ChallengeFind the slow down–answer

time to read 1 min | 144 words

I asked about a slow piece of code and why it is so slow. The answer is pretty simple, I messed up, take a look at this piece of code:

image

When the Value is an int, I’m creating a span from the address of a long variable (initialized to zero). That means that I have a lot of hash collisions, which means that adding to the dictionary turned into a sequential operation, which means that the actual cost here is O(N**2).

Interestingly enough, you can’t write this code without the use of unsafe. I actually didn’t know that the scope of the variable was even valid here to have its address taken. That was very hard to debug, because the problem was hidden away somewhere very far from where we looked at.

More posts in "Challenge" series:

  1. (01 Jul 2022) Find the stack smash bug… – answer
  2. (30 Jun 2022) Find the stack smash bug…
  3. (03 Jun 2022) Spot the data corruption
  4. (06 May 2022) Spot the optimization–solution
  5. (05 May 2022) Spot the optimization
  6. (06 Apr 2022) Why is this code broken?
  7. (16 Dec 2021) Find the slow down–answer
  8. (15 Dec 2021) Find the slow down
  9. (03 Nov 2021) The code review bug that gives me nightmares–The fix
  10. (02 Nov 2021) The code review bug that gives me nightmares–the issue
  11. (01 Nov 2021) The code review bug that gives me nightmares
  12. (16 Jun 2021) Detecting livelihood in a distributed cluster
  13. (21 Apr 2020) Generate matching shard id–answer
  14. (20 Apr 2020) Generate matching shard id
  15. (02 Jan 2020) Spot the bug in the stream
  16. (28 Sep 2018) The loop that leaks–Answer
  17. (27 Sep 2018) The loop that leaks
  18. (03 Apr 2018) The invisible concurrency bug–Answer
  19. (02 Apr 2018) The invisible concurrency bug
  20. (31 Jan 2018) Find the bug in the fix–answer
  21. (30 Jan 2018) Find the bug in the fix
  22. (19 Jan 2017) What does this code do?
  23. (26 Jul 2016) The race condition in the TCP stack, answer
  24. (25 Jul 2016) The race condition in the TCP stack
  25. (28 Apr 2015) What is the meaning of this change?
  26. (26 Sep 2013) Spot the bug
  27. (27 May 2013) The problem of locking down tasks…
  28. (17 Oct 2011) Minimum number of round trips
  29. (23 Aug 2011) Recent Comments with Future Posts
  30. (02 Aug 2011) Modifying execution approaches
  31. (29 Apr 2011) Stop the leaks
  32. (23 Dec 2010) This code should never hit production
  33. (17 Dec 2010) Your own ThreadLocal
  34. (03 Dec 2010) Querying relative information with RavenDB
  35. (29 Jun 2010) Find the bug
  36. (23 Jun 2010) Dynamically dynamic
  37. (28 Apr 2010) What killed the application?
  38. (19 Mar 2010) What does this code do?
  39. (04 Mar 2010) Robust enumeration over external code
  40. (16 Feb 2010) Premature optimization, and all of that…
  41. (12 Feb 2010) Efficient querying
  42. (10 Feb 2010) Find the resource leak
  43. (21 Oct 2009) Can you spot the bug?
  44. (18 Oct 2009) Why is this wrong?
  45. (17 Oct 2009) Write the check in comment
  46. (15 Sep 2009) NH Prof Exporting Reports
  47. (02 Sep 2009) The lazy loaded inheritance many to one association OR/M conundrum
  48. (01 Sep 2009) Why isn’t select broken?
  49. (06 Aug 2009) Find the bug fixes
  50. (26 May 2009) Find the bug
  51. (14 May 2009) multi threaded test failure
  52. (11 May 2009) The regex that doesn’t match
  53. (24 Mar 2009) probability based selection
  54. (13 Mar 2009) C# Rewriting
  55. (18 Feb 2009) write a self extracting program
  56. (04 Sep 2008) Don't stop with the first DSL abstraction
  57. (02 Aug 2008) What is the problem?
  58. (28 Jul 2008) What does this code do?
  59. (26 Jul 2008) Find the bug fix
  60. (05 Jul 2008) Find the deadlock
  61. (03 Jul 2008) Find the bug
  62. (02 Jul 2008) What is wrong with this code
  63. (05 Jun 2008) why did the tests fail?
  64. (27 May 2008) Striving for better syntax
  65. (13 Apr 2008) calling generics without the generic type
  66. (12 Apr 2008) The directory tree
  67. (24 Mar 2008) Find the version
  68. (21 Jan 2008) Strongly typing weakly typed code
  69. (28 Jun 2007) Windsor Null Object Dependency Facility