Tag / SQLOS

by Arvind Shyamsundar

Query of the day: finding SQL Server queries with large memory grants

Quick tip from me today: I recently had to check on which T-SQL query / queries in the system were using up some monster 30GB+ query grants. Luckily the sys.dm_exec_query_memory_grants DMV facilitates this. Here is the query I finally used to figure out what was happening:

SELECT r.session_id
    ,mg.granted_memory_kb
    ,mg.requested_memory_kb
    ,mg.ideal_memory_kb
    ,mg.request_time
    ,mg.grant_time
    ,mg.query_cost
    ,mg.dop
    ,(
        SELECT SUBSTRING(TEXT, statement_start_offset / 2 + 1, (
                    CASE
                        WHEN statement_end_offset = – 1
                            THEN LEN(CONVERT(NVARCHAR(MAX), TEXT)) * 2
                        ELSE statement_end_offset
                        END – statement_start_offset
                    ) / 2)
        FROM sys.dm_exec_sql_text(r.sql_handle)
        ) AS query_text
    ,qp.query_plan
FROM sys.dm_exec_query_memory_grants AS mg
INNER JOIN sys.dm_exec_requests r ON mg.session_id = r.session_id
CROSS APPLY sys.dm_exec_query_plan(r.plan_handle) AS qp
ORDER BY mg.required_memory_kb DESC;

In case you are wondering what memory grants are all about, you should start from this great blog post from Jay Choe, who is part of the SQL Server development team. Jay also posts some useful variants of queries to find out (for example) cached query plans with memory grants. It is a great read.

Happy query tuning!

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by Arvind Shyamsundar

Getting worker thread IDs using DMVs

In SQL 2000, the kpid column in sysprocesses was a convenient way to find out the actual worker thread ID (OS level thread identifier) for a given task. How does one do this using the DMVs?

Here is a quick way:

select R.Session_Id, Th.os_thread_id from sys.dm_exec_requests R
join sys.dm_exec_sessions S on R.session_id = S.session_id
join sys.dm_os_tasks T on R.session_id = T.session_id
join sys.dm_os_workers W on T.worker_address = W.worker_address
join sys.dm_os_threads Th on W.thread_address = Th.thread_address
where S.is_user_process = 1
order by Session_id

The above script will get you the session ID and thread ID for any tasks currently running, and will also filter out any non-user processes. BUT… my hand-written T-SQL looks so crowded and ugly. Let us format it with the T-SQL formatting script I shared a while back on my blog:

SELECT R.Session_Id,
Th.os_thread_id
FROM   sys.dm_exec_requests AS R
INNER JOIN
sys.dm_exec_sessions AS S
ON R.session_id = S.session_id
INNER JOIN
sys.dm_os_tasks AS T
ON R.session_id = T.session_id
INNER JOIN
sys.dm_os_workers AS W
ON T.worker_address = W.worker_address
INNER JOIN
sys.dm_os_threads AS Th
ON W.thread_address = Th.thread_address
WHERE  S.is_user_process = 1;

Looks a lot easier to read! Now, by switching the last is_user_process flag to 0, you can switch to viewing the OS thread IDs of system processes. In the next post, I will show you how to use this knowledge and information to dig even deeper and view the operation of system threads like the lazy writer. Stay tuned!