I have many monitored instances of SQL Server, and on all but one of them, the Plan cache and Buffer cache add up (approximately) to the Total memory. However, one instance, shown below isn't even close to adding up. Where is the other 1600MB? How do I find this out?
The Memory -> Summary drilldown can give you more insight into how memory is being consumed. Take a look at the "Memory Areas" and "Cache" graphs to see the sizes of different sections of memory.
I have usually avoided the drilldowns as the bounds of the graphs never seem to work right. (The scale for the Cache graph by default has a Y-Axis of 0-16TB). After changing the "Bounds" setting to "From Data", I can see the data properly. This data makes sense, but if 2.5GB of data is in "Free Pages", shouldn't the "Memory Used" metric on my original image not be showing 100% in use?
Anyone have suggestions on this? Do I have too much RAM allocated to this instance? It looks like it never uses more than 2GB, but that Memory Used 100% makes me nervous to re-allocate this extra 2GB to another instance that I know needs it.