Q: Performance Dashboard Memory Usage

What exactly does the Memory Usage of Performance Dashboard consider as "used by SQL Server"?

Take a look at these screenshots.

The first one shows the Performance Dashboard saying that of 32GB of memory the server can abuse, there's ~15 GB used by SQL Server, another ~15GB used by something else, and there's ~2 GB free.

The second one says that according to the Task Manager on the same machine, ~26.5GB (~80% of the total, and ~86% of the used memory) is currently being tortured by SQL Server which only leaves around 3.5GB for other processes to play with.

One of these is stretching the truth, to be polite. But which one and why?

There are many different kinds of memory and as far as I know their reporting in the Task Manager is usually difficult to understand. Reserved virtual memory, private set, public, working set, pageable vs not pageable, committed vs not committed, etc... etc.... If you search the web for questions of what memory usage means exactly in the Task Manager, it looks like one of the most common questions is why memory usage reported by the Task Manager doesn't add up to 100%, and doesn't match other things, and the common response it's a result of apples to oranges comparison for different kinds of memory. On top of that SQL Server uses special Address Windowing Extensions (AWE) for it's extended memory management, which isn't even reflected in the Task Manager. The Metric numbers on the top chart come from OS performance counters accessible via sys.dm_os_sys_memory dynamic performance view, and from sys.dm_os_memory_clerks for the SQL Server memory usage. Hope this helps.