I have a server with 96 GB of RAM that has the Buffer Cache Hit Ratio consistently at 100% and the PLE consistently over 500,000, so memory is obviously not a problem.
However, DPA shows the Procedure Cache Hit Ratio is usually in the low 90% range, dropping as low as 70% between 10pm and 7am when the user load is minimal. The Plan Cache Size doesn't vary significantly over the course of 24 hours, staying around 7 MB.
If I run DBCC FREESYSTEMCACHE('SQL Plans'), it drops the Plan Cache Size to less than 1 MB, but seems to have no effect on the Procedure Cache Hit Ratio.
What can I do to improve the Procedure Cache Hit Ratio?