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ChallengeThe loop that leaks
time to read 1 min | 15 words
Consider the following code:
Can you tell me what the output will be? And why?
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Comments
On which framework? In debug or release? I assume it's supposed to leak, but I can't see it. In debug, the 128MB array is apparently not released immediately, but it is released at the next iteration, so that's not really a leak.
@Thomas, There is leak I believe. Spin that inside LINQPad without read line, I can see memory increase from every iteration.
It increase very steadily.
To make my observation more obvious . I run it for 5 min. It increase from 1378756 to 1471260. There are occasion where from execution to execution, increase is 0, sometime -50 to -150. Overall, it is constantly increase.
When increase is 0, there are 2~4 same numbers in a row. Then increase again.
1024 * 1024 * 128 bytes is a large object, so it will go to LOH, so memory consumption will grow unless LOH is compacted
Running the code on a .net Framework 4.7 ConsoleApp didn't show the same numbers or increase in memory consuption.
Without executing the code it should keep a 128MB array around in debug builds because stack local variables are not collected until the method is exited. For Release builds it should not leak though. Although you null out the reference to the array the MD5 method call can get the reference as stack local variable which will not be nulled out and lead the GC to believe that the array is still referenced. On a second thought this might also cause funny issues in release builds depending on how the JIT compiiler decides to pass parameters to managed methods.
MD5 needs to be disposed.
Looks like the first time you write to the console there are some allocations. Here is the same code with a writeline at the start, run it in debug mode and you will get constant results. https://gist.github.com/slang25/39acdb0f404bad691e72f450d9266885
^ I mean release mode sorry, not debug
@Jason the leak might caused by LinqPad. In a real program, I'm not seeing any increase after the 3rd iteration
Thomas, In debug, yes. The key here is that the value is being nulled, but not release.
Valery, Not really, we aren't holding to the value over a long period of time. It should recognize that this is free and reuse the space
Alois, That is indeed the issue, and surprising because the is no actual need to create a temporary stack variable. The scary thing on our part is that this is technically possible on release as well, yes.
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