4.10.0 semi-Random Freeze when TArray tries to resize memory
This started as soon as I upgraded my project to 4.10 from 4.9. I've been catching this repeatedly and its semi random to reproduce.
Basically, I run about 40 threads in the background doing complex AI and signal analysis and out of about a dozen game runs, 1 thread will randomly appear to freeze. The stack always ends with the tbbmalloc/frontend.cpp privatizePublicFreeList() where it is actually stuck in what appears to be a while loop.
in the screenshot below, you can see my stack. I followed it back down to verify I'm not passing garbage around and everything was solid. for reference, the line before TArray<>::CopyToEmpty() is copying 512 floats from one TArray to another, it's here where it gets stuck.
there is no voodoo magic here
So, I have no real way to fix this. It's internal to the engine allocating memory on its data structures. Was not an issue in 4.9, suddenly appeared in 4.10. happens more often in developer mode than in debug.
Has anyone else encountered this? I found a random version of that file online here https://github.com/jckarter/tbb/blob/master/src/tbbmalloc/frontend.cpp which i took a glance at for reference. That file is not exposed to visual studio through UE4 though so I could only assume a few things about the problem. it appears that the buffer being passed in is circular and so never hits the null terminator needed to break out of the loop. is there a rare issue with the memory allocation behind TArray?
asked Nov 30 '15 at 09:08 PM in Bug Reports
We didn't update TBB for 4.10, so it is a little surprising that this issue suddenly started happening for you. We looked through the description you provided, as well as the links that you provided, and came to the same conclusion that Vladimir at Intel mentioned. It seems to be a race condition where two threads attempt to release the same pointer twice. Vladimir mentions that they added some debugging code to identify where this may be happening in version 4.3 update 1. We have not updated to that version or beyond yet, but from what Vladimir says it should be possible to drop the updated version in in place of the currently-used version.
Please download the 4.3 update 1 TBB source code (you can get that here), and paste it into the Engine source code (after making a copy of the existing TBB code). You'll want to run GenerateProjectFiles.bat and build the Engine again at this point. I have not tried debugging TBB before, but I believe you should then be able to run your project in Visual Studio's debugger to help pinpoint where the issue lies. This information may also be helpful.
answered Dec 04 '15 at 03:22 PM
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