There were lots of PAGEIOLATCH_SH waits while running quires, when drilled down I found the statistics were out of date, hence I ran update statistics to all databases. That time usage of physical memory became high and it hold almost full (63.6 GB out of 64 GB). Now Update Statistics activity has been finished and all the queries respond very fast but the usage of physical memory remains 63.6GB only. The Windows Task Manager->Processes->sqlservr.exe shows it holds only 961,360 KB of Memory. Other processes also don't take much memory. It is a Production server - I can't restart. Hence please help me to free up space.