MAXDOP setting

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MAXDOP setting

sotn

SSCrazy

Points: 2672
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December 24, 2012 at 5:07 am

#266545

I know that this is slighty a 'depends' question but our server is heaver user transactional server.
Our SQL 2008R2 server has a Xeon 5765, so that's 2 phsyicals CPUs 6 cores each and hyperthreading.
So in SSMS that shows as NUMA Node 0 with 0-11 processors and NUMA Node 1 with 12-23 processors.
As a percentage we get a lots of CXPACKETS so I have changed MAXDOP from the default of 0 to 12,
CXPACKETS have dropped now, but should I reduce the value futher, as this MS article is not compeltely clear to me?
MS article http://support.microsoft.com/kb/329204 says
•For servers that use more than eight processors, use the following configuration: MAXDOP=8.
•For servers that have eight or less processors, use the following configuration where N equals the number of processors: MAXDOP=0 to N.
•For servers that have NUMA configured, MAXDOP should not exceed the number of CPUs that are assigned to each NUMA node.
•For servers that have hyper-threading enabled, the MAXDOP value should not exceed the number of physical processors
So with all of these rules does this mean that I should set it to a value of 2, (when looking at the last rule)
I guess it depends on the exact meaning of 'processors'.
Thanks In Advance.

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Orlando Colamatteo SSC Guru Points: 182279 More actions · Answer 1

It should say "physical cores" instead of "physical processors." I would set it to 6 for the number of physical cores per NUMA node. If you're still seeing too much parallelism for your taste look into increasing the Cost Threshold for Parallelism.

There are no special teachers of virtue, because virtue is taught by the whole community.
--Plato

jyuan68 Old Hand Points: 310 More actions · Answer 2

A few months ago,there is the same issue in our live DB.

But I do not want to set MAXDOP and the cost threshold for parallelism.

Because such actions are palliatives, so we must find out all poor efficiency query plans in our database.

You can run these commands.

select top 100 b.text,c.client_net_address,a.last_wait_type,a.*

from sys.dm_exec_requests a cross apply sys.dm_exec_sql_text(a.sql_handle) b

inner join sys.dm_exec_connections c on a.session_id=c.session_id

where a.session_id>50 and a.session_id<>@@spid

--

select top 10 total_worker_time/execution_count as avg_cpu_cost,

plan_handle,execution_count,

(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(sql_handle)) as query_text

from sys.dm_exec_query_stats

order by avg_cpu_cost desc

TheSQLGuru SSC Guru Points: 134017 More actions · Answer 3

opc.three (12/24/2012)
It should say "physical cores" instead of "physical processors." I would set it to 6 for the number of physical cores per NUMA node. If you're still seeing too much parallelism for your taste look into increasing the Cost Threshold for Parallelism.

1) physical cores is the MAX you should EVER set MAXDOP on NUMA hardware. Don't leave it at zero either.

2) if you can, test your server with Hyperthreading disabled, especially it if is a data warehouse box (or possibly mixed-use server).

3) I never, ever leave cost threshhold for parallelism at 5 for any client I do performance tuning at. It is universally an inappropriate number for modern hardware. It should be larger. I do analytics to try to identify a 'best start' value for each client's server, but without further information I might try 15 for an oltp box and 40 for a DW box.

Best,
Kevin G. Boles
SQL Server Consultant
SQL MVP 2007-2012
TheSQLGuru on googles mail service

Orlando Colamatteo SSC Guru Points: 182279 More actions · Answer 4

TheSQLGuru (12/25/2012)
opc.three (12/24/2012)
It should say "physical cores" instead of "physical processors." I would set it to 6 for the number of physical cores per NUMA node. If you're still seeing too much parallelism for your taste look into increasing the Cost Threshold for Parallelism.
1) physical cores is the MAX you should EVER set MAXDOP on NUMA hardware. Don't leave it at zero either.

I agree 100%, the documentation should be updated to say:

MS article http://support.microsoft.com/kb/329204 says
•For servers that use more than eight processors, use the following configuration: MAXDOP=8.
•For servers that have eight or less processors, use the following configuration where N equals the number of processors: MAXDOP=0 to N.
•For servers that have NUMA configured, MAXDOP should not exceed the number of ~~CPUs~~ physical cores that are assigned to each NUMA node.
•For servers that have hyper-threading enabled, the MAXDOP value should not exceed the number of physical ~~processors~~ cores

2) if you can, test your server with Hyperthreading disabled, especially it if is a data warehouse box (or possibly mixed-use server).

I have had varied results with disabling Hyperthreading. One experiment with a DW workload on Server 2003 running SQL 2005 did not make one bit of difference. IN other cases it helped performance, in some it hurt. Specific cases are anecdotal at best so it is important to test with your workload. General consensus seems to be to disable it by default if you do not have the time to invest. If you want to know for sure the right way to go test, test, test.

There are no special teachers of virtue, because virtue is taught by the whole community.
--Plato

sotn SSCrazy Points: 2672 More actions · Answer 5

Thanks for the replies, much appreciated.

I have now reduced it to 6 from my 'temp' setting of 12, it was previously the default, 0.

CXPACKET has now dropped from about 170 change per minute to 6.

The phsyical cores/processor mistake in the documentation helps explain this to me.

I will leave HT on for now, but wil investigate that if needed, thanks for the pointer.

The server is OLTP.

For NUMA harware, every SQL Server I've looked at, in processors, says NUMA node 0 etc and then split into the cores/processors. does this mean it is NUMA enabled?

Orlando Colamatteo SSC Guru Points: 182279 More actions · Answer 6

sotn (12/27/2012)
For NUMA harware, every SQL Server I've looked at, in processors, says NUMA node 0 etc and then split into the cores/processors. does this mean it is NUMA enabled?

More like NUMA aware. SQL Server will see the NUMA nodes as they are presented by Windows per the hardware. If there is no NUMA present you'll still see Numa Node 0 as the only CPU grouping.

There are no special teachers of virtue, because virtue is taught by the whole community.
--Plato