Sql-server – Performance impact of Extended Events sessions looking at the exact same event type

extended-eventssql server

I have created an extended events session that watches the module_start event type and filters down based on the object_name: equal_i_sql_unicode_string]([object_name])

The purpose of this session is to simply record basic information whenever a proc in the filter list is called so that I can answer developers questions with 99.9% guarantee whether or not a proc is still called in production. The idea is to run this for ~1 month 24/7 (yes it doesn't account for things that are ran annually, but it is what it is).

The issue I am running into is that the list of procs the developer gave me is about 90 or so long and the filter list of an EE session is limited to 3,000 characters. The only idea I have come up with in order to increase the rate at which we can track the procs is to have 2 separate EE sessions that are identical except the filter predicates are different.

I am not asking "how much of a CPU impact will this be", but more or less is their any concern with running 2 of the same EE sessions with different filter predicates? It is odd to me that Microsoft would limit the filter list to 3,000 characters when 'more filtering == better performance' because the way EE is built into the engine it is very optimized unlike a trace that acts more like a proxy than a "trigger based off an event".

Is it safe to assume that whatever the performance impact of running 1 session is I can multiply it by 2 or is their further concerns I am not considering?

Best Answer

I'm not an expert on Extended Events by any means but I do know there is potential for performance implications if not done carefully. You'll only really be able to find out by testing and monitoring throughout the day if there's any meaningful impact on your current server / database's workload. I would think by filtering it down to specific entities which aren't necessarily even being used might be ok though.

An alternative idea is to modify each stored procedure such that it inserts into a table the name of the procedure and the current datetime.

For some other alternative solutions you can see this StackOverflow post's answers. I would be careful using sys.dm_exec_procedure_stats if you go that route though, as it isn't 100% guaranteed to give you the results you're looking for, if an event has cleared out the plan cache.

Related Solutions

Sql-server – How to determine what is using a particular resource (table, view, or function, etc) on the SQL Server

I did a DuckDuckGo.com search for the like_i_sql_unicode_string since there appears to be no documentation that describes it included with SQL Server, and came across this Connect item, https://connect.microsoft.com/SQLServer/feedback/details/699053/sql-batch-completed-event-returns-0-for-row-count#details - it contains a sample extended event in the "steps to reproduce" section of the Connect item. In the sample extended event is this text, which wraps the search terms in question in the standard SQL Server wildcard, %.

 WHERE ([sqlserver].[like_i_sql_unicode_string]([batch_text],N'%sys.objects%')))

I modified my extended event definition like:

DROP EVENT SESSION FindCaller ON SERVER;

CREATE EVENT SESSION [FindCaller] ON SERVER 
ADD EVENT sqlserver.sql_batch_starting
(
    ACTION
    (
        sqlserver.client_app_name
        , sqlserver.client_hostname
        , sqlserver.database_name
        , sqlserver.nt_username
        , sqlserver.session_id
        , sqlserver.sql_text
    )
    WHERE ([sqlserver].[like_i_sql_unicode_string]([sqlserver].[sql_text],N'%ObjectInQuestion%'))
)
WITH 
(
    MAX_MEMORY=4096 KB
    , EVENT_RETENTION_MODE=ALLOW_SINGLE_EVENT_LOSS
    , MAX_DISPATCH_LATENCY=30 SECONDS
    , MAX_EVENT_SIZE=0 KB
    , MEMORY_PARTITION_MODE=NONE
    , TRACK_CAUSALITY=OFF
    , STARTUP_STATE=OFF
);

Now, when I 'Watch Live Data' I see the culprit statements, along with the actual SQL text being executed, the name of the application, the client machine name, etc. Quite useful.

The take-home for me is the Extended Event GUI needs better documentation!

Sql-server – Performance impact of Extended Events filter [like_i_sql_unicode_string]

I would say you don't need all the start events, just the completed. I don't see that you are capturing data just for the SCOM database either. Extended events can cause an overhead just near to what SQL Server traces will depending on what you are trying to capture.

You are basically capturing every query that comes across the server and telling it to wait so you can first check the text of the query and then grab information through the extended event session, then letting it complete.

As well the constant activity SCOM has, depending on environment configuration, it is likely to have an extreme performance hit either with trace or extended events. I would suggest looking at the plan cache for those types of queries initially. However you might try adding more filters to your session such only grabbing a sample of data from active sessions and then also only capturing data for the particular database. That would look something like this through the GUI:

enter image description here

Or code your WHERE clause would look something like:

WHERE ((([package0].[divides_by_uint64]([sqlserver].[session_id],(5))) 
   AND ([package0].[greater_than_uint64]([sqlserver].[database_id],(4)))) 
   AND ([package0].[equal_boolean]([sqlserver].[is_system],(0)))))

This would capture I think 20% of the active sessions for that database at any given time. However if you want all events then you would have to suffer the overhead of that action.

With it being a Microsoft product I would not be to concerned with what queries it is producing. If it is experiencing performance issues you should follow Microsoft guidelines for the product or consult Microsoft support.

Best Answer

Related Solutions

Sql-server – How to determine what is using a particular resource (table, view, or function, etc) on the SQL Server

Sql-server – Performance impact of Extended Events filter [like_i_sql_unicode_string]

Related Question