MIT licensed
Maintained by Andrew Lelechenko
This version can be pinned in stack with:tasty-bench-0.2.5@sha256:4f00fdafc3aba6e48366a3684c427618d98c0b1b130cd9cab35c9498796c9a65,1497

Module documentation for 0.2.5

tasty-bench Hackage Stackage Nightly

Featherlight benchmark framework (only one file!) for performance measurement with API mimicking criterion and gauge. A prominent feature is built-in comparison against previous runs and between benchmarks.

How lightweight is it?

There is only one source file Test.Tasty.Bench and no non-boot dependencies except tasty. So if you already depend on tasty for a test suite, there is nothing else to install.

Compare this to criterion (10+ modules, 50+ dependencies) and gauge (40+ modules, depends on basement and vector).

How is it possible?

Our benchmarks are literally regular tasty tests, so we can leverage all existing machinery for command-line options, resource management, structuring, listing and filtering benchmarks, running and reporting results. It also means that tasty-bench can be used in conjunction with other tasty ingredients.

Unlike criterion and gauge we use a very simple statistical model described below. This is arguably a questionable choice, but it works pretty well in practice. A rare developer is sufficiently well-versed in probability theory to make sense and use of all numbers generated by criterion.

How to switch?

Cabal mixins allow to taste tasty-bench instead of criterion or gauge without changing a single line of code:

cabal-version: 2.0

benchmark foo
    tasty-bench (Test.Tasty.Bench as Criterion, Test.Tasty.Bench as Criterion.Main, Test.Tasty.Bench as Gauge, Test.Tasty.Bench as Gauge.Main)

This works vice versa as well: if you use tasty-bench, but at some point need a more comprehensive statistical analysis, it is easy to switch temporarily back to criterion.

How to write a benchmark?

Benchmarks are declared in a separate section of cabal file:

cabal-version:   2.0
name:            bench-fibo
version:         0.0
build-type:      Simple
synopsis:        Example of a benchmark

benchmark bench-fibo
  main-is:       BenchFibo.hs
  type:          exitcode-stdio-1.0
  build-depends: base, tasty-bench
  if impl(ghc >= 8.10)
    ghc-options:  "-with-rtsopts=-A32m --nonmoving-gc"
    ghc-options:  "-with-rtsopts=-A32m"

And here is BenchFibo.hs:

import Test.Tasty.Bench

fibo :: Int -> Integer
fibo n = if n < 2 then toInteger n else fibo (n - 1) + fibo (n - 2)

main :: IO ()
main = defaultMain
  [ bgroup "fibonacci numbers"
    [ bench "fifth"     $ nf fibo  5
    , bench "tenth"     $ nf fibo 10
    , bench "twentieth" $ nf fibo 20

Since tasty-bench provides an API compatible with criterion, one can refer to its documentation for more examples.

How to read results?

Running the example above (cabal bench or stack bench) results in the following output:

  fibonacci numbers
    fifth:     OK (2.13s)
       63 ns ± 3.4 ns
    tenth:     OK (1.71s)
      809 ns ±  73 ns
    twentieth: OK (3.39s)
      104 μs ± 4.9 μs

All 3 tests passed (7.25s)

The output says that, for instance, the first benchmark was repeatedly executed for 2.13 seconds (wall time), its mean CPU time was 63 nanoseconds and, assuming ideal precision of a system clock, execution time does not often diverge from the mean further than ±3.4 nanoseconds (double standard deviation, which for normal distributions corresponds to 95% probability). Take standard deviation numbers with a grain of salt; there are lies, damned lies, and statistics.

Note that this data is not directly comparable with criterion output:

benchmarking fibonacci numbers/fifth
time                 62.78 ns   (61.99 ns .. 63.41 ns)
                     0.999 R²   (0.999 R² .. 1.000 R²)
mean                 62.39 ns   (61.93 ns .. 62.94 ns)
std dev              1.753 ns   (1.427 ns .. 2.258 ns)

One might interpret the second line as saying that 95% of measurements fell into 61.99–63.41 ns interval, but this is wrong. It states that the OLS regression (which is not exactly the mean) of wall execution time is most probably somewhere between 61.99 ns and 63.41 ns, but does not say a thing about individual measurements. To understand how far away a typical measurement deviates you need to add/subtract double standard deviation yourself (which gives 62.78 ns ± 3.506 ns, similar to tasty-bench above).

To add to the confusion, gauge in --small mode outputs not the second line of criterion report as one might expect, but a mean value from the penultimate line and a standard deviation:

fibonacci numbers/fifth                  mean 62.39 ns  ( +- 1.753 ns  )

The interval ±1.753 ns answers for 68% of samples only, double it to estimate the behavior in 95% of cases.

When benchmarking multithreaded algorithms, note that tasty-bench reports total elapsed CPU time across all cores, while criterion and gauge print wall-clock time.

Statistical model

Here is a procedure used by tasty-bench to measure execution time:

  1. Set n ← 1.
  2. Measure execution time tₙ of n iterations and execution time t₂ₙ of 2n iterations.
  3. Find t which minimizes deviation of (nt, 2nt) from (tₙ, t₂ₙ).
  4. If deviation is small enough (see --stdev below), return t as a mean execution time.
  5. Otherwise set n2n and jump back to Step 2.

This is roughly similar to the linear regression approach which criterion takes, but we fit only two last points. This allows us to simplify away all heavy-weight statistical analysis. More importantly, earlier measurements, which are presumably shorter and noisier, do not affect overall result. This is in contrast to criterion, which fits all measurements and is biased to use more data points corresponding to shorter runs (it employs n1.05n progression).

An alert reader could object that we measure standard deviation for samples with n and 2n iterations, but report it scaled to a single iteration. Strictly speaking, this is justified only if we assume that deviating factors are either roughly periodic (e. g., coarseness of a system clock, garbage collection) or are likely to affect several successive iterations in the same way (e. g., slow down by another concurrent process).

Obligatory disclaimer: statistics is a tricky matter, there is no one-size-fits-all approach. In the absence of a good theory simplistic approaches are as (un)sound as obscure ones. Those who seek statistical soundness should rather collect raw data and process it themselves using a proper statistical toolbox. Data reported by tasty-bench is only of indicative and comparative significance.

Memory usage

Configuring RTS to collect GC statistics (e. g., via cabal bench --benchmark-options '+RTS -T' or stack bench --ba '+RTS -T') enables tasty-bench to estimate and report memory usage such as allocated and copied bytes:

  fibonacci numbers
    fifth:     OK (2.13s)
       63 ns ± 3.4 ns, 223 B  allocated,   0 B  copied
    tenth:     OK (1.71s)
      809 ns ±  73 ns, 2.3 KB allocated,   0 B  copied
    twentieth: OK (3.39s)
      104 μs ± 4.9 μs, 277 KB allocated,  59 B  copied

All 3 tests passed (7.25s)

Combining tests and benchmarks

When optimizing an existing function, it is important to check that its observable behavior remains unchanged. One can rebuild both tests and benchmarks after each change, but it would be more convenient to run sanity checks within benchmark itself. Since our benchmarks are compatible with tasty tests, we can easily do so.

Imagine you come up with a faster function myFibo to generate Fibonacci numbers:

import Test.Tasty.Bench
import Test.Tasty.QuickCheck -- from tasty-quickcheck package

fibo :: Int -> Integer
fibo n = if n < 2 then toInteger n else fibo (n - 1) + fibo (n - 2)

myFibo :: Int -> Integer
myFibo n = if n < 3 then toInteger n else myFibo (n - 1) + myFibo (n - 2)

main :: IO ()
main = Test.Tasty.Bench.defaultMain -- not Test.Tasty.defaultMain
  [ bench "fibo   20" $ nf fibo   20
  , bench "myFibo 20" $ nf myFibo 20
  , testProperty "myFibo = fibo" $ \n -> fibo n === myFibo n

This outputs:

  fibo   20:     OK (3.02s)
    104 μs ± 4.9 μs
  myFibo 20:     OK (1.99s)
     71 μs ± 5.3 μs
  myFibo = fibo: FAIL
    *** Failed! Falsified (after 5 tests and 1 shrink):
    1 /= 2
    Use --quickcheck-replay=927711 to reproduce.

1 out of 3 tests failed (5.03s)

We see that myFibo is indeed significantly faster than fibo, but unfortunately does not do the same thing. One should probably look for another way to speed up generation of Fibonacci numbers.


  • If benchmarks take too long, set --timeout to limit execution time of individual benchmarks, and tasty-bench will do its best to fit into a given time frame. Without --timeout we rerun benchmarks until achieving a target precision set by --stdev, which in a noisy environment of a modern laptop with GUI may take a lot of time.

    While criterion runs each benchmark at least for 5 seconds, tasty-bench is happy to conclude earlier, if it does not compromise the quality of results. In our experiments tasty-bench suites tend to finish earlier, even if some individual benchmarks take longer than with criterion.

    A common source of noisiness is garbage collection. Setting a larger allocation area (nursery) is often a good idea, either via cabal bench --benchmark-options '+RTS -A32m' or stack bench --ba '+RTS -A32m'. Alternatively bake it into cabal file as ghc-options: "-with-rtsopts=-A32m".

    For GHC >= 8.10 consider switching benchmarks to a non-moving garbage collector, because it decreases GC pauses and corresponding noise: +RTS --nonmoving-gc.

  • If benchmark results look malformed like below, make sure that you are invoking Test.Tasty.Bench.defaultMain and not Test.Tasty.defaultMain (the difference is consoleBenchReporter vs. consoleTestReporter):

      fibo 20:       OK (1.46s)
        Response {respEstimate = Estimate {estMean = Measurement {measTime = 87496728, measAllocs = 0, measCopied = 0}, estStdev = 694487}, respIfSlower = FailIfSlower Infinity, respIfFaster = FailIfFaster Infinity}
  • If benchmarks fail with an error message

    Unhandled resource. Probably a bug in the runner you're using.

    this is probably caused by env or envWithCleanup affecting benchmarks structure. You can use env to read test data from IO, but not to read benchmark names or affect their hierarchy in other way. This is a fundamental restriction of tasty to list and filter benchmarks without launching missiles.

Isolating interfering benchmarks

One difficulty of benchmarking in Haskell is that it is hard to isolate benchmarks so that they do not interfere. Changing the order of benchmarks or skipping some of them has an effect on heap’s layout and thus affects garbage collection. This issue is well attested in both criterion and gauge.

Usually (but not always) skipping some benchmarks speeds up remaining ones. That’s because once a benchmark allocated heap which for some reason was not promptly released afterwards (e. g., it forced a top-level thunk in an underlying library), all further benchmarks are slowed down by garbage collector processing this additional amount of live data over and over again.

There are several mitigation strategies. First of all, giving garbage collector more breathing space by +RTS -A32m (or more) is often good enough.

Further, avoid using top-level bindings to store large test data. Once such thunks are forced, they remain allocated forever, which affects detrimentally subsequent unrelated benchmarks. Treat them as external data, supplied via env: instead of

largeData :: String
largeData = replicate 1000000 'a'

main :: IO ()
main = defaultMain
  [ bench "large" $ nf length largeData, ... ]


import Control.DeepSeq (force)
import Control.Exception (evaluate)

main :: IO ()
main = defaultMain
  [ env (evaluate (force (replicate 1000000 'a'))) $ \largeData ->
    bench "large" $ nf length largeData, ... ]

Finally, as an ultimate measure to reduce interference between benchmarks, one can run each of them in a separate process. We do not quite recommend this approach, but if you are desperate, here is how.

Assuming that a benchmark is declared in cabal file as benchmark my-bench component, let’s first find its executable:

cabal build --enable-benchmarks my-bench
MYBENCH=`cabal list-bin my-bench`

Now list all benchmark names (hopefully, they do not contain newlines), escape quotes and slashes, and run each of them separately:

$MYBENCH -l | sed -e 's/[\"]/\\\\\\&/g' | while read name; do $MYBENCH -p '$0 == "'$name'"'; done

Comparison against baseline

One can compare benchmark results against an earlier baseline in an automatic way. To use this feature, first run tasty-bench with --csv FILE key to dump results to FILE in CSV format (it could be a good idea to set smaller --stdev, if possible):

Name,Mean (ps),2*Stdev (ps)
All.fibonacci numbers.fifth,48453,4060
All.fibonacci numbers.tenth,637152,46744
All.fibonacci numbers.twentieth,81369531,3342646

Note that columns do not match CSV reports of criterion and gauge. If desired, missing columns can be faked with awk 'BEGIN {FS=",";OFS=","}; {print $1,$2,$2,$2,$3/2,$3/2,$3/2}' or similar.

Now modify implementation and rerun benchmarks with --baseline FILE key. This produces a report as follows:

  fibonacci numbers
    fifth:     OK (0.44s)
       53 ns ± 2.7 ns,  8% slower than baseline
    tenth:     OK (0.33s)
      641 ns ±  59 ns
    twentieth: OK (0.36s)
       77 μs ± 6.4 μs,  5% faster than baseline

All 3 tests passed (1.50s)

You can also fail benchmarks, which deviate too far from baseline, using --fail-if-slower and --fail-if-faster options. For example, setting both of them to 6 will fail the first benchmark above (because it is more than 6% slower), but the last one still succeeds (even while it is measurably faster than baseline, deviation is less than 6%). Consider also using --hide-successes to show only problematic benchmarks, or even tasty-rerun package to focus on rerunning failing items only.

Comparison between benchmarks

You can also compare benchmarks to each other without reaching to external tools, all in the comfort of your terminal.

import Test.Tasty.Bench

fibo :: Int -> Integer
fibo n = if n < 2 then toInteger n else fibo (n - 1) + fibo (n - 2)

main :: IO ()
main = defaultMain
  [ bgroup "fibonacci numbers"
    [ bcompare "tenth"  $ bench "fifth"     $ nf fibo  5
    ,                     bench "tenth"     $ nf fibo 10
    , bcompare "tenth"  $ bench "twentieth" $ nf fibo 20

This produces a report, comparing mean times of fifth and twentieth to tenth:

  fibonacci numbers
    fifth:     OK (16.56s)
      121 ns ± 2.6 ns, 0.08x
    tenth:     OK (6.84s)
      1.6 μs ±  31 ns
    twentieth: OK (6.96s)
      203 μs ± 4.1 μs, 128.36x

Locating a baseline benchmark in larger suites could get tricky;

bcompare "$NF == \"tenth\" && $(NF-1) == \"fibonacci numbers\""

is a more robust choice of an awk pattern here.

Plotting results

Users can dump results into CSV with --csv FILE and plot them using gnuplot or other software. But for convenience there is also a built-in quick-and-dirty SVG plotting feature, which can be invoked by passing --svg FILE.

Command-line options

Use --help to list command-line options.

  • -p, --pattern

    This is a standard tasty option, which allows filtering benchmarks by a pattern or awk expression. Please refer to tasty documentation for details.

  • -t, --timeout

    This is a standard tasty option, setting timeout for individual benchmarks in seconds. Use it when benchmarks tend to take too long: tasty-bench will make an effort to report results (even if of subpar quality) before timeout. Setting timeout too tight (insufficient for at least three iterations) will result in a benchmark failure.

  • --stdev

    Target relative standard deviation of measurements in percents (5% by default). Large values correspond to fast and loose benchmarks, and small ones to long and precise. If it takes far too long, consider setting --timeout, which will interrupt benchmarks, potentially before reaching the target deviation.

  • --csv

    File to write results in CSV format.

  • --baseline

    File to read baseline results in CSV format (as produced by --csv).

  • --fail-if-slower, --fail-if-faster

    Upper bounds of acceptable slow down / speed up in percents. If a benchmark is unacceptably slower / faster than baseline (see --baseline), it will be reported as failed. Can be used in conjunction with a standard tasty option --hide-successes to show only problematic benchmarks.

  • --svg

    File to plot results in SVG format.



  • Fix comparison against baseline.


  • Add a simplistic SVG reporter.
  • Add bcompare to compare between benchmarks.
  • Throw a warning, if benchmarks take too long.


  • Prohibit duplicated benchmark names in CSV reports.


  • Remove NFData constraint from whnfIO.


  • Fix integer overflow in stdev computations.


  • Add env and envWithCleanup.
  • Run console and CSV reporters in parallel.
  • Extend console reporter and export it as consoleBenchReporter.
  • Add comparison against baseline and relevant options.
  • Export RelStDev option.
  • Export benchIngredients.


  • Initial release.