Query: Entities not being released due to leak in query caching

[dkrooke]

Apologies if this has been mentioned before:

The problem I am seeing, is that entities are still being held in memory, even after the Context has been disposed.

I have tried the following:

private void Method() {
     Test();
}

 private void Test()
 {
            using (var Context = new ExampleEntities())
            {
                Context.ExampleTable.ToList();  
          }
}

When you view the memory heap after the method Test has completed, ExampleTable rows are still held in memory.

[rowanmiller]

Are you running in release mode? And are you sure that garbage collection has run?

[dkrooke]

I was debugging and I'm 90% certain as other objects (non ef) are released when I force a collection.

Sent from my iPhone

On 10 Oct 2016, at 19:46, Rowan Miller notifications@github.com<mailto:notifications@github.com> wrote:

Are you running in release mode? And are you sure that garbage collection has run?

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[divega]

A few data points on this in case it helps:

• By default, entities returned in query results are tracked by default in the DbContext you used to execute the query. This implies the DbContext instance contains references to the entities. There are ways to opt out, e.g. call the AsNoTracking() method in the query.
• By design, DbContext won't release referenced entities when Dispose() is invoked (e.g. a the end of the using block). You will rely on garbage collection to collect the context and all its children objects instead.
• I can't remember from the top of my mind if this actually applies to this specific case, but in the past I have definitively seen Debug compilations alter the standard scoping rules of variables. Assuming that was happening here, the variable Context could still be in the method stack even after the using block is finalized. In my experience, if you are paying very close attention you will see some bizarre behaviors in Debug that won't actually occur when you compile for Release.

A more reliable way to tell if the entity is still in memory is to look after the Test() and you force a garbage collection. If you can still observe any unexpected behavior when doing that, it would be great if you could provide a full repro.

[dkrooke]

Hi,

Thanks for the comments.

• Unfortunately AsNoTracking() does not help me, as I have found that by using it, the entity relationships are not loaded automatically.

Rightly or wrongly I have found that by doing the following:

class TableA { [key] public Guid Oid {get;set;} public string Code {get;set;} }

class TableB{ [key] public Guid Oid {get;set;} public Guid TableA {get;set;} public TABLEA TABLEA {get;set;} public string Code {get;set;} }

Context.TableA.ToList(); Context.TableB.ToList();

Significantly out performs

Context.TableB.Include(row => row.TABLEA). AsNoTracking().ToList();

On my more complex model, I also got out of memory exceptions using the AsNoTracking() technique, where I didn’t get anywhere close using the first method.

• I first noticed this when I ran my app in release mode. As you may gather I am loading a large amount of data for a year, If I want to load another years data in, I reset all my objects back to null and restart the load process, I noticed that the amount of memory being used was increasing rather than staying at a constant (as best as) level. Eventually, by keep loading data from 1 year to the next, Out Of Memory exceptions are thrown..

• I looked at the memory, when the USING had completed, then when Test() had completed and then after a system.gc.collect() completed. Each time, the objects were still in memory.

I have attached an sample project hopefully demonstrating my issue. EFTest.zip

You will need to add the Framework and adjust the DB Connection.

When the Main Method (EFTest) or MainWindow Constructor (WPFApplication) has got to the end, there are multiples of the same rows from TableA & TableB still in memory.

It's probably my complete misunderstanding of the garbage collection

Hope this makes sense

[rowanmiller]

Before we get someone to dig into this, I just want to double check that this isn't because the entities are databound to a form? If there is no databinding, then we'll get someone to look at it.

[dkrooke]

The data items are not bound

[rowanmiller]

Thanks, we'll take a look at it.

[ajcvickers]

This seems to be a leak in query caching, which is keeping references to the context. There may be other paths, but the one I have been able to trace is this:

• MemoryCache uses CacheKey instances (SqlServerCompiledQueryCacheKey in this case)
• CacheKey instances have an Expression, which in this case is an ConstantExpression holding an EntityQueryable instance
• EntityQueryable instances hold on to the EntityQueryProvider
• EntityQueryProvider holds on to the QueryCompiler
• QueryCompiler is a scoped provider service that uses the current DbContext through several paths

Marking the bug for re-triage.

[schwietertj]

I can confirm with @ajcvickers that the issue is still happening. This seems like a decent bug because it is going to bogart resources/DTUs on Azure until this is patched.

[divega]

@anpete just confirm, is the bug fix included in 1.1.0-preview1?

[anpete]

No, didn't make it.

[schwietertj]

@anpete Is there a time frame for the patch release and if there is a work around? Is this an issue with IDisposable or just the implementation with EF?

[anpete]

@schwietertj The problem was with EF. You can see the details in this PR https://github.com/aspnet/EntityFramework/pull/6819

I think there may be a workaround, which is to Detach all entities from the leaked DbContext. I.e. The context instance that was used to initially execute the query. cc @ajcvickers who should be able to confirm.

[ajcvickers]

@schwietertj @anpete That should work to release the vast majority of the memory.

[schwietertj]

@ajcvickers @anpete Thank you for pointing me in a good direction. I will try it out at work tomorrow and let you know what I find.

[schwietertj]

@ajcvickers @anpete I believe it worked when a variable was declared and set to the entity. In the following instance there would not be an entity to detach and thus a connection will still persist.

dynamic result = new ExpandoObject();
using (var ctx = new CompanyDbContext())
            {
                result.employees = await ctx.Employees.Where(x => x.CompanyId == company_id && x.Active).CountAsync();
            }

I am thinking I will have to wait for the patch to be released and monitor production. The only other work around I can think of would be to set up a service that queries azure for any connection that has been quiet for 30 seconds and kill it.

[anpete]

@schwietertj You can enumerate all of the entities via DbContext.ChangeTracker.Entries()

[divega]

@schwietertj are you saying that you are seeing database connections leaked? The original issue that was reported here and that was fixed and for which the workaround applies is about a memory leak and not related to database connections.

[schwietertj]

@divega Very good point. I have experienced both memory leaks and connections that should be disposed that are not being disposed of in the application.

After a request is made in the mvc project, I can look at the activity monitor in ssms and still see an active process. The process should not be active since the db context has been disposed.

I originally thought it was due to DI the db context. I removed the injection and refactored with the "using" statement.

Please correct me if I am wrong but the database connection should be closed after the using block has been executed.

If need be I can open a new issue if it is deemed to be a separate issue than this one.

[divega]

@schwietertj It could be #6581 which causes some connections to be left open. It has been fixed for 1.0.2 (not yet released) and in 1.1.0-preview1. My understanding is that the workarounds for it are (a) enable MARS in the connection string and (b) avoid using Include() with collections and use multiple queries to retrieve the same data instead.

If you think that is not what you are hitting then yes please create a new issue.

[schwietertj]

@divega I just tried enabling MARS and the connection count increased 8 fold per active user. We also have to use includes since lazy loading isn't implemented. Is there an ETA on the 1.0.2 release?

[Eilon]

This patch is approved, please ensure it is merged into the correct branch and building as part of the patch train.

[joshmouch]

@rowanmiller I know this is an old thread, but about halfway through you ask:

Are you running in release mode?

Does running in debug mode cause some sort of known memory leak? I'm trying to track down an EF memory leak and am grasping at straws.

[ajcvickers]

@joshmouch No known issue like that,