How many sockets does my machine have in SQL Server?

select

cpu_count/hyperthread_ratio AS sockets

from

sys.dm_os_sys_info

Buffer Cache Hit Ratio

Buffer Cache Hit Ratio shows how SQL Server utilizes buffer cache
“Percent of page requests satisfied by data pages from the buffer pool”

SELECT object_name, counter_name, cntr_value
FROM sys.dm_os_performance_counters
WHERE [object_name] LIKE '%Buffer Manager%'
AND [counter_name] = 'Buffer cache hit ratio'

The recommended value for Buffer Cache Hit Ratio is over 90. A lower value indicates a memory problem.

Page Life Expectancy

“Duration, in seconds, that a page resides in the buffer pool”

SELECT object_name, counter_name, cntr_value
FROM sys.dm_os_performance_counters
WHERE [object_name] LIKE '%Buffer Manager%'
AND [counter_name] = 'Page life expectancy'

The normal values are above 300 seconds (5 minutes) and the trend line should be stable. If the value is below 300, it’s a clear indication that something is wrong
Also, a value drop of more than 50% is a sign for deeper investigation.