Meaning of Cached Available and Free Memory in Task Manager’s Performance “Physical Memory” section

Don't confuse free memory with unused memory. Free memory, in the unix world is a page of physical memory that has no logical data mapped to it. Unused memory does have some data mapped to it, but it is currently not in active use by a running process.

% free -m
             total       used       free     shared    buffers     cached
Mem:           997        942         55          0         71        366
-/+ buffers/cache:        504        492
Swap:         2015        618       1397

Linux (and all Unix OS'es) try to have as little free memory as possibly. Instead they use memory which is not actively mapped to processes in the running OS for things like file cache and buffers for various IO transfer operations.

Something else that may be confusing you is you cannot simply add up the memory in use by all running processes to get a total memory in use figure. If you attempted this you would quickly discover that you applications appear to be using more memory than actually exists on the machine. This is for two reasons

1. Memory can be shared between various processes, through Copy-On-Write memory allocation, memory mapped IO and shared dynamic libraries.
2. The operating system is at liberty to promise more memory to the application than it has actually supplied. The theory being that most application writers prefer to ask for large amounts of memory in one go, to avoid overhead, and may not actually use all that memory.

There is a recent article on lwn.net discussing this issue.

This is normal. SQL server will always use all available memory, and scale back when the system needs more resources.

THIS article describes what is going on.

When you start SQL Server, SQL Server memory usage may continue to steadily increase and not decrease, even when activity on the server is low. Additionally, the Task Manager and Performance Monitor may show that the physical memory available on the computer is steadily decreasing until the available memory is between 4 to 10 MB.

This behavior alone does not indicate a memory leak. This behavior is normal and is an intended behavior of the SQL Server buffer pool.

By default, SQL Server dynamically grows and shrinks the size of its buffer pool (cache) depending on the physical memory load reported by the operating system. As long as enough memory is available to prevent paging (between 4 - 10 MB), the SQL Server buffer pool will continue to grow. As other processes on the same computer as SQL Server allocate memory, the SQL Server buffer manager will release memory as needed. SQL Server can free and acquire several megabytes of memory each second, allowing it to quickly adjust to memory allocation changes.