ChallengeThe loop that leaks–Answer
time to read 2 min | 328 words

In my previous post, I asked about the following code and what its output will be:

As it turns out, this code will output two different numbers:

  • On Debug – 134,284,904
  • On Release – 66,896

The behavior is consistent between these two modes.

I was pretty sure that I knew what was going on, but I asked to verify. You can read the GitHub issue if you want the spoiler.

I attached to the running program in WinDBG and issued the following command:

We care about the last line. In particular, we can see that all the memory is indeed in the byte array, as expected.

Next, let’s dump the actual instances that take so much space:

There is one large instance here that we care about, let’s see what is holding on to this fellow, shall we?

It looks like we have a reference from a local variable. Let’s see if we can verify that, shall we? We will use the clrstack command and ask it to give us the parameters and local variables, like so:

The interesting line is 16, which shows:

image

In other words, here is the local variable, and it is set to null. What is going on? And why don’t we see the same behavior on release mode?

As mentioned in the issue, the problem is that the JIT introduce a temporary local variable here, which the GC is obviously aware of, but WinDBG is not. This cause the program to hold on to the value for a longer period of time than expected.

In general, this should only be a problem if you have a long running loop. In fact, we do in some case, and in debug mode, that actually caused our memory utilization to go through the roof and led to this investigation.

In release mode, these temporary variables are rarer (but can still happen, it seems).

Tweet Share Share 3 comments
Tags:

More posts in "Challenge" series:

  1. (21 Apr 2020) Generate matching shard id–answer
  2. (20 Apr 2020) Generate matching shard id
  3. (02 Jan 2020) Spot the bug in the stream
  4. (28 Sep 2018) The loop that leaks–Answer
  5. (27 Sep 2018) The loop that leaks
  6. (03 Apr 2018) The invisible concurrency bug–Answer
  7. (02 Apr 2018) The invisible concurrency bug
  8. (31 Jan 2018) Find the bug in the fix–answer
  9. (30 Jan 2018) Find the bug in the fix
  10. (19 Jan 2017) What does this code do?
  11. (26 Jul 2016) The race condition in the TCP stack, answer
  12. (25 Jul 2016) The race condition in the TCP stack
  13. (28 Apr 2015) What is the meaning of this change?
  14. (26 Sep 2013) Spot the bug
  15. (27 May 2013) The problem of locking down tasks…
  16. (17 Oct 2011) Minimum number of round trips
  17. (23 Aug 2011) Recent Comments with Future Posts
  18. (02 Aug 2011) Modifying execution approaches
  19. (29 Apr 2011) Stop the leaks
  20. (23 Dec 2010) This code should never hit production
  21. (17 Dec 2010) Your own ThreadLocal
  22. (03 Dec 2010) Querying relative information with RavenDB
  23. (29 Jun 2010) Find the bug
  24. (23 Jun 2010) Dynamically dynamic
  25. (28 Apr 2010) What killed the application?
  26. (19 Mar 2010) What does this code do?
  27. (04 Mar 2010) Robust enumeration over external code
  28. (16 Feb 2010) Premature optimization, and all of that…
  29. (12 Feb 2010) Efficient querying
  30. (10 Feb 2010) Find the resource leak
  31. (21 Oct 2009) Can you spot the bug?
  32. (18 Oct 2009) Why is this wrong?
  33. (17 Oct 2009) Write the check in comment
  34. (15 Sep 2009) NH Prof Exporting Reports
  35. (02 Sep 2009) The lazy loaded inheritance many to one association OR/M conundrum
  36. (01 Sep 2009) Why isn’t select broken?
  37. (06 Aug 2009) Find the bug fixes
  38. (26 May 2009) Find the bug
  39. (14 May 2009) multi threaded test failure
  40. (11 May 2009) The regex that doesn’t match
  41. (24 Mar 2009) probability based selection
  42. (13 Mar 2009) C# Rewriting
  43. (18 Feb 2009) write a self extracting program
  44. (04 Sep 2008) Don't stop with the first DSL abstraction
  45. (02 Aug 2008) What is the problem?
  46. (28 Jul 2008) What does this code do?
  47. (26 Jul 2008) Find the bug fix
  48. (05 Jul 2008) Find the deadlock
  49. (03 Jul 2008) Find the bug
  50. (02 Jul 2008) What is wrong with this code
  51. (05 Jun 2008) why did the tests fail?
  52. (27 May 2008) Striving for better syntax
  53. (13 Apr 2008) calling generics without the generic type
  54. (12 Apr 2008) The directory tree
  55. (24 Mar 2008) Find the version
  56. (21 Jan 2008) Strongly typing weakly typed code
  57. (28 Jun 2007) Windsor Null Object Dependency Facility