Performance issues, can I recover memory? In the past month I am having some serious performance issues with my MacBook Pro (2008 model).
From the activity monitor, I get that the CPU usage is low (25%), disk usage is low (256 GB SSD drive, 50% free), network activity almost null.
What concerns me is system memory usage; of the 4GB RAM I have:  

Free: 35 MB
Wired: 1.1 GB
Active: 1.9 GB
Inactive: 900 MB
Used: 3.99 GB
VM size: 650 GB
Swap used: 11 GB

I have nothing particularly heavy running. Could it be that, since I have not rebooted for a few months, some executables keep requiring memory slowing the system down (memory leak)?  

Is it possible to recover unused but locked memory, without a reboot?
To answer your question, yes, applications and processes continue to start up and if you have not manually closed them or restarted the computer, you will be "losing" RAM. You can turn the inactive memory into free memory with this command in Terminal:

sudo purge


You can also run Activity Monitor to diagnose what apps are running and sort the list by most memory.