Definitely consider doing some MySQL performance tuning. There are scripts you can download that check your environment and offer suggestions for changes in you my.cnf file.
MySQL allocates memory in tons of places. Especially look at
table cachePerformance_schema Run this:
show engine performance_schema status
and look at the last line. That may be the cause for the systems with small amount of RAM, i.e. 1G or less
InnoDB Run this:
show engine innodb status
check the
buffer pool section, memory allocated for
buffer_pool and related caches)
Temporary tables in RAMFind all in-memory tables by running
select * from information_schema.tables where engine='MEMORY'
Prepared statements, when it is not deallocated (check the number of prepared commands via
deallocate command by running
show global status like ‘ Com_prepare_sql';show global status like 'Com_dealloc_sql'
Welcome to the deep end