Comments - Would running multiple server processes increase performance or security?
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If you are short on hardware resources, cpus memory etc, there will be increased overhead as the OS context switched (small) or paging out of swap (large) between instances resulting in a degraded of performance.
The PHP approach of pooling allows a fixed amount of resources to be available for each application. MariaDB is very good and ensuring a fair use of resources between multiple connections however inadequite tuning/monitoring of connections/table cache elements can cause some contention between different applications. Running separate instances will still require these to be tuned per application.
MariaDB access control with a user per db application (with limited grants) does a very good job of security for many applications. For protection against unknown MariaDB vulnerabilities there are reasonable amount of security benefits of running multiple instances in protecting the security of one application from similar exploitation in a different application (assuming that they are a different applications meaning the same web application exploit that results in a database security issues cannot be applied by the same user to a different instance).