Question

I have an ASP.NET MVC2 site connecting to a MySQL database via DbLinq. There is a particular set of actions that get done at regular intervals on the site, which includes looping over a particular set of records in a few tables and updating them, and adding some new records in some other tables.

I’ve been testing with a moderate sized set of data. In my particular test set right now, on an update it ends up inserting 44 new rows and updating 81 other rows. But my call to SubmitChanges() ends up taking a very long time – ~3-4 minutes, which seems like a long time to push (what I thought was) a relatively small number of changes to the DB.

I eventually did some simple profiling, and I found that the problem does not seem to be with executing the query on the database, or even building the query. The majority of the time seems to be taken up by a call inside of UpdateEntity to AllTrackedEntities.ContainsReference().

To give some actual numbers, from a recent test run I had:

• Time in SubmitChangesImpl: 204884 ms
  • Time in UpdateEntity: 200908 ms
    • Time in ContainsReference: 148173 ms
    • Time in QueryBuilder.GetUpdateQuery: 685 ms
    • Time in QueryRunner.Update: 28 ms
    • Time in UpdateReferencedObjects: 49958 ms

As you can see, building and running the SQL query is dwarfed by the amount of time spent checking to see if there exists a reference to the entity we are updating (if there is no reference, the entity is inserted, although in this case all of the updated entities exist). While I understand why this happens, to maintain data integrity and so on, this is killing the performance of these regular update operations.

I looked at setting ObjectTrackingEnabled to false, but that makes it so the DataContext is read-only, and that is of no use to me – my issue is with the performance on updates specifically.

Is there anything that can be done to improve the performance of updates? Am I using DbLinq in a less than optimal way in terms of trying to push through 40-50 inserts and 80+ updates in a single submit? If so, is there a better way to go about this?

Answer 1

If you use long-lived DataContexts, where you read data, modify data, submit changes, then repeat using the same DataContext object, this can negatively impact performance. Once a DataContext has materialized an entity, it keeps it internally for the life of the DataContext as part of its object tracking. Over time, this internal cache can become large, in some cases effectively becoming an in-memory cache of a large portion of your database. This can slow things down, and cause more work for the DataContext during SubmitChanges.

DataContext is meant to be short-lived. It’s lifespan should be a unit of work. Create it, use it for something, then dispose it.

Here is some more detail:

Why would reusing a DataContext have a negative performance impact?

and this by someone close to the product:

http://blogs.msdn.com/b/dinesh.kulkarni/archive/2008/04/27/lifetime-of-a-linq-to-sql-datacontext.aspx

Long-lived usage:DataContext does not itself overwrite the objects
once you retrieve them through queries. So as time passes, the
retrieved objects can become stale if they are frequently changed.

Life after SubmitChanges(): DataContext could be used after
SubmitChanges() but one has to be careful. SubmitChanges() does all
the hard work of figuring out all the changes you have made to the
object graph. It orders the CUD operations for you and provides
optimistic concurrency check at the granularity of each changed
object.