Bug 18810 - Memory leaks when using Microsoft.Bcl.Async in PCL
Summary: Memory leaks when using Microsoft.Bcl.Async in PCL
Status: RESOLVED DUPLICATE of bug 18398
Alias: None
Product: iOS
Classification: Xamarin
Component: Xamarin.iOS.dll ()
Version: 7.2.0
Hardware: Macintosh Mac OS
: Normal normal
Target Milestone: Untriaged
Assignee: Marek Safar
Depends on:
Reported: 2014-04-04 04:38 UTC by Johannes Rudolph
Modified: 2014-04-11 12:23 UTC (History)
4 users (show)

Is this bug a regression?: ---
Last known good build:

Sample Project reproducing the Memory Leak (2.78 MB, application/zip)
2014-04-04 04:39 UTC, Johannes Rudolph
Heapshot (1.59 MB, image/png)
2014-04-04 04:40 UTC, Johannes Rudolph
Further simplified repro. (4.52 MB, application/zip)
2014-04-05 10:26 UTC, Johannes Rudolph

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Description Johannes Rudolph 2014-04-04 04:38:22 UTC
I'm experiencing a massive memory leak when await'ing Tasks in a PCL that uses Microsoft.Bcl.Async. It seems as though the async state machine / support code generated by the compiler keeps holding on to finished tasks and they never get garbage collected. When I use the same code in a Xamarin.iOS App exclusively (i.e. use code compiled against the Xamarin.iOS async framework libraries) the memory leak does not occur. 

I have attached a project that reproduces this consistently, as well as a screenshot of a HeapShot demonstrating the Memory Leak. The sample uses a simple Trigger/Timer implemented using await Task.Delay that will cause this memory leak. In AsyncMemoryLeakViewController.ViewDidLoad you can switch between the PCL/iOS version of the Trigger class.

            // When we use TriggerDefinedInApp there's no memory leak
            // TriggerDefinedInPcl leaks _a lot_ 
            // app = new TriggerDefinedInApp( d => 
            pcl = new TriggerDefinedInPcl( d => 

I'm at loss to pinpoint what is at fault here - it may be Microsoft.Bcl.Async (I'll try reproducing on Windows), the Async state machine code generated by MCS or something entirely different. However, this has caused serious headaches for us and I consider this a major problem, wherever this is rooted.
Comment 1 Johannes Rudolph 2014-04-04 04:39:41 UTC
Created attachment 6492 [details]
Sample Project reproducing the Memory Leak
Comment 2 Johannes Rudolph 2014-04-04 04:40:16 UTC
Created attachment 6493 [details]
Comment 3 Johannes Rudolph 2014-04-04 05:23:37 UTC
Fyi: I could not reproduce the bug using a WindowsFormsApplication which in turn used TriggerDefinedInPcl from the sample project, so this seems to be related to Xamarin exclusively.
Comment 4 Johannes Rudolph 2014-04-05 10:26:00 UTC
I could strip down the sample even further, so basically it appears every await inside a PCL leaks the TaskAwaiter, which holds on to the task (via an Action) and eventually the whole state machine. 

I have verified that the statemachines generated for the PCL and the App assembly are identical, so it seems we can rule out MCS here. 

Interestingly enough, the state machine generated by the Microsoft C# compiler for the PCL looks a little different. One major difference in the state machine there is that it seems to zero out the TaskAwaiter held by the state machine when it is done with it. 

So I went ahead and referenced the PCL compiled with CSC against the project (so now using the State machine that zeros out the TaskAwaiter, but got the same memory leaks. I think that means we can rule out the differences in the sate machine for causing this. 

My gut feeling is that this is either something going wrong with the AOT compiler (I will check on Android to be sure), assembly unification (if that applies here) or the garbage collector (which would be pretty... bad).
Comment 5 Johannes Rudolph 2014-04-05 10:26:34 UTC
Created attachment 6502 [details]
Further simplified repro.
Comment 6 Udham Singh 2014-04-07 08:45:32 UTC
I have checked this issue with the Sample Project attached into comment 1 and observed that we are getting memory leak in both the cases either we use "pcl = new TriggerDefinedInPcl( d => " in ViewDidLoad() method or we use "app = new TriggerDefinedInApp( d => ". To check this issue I have deployed application on iOS device and then check memory leak with "Instruments" by launching instruments.

Screencast : http://screencast.com/t/HqDr3zSF

Environment Info :

=== Xamarin Studio ===

Version 4.2.3 (build 60)
Installation UUID: 011d70a5-dede-428b-ab04-ef451c2e539d
	Mono 3.2.6 ((no/9b58377)
	GTK+ 2.24.23 theme: Raleigh
	GTK# (
	Package version: 302060000

=== Apple Developer Tools ===

Xcode 5.1 (5084)
Build 5B130a

=== Xamarin.iOS ===

Version: (Trial Edition)
Hash: 58c3efa
Build date: 2014-10-03 18:02:26-0400

=== Xamarin.Mac ===

Xamarin.Mac: Not Installed

=== Xamarin.Android ===

Version: 4.12.1 (Starter Edition)
Android SDK: /Users/MM/Desktop/android-sdk-macosx
	Supported Android versions:
		2.1   (API level 7)
		2.2   (API level 8)
		2.3   (API level 10)
		3.1   (API level 12)
		3.2   (API level 13)
		4.0   (API level 14)
		4.0.3 (API level 15)
		4.1   (API level 16)
		4.2   (API level 17)
		4.3   (API level 18)
		4.4   (API level 19)
Java SDK: /usr
java version "1.6.0_65"
Java(TM) SE Runtime Environment (build 1.6.0_65-b14-462-11M4609)
Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-462, mixed mode)

=== Build Information ===

Release ID: 402030060
Git revision: 30c4afc300c2a39ec5300851357ce02e49dd217e
Build date: 2014-03-05 22:09:33+0000
Xamarin addins: f8a9589b57c2bfab2ccd73c880e7ad81e3ecf044

=== Operating System ===

Mac OS X 10.9.2
Darwin MacMini.local 13.1.0 Darwin Kernel Version 13.1.0
    Thu Jan 16 19:40:37 PST 2014
    root:xnu-2422.90.20~2/RELEASE_X86_64 x86_64
Comment 7 Rolf Bjarne Kvinge [MSFT] 2014-04-07 12:00:14 UTC
I can reproduce the issue with the sample from comment #5.

From what I can see the only thing that differs is that the PCL version is calling TaskEx.Delay (from Microsoft.Threading.Tasks.dll), instead of Task.Delay, so presumably there's something different with that assembly.
Comment 8 Johannes Rudolph 2014-04-07 12:10:15 UTC
I could consistently reproduce with the simplified sample from comment #5. The reproduction in comment number 6 is incorrect - I am not referring to leaks on the unmanaged heap (as reported by Instruments) but am referring to leaks in the managed heap as made evident by using the Mono Profiler (in X Studio Project - Profile - Mono).
Comment 9 Marek Safar 2014-04-10 14:45:52 UTC

*** This bug has been marked as a duplicate of bug 18398 ***
Comment 10 Johannes Rudolph 2014-04-11 08:36:09 UTC
Not sure I agree on this - how does this explain that I get leaks with code from a PCL only and not with code compiled e.g. into an iOS assembly? Is it linking to two different implementations of Task? I thought the implementation the PCL links against is the Microsoft library, which should not have this bug. Even though, I get leaks when compiling my await code in a PCL only.

Also, can you indicate when the patch will be in xamarin stable?
Comment 11 Marek Safar 2014-04-11 11:07:54 UTC
You don't have to agree.

PCL async assembly still uses BCL task implementation. The fix won't make it to 7.2.1 release and will probably appear in 7.4 series earliest.

You can check Mono commits for the change
Comment 12 Johannes Rudolph 2014-04-11 12:17:15 UTC
Marek, thanks for the explanation. I still don't get why the code compiled into the iOS assembly does not leak when the same code compiled into a PCL assembly does leak. I realize that the PCL is implemented using TypeForwardedTo mscorlib, so both compiled assemblies should eventually rely on the same Task implementation. 

Obviously this isn't the case, since using await on a Task from a PCL does leak the Task whereas using a wait on a Task inside an iOS assembly does not leak the task. Hence my question for clarification. 

I'm just questioning whether the fix referenced in bug 18398 would really resolve this issue. 7.4 probably still has a long way to go until it hits stable and I'd hate to find out the issue is not fixed once it rolls out. Following Xamarins promise of supporting async and PCLs we've started to rely heavily on that combination only to find out that it fails in production. 

I'd love to run the sample with mono trunk myself, but I do currently not see how I'd be able to do so until the Brian's commit makes its way into alpha at least.
Comment 13 Marek Safar 2014-04-11 12:23:02 UTC
What causes the leak is ConfigureAwait (false), when you remove it or change it to ConfigureAwait (true) the leaks does not occur. ConfigureAwait is most likely implemented differently in your nuget vesion of async than in mono BCL. 

I ran the test locally with the change and it does not leak for me. This has nothing to do with PCL it's just that different Mono Task code paths are executed.