1.4. How To Debug

This chapter presents a recipe for debugging performance regressions in Haskell. Often, when we debug code, it becomes too easy to begin shotgun debugging; we apply a bunch of best-guess changes and retest to see if our stimulus induced a response. You should do your best to avoid these urges. Instead, a more effective method is to use a scientific approach, and develop a hypothesis and conceptual model of how the bug manifested. Every bug or performance regression is a learning opportunity and should be considered as such. By treating regressions as learning opportunities, you gain knowledge of your system, the quality of its design, and how the system interacts with its environment. But more importantly you become a better software engineer. This chapter provides a guide to aid you in debugging performance regression. We base it off of David Agan’s 9 Rules for Debugging book [1] and apply his insights to Haskell programs.

1.4.1. Vocabulary

Unless otherwise noted, we use the following vocabulary to describe aspects of our optimization journey. Because these do not have a formal definition, we present them here instead of in the Glossary:

  1. The system: The system is the entire computational edifice that you’re constructing. This includes your operating system, your CPU, your memory controller and the program that you have written.

  2. The program: The program is the program we are trying to optimize that runs on the system.

  3. The problem or the bug: The problem is an observable phenomenon of the program. It is the performance regression we are trying to characterize, understand, fix and prevent.

  4. The failure mode: The failure mode is the sequence of interactions between sub-systems or external systems and your system that manifest the problem.

  5. The baseline: The baseline is the observable, measurable behavior of the program which constitutes normal and acceptable operation. This what you compare with to know you have a problem.

1.4.2. The Goal

We have two goals when debugging. First, we wish to repair [2] our system as fast as possible. Second, and more importantly, we wish to gain a deeper understanding of our system. The value communicated by the second goal far outweighs the first. By taking the opportunity to gain a deeper understanding, we are empowering ourselves and everyone who reads our documentation to write more effective code in the future. Ideally this translates to preventing regressions before they can manifest, which will always cost less engineering time and resources than reacting to regressions after they occur.

The best way to achieve these goals is, paradoxically, to work slowly, contemplatively and deliberately. By working deliberately, you can ensure that your debugging is making progress and can rigorously test your mental model of the system. Changes to your code should verify that your mental model is either correct or incorrect.

1.4.3. Agan’s Rules of Debugging

There are nine rules, ordered from most important to least. We’ll take them one by one and relate each to Haskell. Note that we do not repeat all of Agan’s advice, only the central ideas and how they relate to Haskell. This way all readers will still benefit from Agan’s work, which we strongly encourage. Understand the System

To debug, optimize, iterate and improve, you must understand the current system, otherwise how will you know if the system is operating correctly and therefore if your performance is real [3] . Furthermore, not knowing the system will limit the optimizations that you will be able to conceive and implement.

Understanding the system is challenging for a high-level language such as Haskell and is often a barrier to optimizing and debugging performance of Haskell code. For example, GHC might surprise you by not inlining a particular function and therefore a cascade of optimizations have not taken place.

Fear not, this is why this book exists. For our purposes, learning the system means understanding your program, how GHC compiles your program, and likely some aspects of GHC itself. Note that this is not unique to Haskell. If one were trying to optimize a C program they would need to understand the program, its interaction with whichever C compiler, and perhaps the interaction with the operating system or even the CPU caches depending on the optimizations they desire to implement and their performance goals.

This book cannot help you learn your own program, but it can help you learn GHC. To understand the system, begin with learning to read the intermediate representations of your program such as Core, or Stg. Reading the intermediate representations are prerequisites for understanding the GHC Optimizations that GHC performs which make Haskell so fast; and so slow when they do not fire. Make it Fail

To make it fail, means to identify and have control over a stimulus that induces incorrect system behavior. This is the phase where one is searching for a small program, called a reproducer, that induces the malformed behavior. A reproducer is imperative because it gives control and a litmus test. With a reproducer one can observe the problem at will and repeatedly test the bug to determine when it is fixed in preparation for a repair.

In Haskell, the search for a reproducer is no different than in any other language. Try to start from a known state [4]; use the exact hardware and software if possible. The closer the hardware and software matches, the less variables you have to consider. Then, try to automate as much as possible with a script. A script pays dividends in the long run. It details the exact sequence of events that produces the bug and leaves out any guess work. You should expect to run the reproducer numerous times over weeks and months in the worst case and a script will help you keep things tidy and controlled. Don’t Think, Look

Of all of Agan’s rules, this is the rule that Haskellers struggle to do the most often. We enjoy thinking with types and abstractions rather than instrumentation and measurement. To don’t think, look means to check the instrumentation or add instrumentation to check, and then use the instrumentation to confirm or reject your hypothesis. Measurement, and observation will be faster on average than shotgun debugging unless you are very lucky (which of course is not reliable).

So what is our instrumentation? We can directly observe by reading the intermediate representations such as Core, Stg, or Cmm. Reading the intermediate representations works well if you suspect an optimization is not firing; which can often happen during upgrades of GHC or dependencies. Or we can use a probe to inspect the system. The available probes range across the entire Haskell software stack, from binary probes, such as perf and valgrind, to GHC provided probes such as eventlog and GHC-debug. Note that there is no best probe, rather the right probe will depend on the bug and your exact situation. For example, using eventlog to inspect your program’s heap is typically the first check of instrumentation if you suspect a memory leak.


See the Measurement, Profiling, and Observation for a complete list of instrumentation.

Once you know which instrumentation to use and how to interpret its output, search its output until you identify a handful of causes and have at least one failure mode hypothesis to test. Remember that you are not observing the bug with the instrumentation, rather you are observing the effect of the bug in order to formulate a hypothesis. For example, with a memory leak, the bug’s effect could be a high amount of memory usage reported by GHC, or your operating system, or a pyramid shaped heap profile. For a missed optimization, the effect could be redundant boxing or a missed rule that produces poor performing Core, and consequently a higher Mutator time reported by the RTS. Divide and Conquer

Imagine the system execution as an ordered linear sequence of causal events \(e_{0} \rightarrow \ldots{} \rightarrow e_{halt}\), where \(e_{0}\) is the first event to take place and \(e_{halt}\) the last. When the system has a bug, the sequence of events, also called a causal chain, diverges from its expected behavior at some event, \(e_{bug}\). In this view, debugging is searching the causal chain for \(e_{bug}\). Thus, to divide and conquer means to search the causal chain with the divide and conquer strategy.

To divide and conquer the causal chain, start with the anomalous end. Think of \(e_{bug}\) as a pivot point, after the bug the chain is: \(e_{bug} \rightarrow \ldots{} \rightarrow e_{halt}\) and the system is in an anomalous operating state, before the bug: \(e_{0} \rightarrow \ldots{} \rightarrow e_{bug-1}\) the system is in an acceptable operating state. So if we start from \(e_{0}\) then we must verify the system state at every event \(e_{0} \ldots e_{bug-1}\) on all possible control flow branches. That is a lot of work (and is better left to assertions ). However, by beginning the search on the anomalous side we only have to find one anomalous state on one branch to begin to work backwards to \(e_{bug}\). Thus, searching from the anomalous end is faster because there are less possible system states to check.

A good tactic to make the search for \(e_{bug}\) easier is to exacerbate the effect of the bug, or in the word’s of David Agan: “Make it obvious.”. Our Haskell programs, like all programs, obey this causal chain. But Haskell is a pure lazy language, so the causal chain forms via data dependency rather than forming via the observable ordering of side-effects [5]. This simplifies debugging because we control the data and therefore the causal chain. So by changing the input to the system, we can make the rough location of \(e_{bug}\) more obvious. Making the bug’s effect obvious can be as simple as making the load on the system larger. For example, imagine trying to optimize the Fibonacci function, instead of testing with fib 10 one can use fib 200 to exacerbate memory or runtime issues. This will create a larger response signal in the instrumentation which is easier to find, diagnose and analyze.

A similar method is to input data that has an easily recognizable pattern. This technique is useful when reading Core, Stg, or Cmm. GHC generates names based on user provided names and keeps the original names in the intermediate representations. For example, consider this code

{-# OPTIONS_GHC -dsuppress-all -ddump-simpl  #-} -- dump the Core

module Main where

main :: IO ()
main = print $ f 0 True
    f x y = let j :: Int -> Int
                j 0 = 0
                j n = j (n-1)
            in case y of
              True -> j 22
              False -> j 33

and its Core output (the 0 at the end is the program’s result):

==================== Tidy Core ====================
Result size of Tidy Core
  = {terms: 56, types: 31, coercions: 0, joins: 0/1}

$trModule1_r17l = "main"#

$trModule2_r17m = TrNameS $trModule1_r17l

$trModule3_r17n = "Main"#

$trModule4_r17o = TrNameS $trModule3_r17n

$trModule = Module $trModule2_r17m $trModule4_r17o

  = \ @p_aLV _ y_azl ->
      letrec {
          = \ ds_d17b ->
              case ds_d17b of wild_X1E { I# ds1_d17c ->
              case ds1_d17c of {
                __DEFAULT -> j_azm (- $fNumInt wild_X1E (I# 1#));
                0# -> I# 0#
              }; } in
      case y_azl of {
        False -> j_azm (I# 33#);
        True -> j_azm (I# 22#)

main = $ (print $fShowInt) (f_r17p (IS 0#) True)

main = runMainIO main


Notice that the user names f and j are still in the Core output as f_r17p and j_azm, but both have been transformed into an Occurrence Name. So we can use more obvious names to make searching the intermediate representations faster. For example, instead of f and j we can use the obnoxious fFINDME or jLOOKDONTTHINK:

{-# OPTIONS_GHC -dsuppress-all -ddump-simpl  #-} -- dump the Core

module Main where

main :: IO ()
main = print $ fFINDME 0 True
    fFINDME x y = let jLOOKDONTTHINK :: Int -> Int
                      jLOOKDONTTHINK 0 = 0
                      jLOOKDONTTHINK n = jLOOKDONTTHINK (n-1)
                  in case y of
                    True -> jLOOKDONTTHINK 22
                    False -> jLOOKDONTTHINK 33

==================== Tidy Core ====================
Result size of Tidy Core
  = {terms: 56, types: 31, coercions: 0, joins: 0/1}

$trModule1_r17l = "main"#

$trModule2_r17m = TrNameS $trModule1_r17l

$trModule3_r17n = "Main"#

$trModule4_r17o = TrNameS $trModule3_r17n

$trModule = Module $trModule2_r17m $trModule4_r17o

  = \ @p_aLV _ y_azl ->
      letrec {
          = \ ds_d17b ->
              case ds_d17b of wild_X1E { I# ds1_d17c ->
              case ds1_d17c of {
                __DEFAULT -> jLOOKDONTTHINK_azm (- $fNumInt wild_X1E (I# 1#));
                0# -> I# 0#
              }; } in
      case y_azl of {
        False -> jLOOKDONTTHINK_azm (I# 33#);
        True -> jLOOKDONTTHINK_azm (I# 22#)

main = $ (print $fShowInt) (fFINDME_r17p (IS 0#) True)

main = runMainIO main


And now it is much easier to recognize or search for these names. Change One Thing at a Time

To some extent, everyone understands that changing only one thing at a time is good practice. It simplifies keeping a log of changes, correlating cause and effect, and more importantly, it reduces the probability of creating an abnormal system. Every change to the system has a chance to move that system’s operation from inside the system’s engineering tolerance (the expected range of operation) to outside; into new and unexplored operating conditions. By changing only one thing at a time we mitigate this risk while debugging.

Doing this on a Haskell code base will be identical to any other programming environment; Haskell is not unique here. But we have two recommendations: first, make sure you have a baseline, a working master copy, so that you can always compare your working copy to it. Second, if you begin changing lots of parts of the system semi-randomly (such as adding a bunch of strictness) to check if these changes affect the bug’s effect, then you are guessing, and instead should Don’t Think, Look. Keep an Audit Trail

To keep an audit trail means to maintain a log of your debugging work. Think like a scientist who wants others to be able to replicate their work. Be meticulous, you should write down the exact sequence of what you did, and then what and how much happened. You should write your log as if you would return to it years or months later. You should record your theories. Theories are the background context that informed your change; the reason, the why that you made the change you did. In addition, record the commands you ran, how you first observed the bug’s effect, the instrumentation you used to monitor the bug’s effect and what you expected to observe from a change. A good log should tell a story; it should read like the laboratory journal of a scientist or engineer. Lastly, if the effect of the bug is some special piece of output, then be sure to include it so that you create a searchable document, you’re future self will be thankful.

An example log might look something like this:


In the example, I use foo and bar as meta-variables that stand for a subsystem or test. Similarly, I use angle-bracket notation <...> to represent pieces of important data that the log should record.

* Mutator regression in commit <some-hash> | System version <i> | Feb. 02, 2024

  ** Bug's Effect:

     - We've observed in increase of <n> seconds (<m> %) in Mutator time as
     reported by the RTS's -S flag when running test foo on debian 10 at
     commit <hash> in CI.

  ** Background:

     Test foo is memory intensive, testing a pathological case where user
     input results is subsystem bar performing a lot of IO operations

  ** Sanity Checks:

     - Do we observe this regression on other platforms, e.g., Windows,
       Fedora or Mac?

     - Has the CI runner changed?

  ** Theory: Regression caused by missed optimizations resulting in a GHC
  version bump that occurred at commit <hash>.

  *** Possible Instrumentation

     - Compare Mutator time with the baseline using -O0. I expect to see the
       baseline and regressed versions perform similarly at -O0. If that is
       the case then we should disable the optimizations implied by -O1 one
       by one. Why -O1? Because that is -O2 with two less passes of GHC's
       optimizer. If that is not the case, then we should check versioning
       differences and bisect the commits to find the commit where the
       mutator regresses. In this scenario it is likely that we've slowed the
       system rather than some interaction with GHC. We could also check for
       a stack leak in this case.

     - Revert to known working GHC version, then run test foo on debian 10 at
       commit we observed the bug. If we observe the bug's effect then we
       know its a regression that in our code base. If not then its a
       regression in a dependency and we can systematically test each one.

     - Inspect the tickyticky output to compare the baseline and regressed
       branch. Check for a change in the number of unboxed tuples and data
       constructors. Unsure how stable these numbers are between runs of the
       same test.

     - Compare the output of the baseline branch and the regressed branch
       when compiling with `-Wall-missed-specializations`. Changes in this
       output could point to missed specializations which would also be
       observable in Core. However, this output also changes along dependency
       versioning. I expect to see minor differences in the output. If these
       differences occur in the code the foo exercises then they are likely
       candidates for the regression. Will have to verify by reading Core.

  *** Tests

     - Compare Mutator time with baseline version using -O0:
         Note taken on [2021-12-03 Fri 15:55]

         With `ghc --version`: 9.8.1

         Ran `cabal
         test --show-details=streaming --pattern='foo' --ghc-options='+RTS -S
         -RTS'` on both baseline (commit <hash>) and regressed (commit
         <hash>). Results differed by less than 1%.

  ** Theory: Regression caused by regression in a dependency
  ... Check the Plug

To check the plug means to verify your assumptions by performing sanity checks. Assumptions could be library and GHC versions, your operating systems’ available resources and settings; such as the CPU governor (be sure to make sure your laptop is plugged in). But assumptions can also be environmental, for example, the input data is what you expect, or that you are working from the commit you expect to be working from.

Checking the plug enables one to divide and conquer. Without checking the plug, the anomalous region of the system’s causal chain is the whole chain! So make sure you take the time to check the plug. To check the plug, start at the beginning of the causal chain; check the tools, the dependencies, the inputs and default settings; as David Agan states: “Many anomalous systems are created by default settings.”

Ideally you would check the plug at the beginning of your investigation so that your investigation does not proceed with bad assumptions. Another time to check the plug is when your investigation has led you into contradiction. This happens when you follow two hypotheses that are both confirmed with testing, but which contradict each other. This is typically an indication that something is egregiously wrong, and that the system you are inspecting is far outside its engineering tolerances. Such deviations are more times than not (but not always!) caused by unplugged things in the system. Get a Fresh View

Sometimes, despite your best efforts you’ll exhaust all leads and become stuck. To get a fresh view, means to recognize when you’re stuck and ask for help, especially from experts.

The value in asking for help is a new perspective on the problem. The new perspective may highlight features of the problem that you missed or overlooked, and often times describing and explaining the issue to another person can lead you to new insights.

When asking for help, do not communicate your theories, instead communicate the symptoms you’ve observed. If you communicate your theories then you’ll inevitably lead your interlocutor down the path your investigation took; you’ll accidentally coerce them to your perspective. So resist the temptation, allow them to come to their own conclusions and formulate their own theories.

Fortunately, the Haskell ecosystem is full of enthusiastic, helpful people from all over the world. Don’t be afraid to participate! The worst case outcome is no one responds, the best case is that you connect with others who share your passion and help you fix the problem. Here are the best forums to reach out: If You Didn’t Fix It, Then It Ain’t Fixed

Repeat it with me: “The problem will not fix itself nor will the system correct itself.” We wish it were otherwise, but the cost of inaction almost always outweighs the cost of action because inaction prevents quality control.

So once you have a fix in mind how do you know it will correct the system? You check. You must Don’t Think, Look! To verify that the fix corrects the system, toggle the fix on and off, and observe if the toggle also toggles the bug’s effect. If the bug’s effect also toggles then you should have high confidence that you’ve found a fix because you can now affect the bug at will, and therefore you have regained control of the system.

Control of the system is crucial; if you do not regain control, then you cannot be sure the bug will not manifest again and have lost some understanding of what the system is. In other words, it is only with control of the system that you are able to make correctness and performance guarantees to your end-users. Furthermore, it is only with control that you can begin to craft a repair, complete with instrumentation to capture more system details for next time.

1.4.4. Summary

We’ve come a long way, let’s review, the nine rules of debugging are:

  1. Understand the System: If you do not understand the system then you cannot debug it.

  2. Make it Fail: Find a reproducer. This is the beginning of regaining control of the system.

  3. Don’t Think, Look: Check your instrumentation, if you do not have instrumentation then add it. Observation is reliably faster than guess work.

  4. Divide and Conquer: Search for the bug by dividing and conquering. start from the anomalous end of the system, verify its anomalous, then go to sound end of the system, verify its sound, now repeat until you close in on the bug.

  5. Change One Thing at a Time: Don’t shotgun debug, change only one thing at a time to mitigate the risk of moving the system so far outside its engineering tolerances that the bug’s effect is obscured by newly induced bugs.

  6. Keep an Audit Trail: You are a scientist. Keep a laboratory journal of your work so that you can reproduce it months or years later. Make it searchable, make it precise, give it background context and record your theories.

  7. Check the Plug: Verify your assumptions before diving into a rabbit hole and when you conclude in a contradiction. And always check the default settings!

  8. Get a Fresh View: Ask for help, especially from an expert. Only report your observations and data, don’t report your theories.

  9. If You Didn’t Fix It, Then It Ain’t Fixed: The system will not repair itself, and even if it did you would not regain control of the product that you ship. Take the time and put in the work to find a repair, this will always be faster in the long run than ignoring the bug and relying on hope.

Why do we follow these rules? Because doing so is more efficient than shotgun debugging, guessing, or living with a buggy system. Recall our goals: in the short term, to repair the system; and in the long term, to gain a deeper understanding of the system. We are thinking on a time scale of years. On that time scale bugs are inevitable. By gaining a deeper understanding of the system we slowly master the system. Mastering the system, in turn, enables more efficient, better engineered systems, more communicative documentation, and the ability to avoid future bugs before they manifest. So work slowly, deliberately and carefully. The investment pays off in the long run.