The following was posted by Dimitry Yemanov on the Firebird Development list in response to some questions about Memory Usage within Firebird (Classic)
"Need some help understanding how memory allocation is done per process on Linux 1.5.5 CS. It's understandable that there is some amount of working memory needed to support fb process, plus per connection amount, plus cache depending on database page size and buffer count."
Per process (independent) memory usage:
- page buffers (40MB in your case)
- metadata cache (cannot be predicted/controlled, depends on the schema complexity, especially number of interdependent PSQL objects)
- sort buffers (turned off by default on Classic, see SortMemUpperLimit in firebird.conf)
- undo log (cannot be predicted/controlled, wastes memory mostly for multiple batch updates in the same transaction)
Shared memory usage (depends on other connections):
- lock table (starts with LockMemSize and auto-growing)
- event table (starts with EventMemSize and auto-growing)
"What makes me wander is how concurrent user connection count affects memory utilization per process. Somehow the more users we have on the server - the bigger overhead is added per connection. When comparing mem utilization on same database having single connection and 55 for example - utilized memory delta increases by ~33MB. On server having 160 connections it's almost 2-3 times more. Is this lock manager related data? 30MB just for that?"
The lock manager data is shared, but it's mapped into the address space of the every Classic process. I dunno whether the Linux tools include the mmap'ed memory into the counters you check or not. If so, it gets easy to explain: yes, this increase is caused by the lock table. You can draftly calculate the lock manager memory requirements by this formula:
number of connections * number of page buffers * 100 bytes.
The exact size could be found in the output of fb_lock_print (header section) at runtime. With your quite large (for Classic) page cache settings, the numbers should be nearly the same as you experience.