on November 22, 2016
I recently got a fantastic question from a reader regarding lock usage in SQL Server:
One of my production databases has a total lock count around 25,000 (select count(*) from sys.dm_tran_locks). The configuration setting for locks is set to the default of zero. This lock count is due to multiple procedures which frequently run and use the same 2-3 tables, repeatedly taking out and releasing locks. Do I need to change the configuration for locks or look into the SP’s so they can finish more quickly, rather than creating locks?
Our friend is correct to leave the ‘locks’ setting in sp_configure alone
The default setting of zero lets SQL Server manage the memory for locks dynamically. When use this dynamic setting, SQL Server will:
- Allocate memory for 2,500 locks on startup
- Acquire more memory for locks when it needs to do so (unless you’re in a memory pressure situation and it would cause paging)
- Not allocate more than 60% of the memory allocated to the instance for locks
- Trigger lock escalation when lock memory hits 40% of the memory allocated to the instance
If you change the setting for ‘locks’, you’re giving SQL Server a maximum number of locks that it can use.
Microsoft recommends leaving this at zero.
You could change ‘locks’ to raise the number of locks allowed. But is it a good idea to use more than 60% of your memory for locks? Even if you’re using that as a temporary band-aid, leaving less than 40% of your memory for buffer pool (data cache), execution plans, and everything else is going to be a world of hurt for most instances..
You could change ‘locks’ to lower the number of locks allowed. But not allocating locks means that the queries asking for locks are going to throw errors and fail. That’s not attractive, either.
So if you’re concerned about the number of locks you have, changing the ‘locks’ configuration setting isn’t likely to help you out.
The ‘locks’ configuration is also marked as slated to be removed in a future version. Microsoft doesn’t want you to be dependent on it.
What’s the memory overhead of those locks?
Each lock uses 96 bytes of memory. On the instance in question, 25,000 locks = 2,400,000 bytes.
That’s only 2.3 MB of memory devoted to locks. Even though 25K sounds like a lot, the memory footprint for that is pretty darn small.
I checked back with our questioner, and their instance has 32GB of memory. That’s a pretty small amount in the grand scheme of things (as of SQL Server 2014, Standard Edition can use up to 128GB of memory for the Buffer Pool), but 2.3 MB isn’t anything to worry about, percentage wise.
Do you have a high number of locks because you need better indexes?
Good indexes can dramatically reduce your lock overhead. Here’s a simple example using the SQLIndexWorkbook sample database – now renamed to BabbyNames.
For this sample query, run under the default read committed isolation level:
SELECT COUNT(*) FROM agg.FirstNameByYear WHERE FirstNameId=1;
GO
When this needs to do a clustered index scan, it requires 5,061 page locks.
After creating a nonclustered index on FirstNameId, the query requires only one page lock.
Indexes that help SQL Server find rows more quickly can dramatically reduce the number of locks that are taken out.
Are you waiting for locks because of blocking?
SQL Server is fast at acquiring locks – unless there’s a conflict, and you have to wait to get the lock because someone else is already using it.
In this case, the first step is to figure out when there is blocking, and who is blocking whom. I like to use alerts and the Blocked Process Report to help figure this out.
Do you have a high number of locks because of estimate problems?
One reason you might get a high number of locks is an inefficient execution plan based on poor estimates. If SQL Server thinks it’s only going to get a small number of rows, it may design a plan based on “lookups”. If it turns out that it’s got a lot more rows than it thought, it might have to execute this loop over and over and over– slowly looping and acquiring lots of little locks.
In this case, the stored procedures using this database are making heavy use of table variables. Table variables often lead to incorrect estimates in complex procedures, and could result in inefficient plans.
In this case, I wasn’t too worried about the 25,000 locks, but I thought it was possible that the performance of the procedures might be able to be improved if they have better estimates. I recommended:
- Testing out the procedures in a dev environment with temporary tables instead of table variables
- Evaluating how the procedures use indexes after the change – they likely will need different indexes for the new execution plans
If you have heavy use of table variables and can’t test out temporary tables, you can test out Trace Flag 2453 on SQL Server 2012 SP2 and higher. This trace flag doesn’t give table variables the full statistics support which temporary tables have, but does try to make SQL Server smarter about the number of rows in the table variable.
Disclaimer: changing from table variables to temporary tables doesn’t always make things faster. I wasn’t doing live troubleshooting here and I didn’t have actual execution plans– it’s possible that the rowsets are small and the table variables were doing well. You never know until you test!
Sometimes you should just take out a tablock
I don’t think this is the case for the person asking this question, but there are some cases when you just want to go ahead and take out an exclusive lock on a table. Not only can it simplify the number of locks for the table, but it can help make data loading more efficient.
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