why I cannot see memory related waits while server is under pressure?

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why I cannot see memory related waits while server is under pressure?

radekg

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May 27, 2022 at 3:50 pm

#4040314

Hi,
It may be trivial question but I am really confused at this moment.
At one of my servers I noticed that cached plans are deleted pretty much every 4 hours. Data files on that server consume a little bit more than 700 GB and SQL's memory is limited to 25 GB. I found one process that runs every few hours for which memory grant is about 3 GB. I know that there is huge room for improvement regarding query optimization and indexing...
But... I do not see any waits related to the memory. How it is possible that all plans are cleared out every 4 hours (which suggest heavy memory pressure) and at the same time I cannot see confirmation of that fact in wait statistics?

May 29, 2022 at 11:49 am

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May 30, 2022 at 10:31 am

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Site Owners SSC Guru Points: 80330 More actions · Answer 1

Thanks for posting your issue and hopefully someone will answer soon.

This is an automated bump to increase visibility of your question.

Jeff Moden SSC Guru Points: 1004329 More actions · Answer 2

See the following article and the link to the other article list in it.

https://blog.sqlauthority.com/2020/06/18/sql-server-queries-waiting-for-memory-grant-performance-tuning/

That will help you provide the proof you need to increase your RAM.

--Jeff Moden

RBAR is pronounced "ree-bar" and is a "Modenism" for Row-By-Agonizing-Row.
First step towards the paradigm shift of writing Set Based code:
________Stop thinking about what you want to do to a ROW... think, instead, of what you want to do to a COLUMN.

Change is inevitable... Change for the better is not.

Helpful Links:
How to post code problems
How to Post Performance Problems
Create a Tally Function (fnTally)

frederico_fonseca SSCoach Points: 16154 More actions · Answer 3

on some of my busy and misbehaved servers I have the following query running every 10 seconds with output to a table.

with this table its easy to identify queries that may require rewriting as well queries that may request incorrect memory grant due to bad statistics

insert into MemoryGrants
select getdate() as report_time
     , qmg.session_id
     , qmg.request_id
     , qmg.scheduler_id
     , qmg.dop
     , qmg.request_time
     , qmg.grant_time
     , qmg.requested_memory_kb
     , qmg.granted_memory_kb
     , qmg.required_memory_kb
     , qmg.used_memory_kb
     , qmg.max_used_memory_kb
     , qmg.query_cost
     , qmg.timeout_sec
     , qmg.resource_semaphore_id
     , qmg.queue_id
     , qmg.wait_order
     , qmg.is_next_candidate
     , qmg.wait_time_ms
     , qmg.plan_handle
     , qmg.sql_handle
     , qmg.group_id
     , qmg.pool_id
     , qmg.is_small
     , qmg.ideal_memory_kb
     , t.text
from sys.dm_exec_query_memory_grants qmg
cross apply sys.dm_exec_sql_text(qmg.sql_handle) as t
where qmg.requested_memory_kb > 100000 -- queries requesting more than 100MB of memory
;