 cache_disk_module (static)
 cache_socache_module (static)
 socache_shmcb_module (static)
 socache_dbm_module (static)
 socache_memcache_module (static)
 so_module (static)
 macro_module (static)
 dbd_module (static)
 dumpio_module (static)
 buffer_module (static)
 ratelimit_module (static)
 reqtimeout_module (static)
 ext_filter_module (static)
 request_module (static)
 include_module (static)
 filter_module (static)
 substitute_module (static)
 sed_module (static)
 deflate_module (static)
 http_module (static)
 mime_module (static)
 log_config_module (static)
 log_debug_module (static)
 logio_module (static)
 env_module (static)
 expires_module (static)
 headers_module (static)
 unique_id_module (static)
 setenvif_module (static)
 version_module (static)
 remoteip_module (static)
 proxy_module (static)
 proxy_connect_module (static)
 proxy_ftp_module (static)
 proxy_http_module (static)
 proxy_fcgi_module (static)
 proxy_scgi_module (static)
 proxy_wstunnel_module (static)
 proxy_ajp_module (static)
 proxy_balancer_module (static)
 proxy_express_module (static)
 session_module (static)
 session_cookie_module (static)
 session_dbd_module (static)
 slotmem_shm_module (static)
 ssl_module (static)
 lbmethod_byrequests_module (static)
 lbmethod_bytraffic_module (static)
 lbmethod_bybusyness_module (static)
 lbmethod_heartbeat_module (static)
 unixd_module (static)
 dav_module (static)
 status_module (static)
 autoindex_module (static)
 info_module (static)
 suexec_module (static)
 cgi_module (static)
 dav_fs_module (static)
 dav_lock_module (static)
 vhost_alias_module (static)
 negotiation_module (static)
 dir_module (static)
 actions_module (static)
 speling_module (static)
 userdir_module (static)
 alias_module (static)
 rewrite_module (static)
 suphp_module (shared)
 ruid2_module (shared)
 mpm_event_module (shared)