I tried the techniques those are mentioned in the link already but nothing released the memory.
Again, why are you trying to release memory? SQL needs that memory to perform well. If it's using too much, reduce max server memory slightly
But still SOMETHING which makes Task manager to show the wrong results means - that SOMETHING should be the root casuse of problem - I trying to figure out that SOMETHING.
Task manager only shows memory allocated by the VirtualAlloc API call. When you have locked pages in memory, SQL does not use VirtualAlloc to allocate memory, it uses a different API call, hence Task Manager doesn't show that memory. It's a bug (or limitation) in Task Manager, nothing more.
As for the root cause of the problem, well there's no problem here. SQL's behaving as expected.
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: Discussions on DB performance with occasional diversions into recoverability
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