SQL Server RAM usage is increasing and query executions is slowing down with time

Question

I'm using SQL server on a server with 48 GB of RAM.

When I start the SQL Server service, it's using 12% of the RAM at first, but after 24 hours it's reaching 90% and query executions begin slowing down. We have to restart the service so we can serve users.

What could be the problem and how we can fix it?

Example images:

and (select value_in_use from sys.configurations where name = 'max degree of parallelism')

and my server cores are 47.

Answer 1

Based on the waits statistics in your system you have incorrect settings for parallelism.

Maxdop in your system should not be more than 16, you can find the correct value based on this MS article: Recommendations and guidelines for the "max degree of parallelism" configuration option in SQL Server

Here are some T-SQL scripts to do it (I don't know your exact NUMA configuration but the scripts alilyze it): MAXDOP setting algorithm for SQL Server

Here is the explanation given by Max Vernon on which I agree. He explains why incorrect setting of maxdop slows down the execution.

If you set MAXDOP higher than the number of cores/numa node, you end up with calls into far memory which are many many times slower than calling near memory. This is because each numa node has its own memory; having a query use more threads than are present in a single numa mode will spread the CPU load over multiple cores, and therefore multiple memory nodes.

Answer 2

SQL will always use up the RAM it has to cache data. Setting a min/max RAM for your instance is always a good idea (especially MAX, so you keep some RAM for your OS) See this good answer on dba.SE:Why is SQL Server consuming more server memory?

Right click your instance in SSMS -> Memory -> Fill in min/max memory. It's a good idea to leave at least 4 GB for your OS on a SQL dedicated machine. If you have other services running, keep those in mind as well.

Now, you're saying your queries are slowing down, which should not happen. Perhaps you should make a different question asking for performance advice on specific queries.