Using airspeed velocity

airspeed velocity is designed to benchmark a single project over its lifetime using a given set of benchmarks. Below, we use the phrase “project” to refer to the project being benchmarked, and “benchmark suite” to refer to the set of benchmarks – i.e., little snippets of code that are timed – being run against the project.

The benchmark suite may live inside the project’s repository, or it may reside in a separate repository – the choice is up to you and is primarily a matter of style or policy. Note also that the result data is stored in JSON files alongside the benchmark suite and may grow quite large, and you may want to plan where to store it.

You can interact with airspeed velocity through the asv command. Like git, the asv command has a number of “subcommands” for performing various actions on your benchmarking project.

Setting up a new benchmarking project

The first thing to do is to set up an airspeed velocity benchmark suite for your project. It must contain, at a minimum, a single configuration file, asv.conf.json, and a directory tree of Python files containing benchmarks.

The asv quickstart command can be used to create a new benchmarking suite. Change to the directory where you would like your new benchmarking suite to be created and run:

$ asv quickstart
· Setting up new Airspeed Velocity benchmark suite.

Which of the following template layouts to use:
(1) benchmark suite at the top level of the project repository
(2) benchmark suite in a separate repository

Layout to use? [1/2] 1

· Edit asv.conf.json to get started.

Answer ‘1’ if you want a default configuration suitable for putting the benchmark suite on the top level of the same repository where your project is, or ‘2’ to get default configuration for putting it in a separate repository.

Now that you have the bare bones of a benchmarking suite, let’s edit the configuration file, asv.conf.json. Like most files that airspeed velocity uses and generates, it is a JSON file.

There are comments in the file describing what each of the elements do, and there is also a asv.conf.json reference with more details. The values that will most likely need to be changed for any benchmarking suite are:

  • project: The Python package name of the project being benchmarked.

  • project_url: The project’s homepage.

  • repo: The URL or path to the DVCS repository for the project. This should be a read-only URL so that anyone, even those without commit rights to the repository, can run the benchmarks. For a project on github, for example, the URL would look like:

    The value can also be a path, relative to the location of the configuration file. For example, if the benchmarks are stored in the same repository as the project itself, and the configuration file is located at benchmarks/asv.conf.json inside the repository, you can set "repo": ".." to use the local repository.

  • show_commit_url: The base of URLs used to display commits for the project. This allows users to click on a commit in the web interface and have it display the contents of that commit. For a github project, the URL is of the form$OWNER/$REPO/commit/.

  • environment_type: The tool used to create environments. May be conda or virtualenv or mamba. If Conda supports the dependencies you need, that is the recommended method. Mamba is faster but needs a newer Python version (3.8 or greater). See Environments for more information.

  • matrix: Dependencies you want to preinstall into the environment where benchmarks are run.

  • default_benchmark_timeout: The maximum time in seconds that a benchmark is allowed to run before it is terminated. This is useful to prevent benchmarks from running indefinitely if they get stuck. The default is 60 seconds.

The rest of the values can usually be left to their defaults, unless you want to benchmark against multiple versions of Python or multiple versions of third-party dependencies, or if your package needs nonstandard installation commands.

Once you’ve set up the project’s configuration, you’ll need to write some benchmarks. The benchmarks live in Python files in the benchmarks directory. The quickstart command has created a single example benchmark file already in benchmarks/

# Write the benchmarking functions here.
# See "Writing benchmarks" in the asv docs for more information.

class TimeSuite:
    An example benchmark that times the performance of various kinds
    of iterating over dictionaries in Python.
    def setup(self):
        self.d = {}
        for x in range(500):
            self.d[x] = None

    def time_keys(self):
        for key in self.d.keys():

    def time_values(self):
        for value in self.d.values():

    def time_range(self):
        d = self.d
        for key in range(500):

class MemSuite:
    def mem_list(self):
        return [0] * 256

You’ll want to replace these benchmarks with your own. See Writing benchmarks for more information.

Running benchmarks

Benchmarks are run using the asv run subcommand.

Let’s start by just benchmarking the latest commit on the current main branch of the project:

$ asv run

Machine information

If this is the first time using asv run on a given machine, (which it probably is, if you’re following along), you will be prompted for information about the machine, such as its platform, cpu and memory. airspeed velocity will try to make reasonable guesses, so it’s usually ok to just press Enter to accept each default value. This information is stored in the ~/.asv-machine.json file in your home directory:

I will now ask you some questions about this machine to identify
it in the benchmarks.

1. machine: A unique name to identify this machine in the results.
   May be anything, as long as it is unique across all the
   machines used to benchmark this project.  NOTE: If changed from
   the default, it will no longer match the hostname of this
   machine, and you may need to explicitly use the --machine
   argument to asv.
machine [cheetah]:
2. os: The OS type and version of this machine.  For example,
   'Macintosh OS-X 10.8'.
os [Linux 3.17.6-300.fc21.x86_64]:
3. arch: The generic CPU architecture of this machine.  For
   example, 'i386' or 'x86_64'.
arch [x86_64]:
4. cpu: A specific description of the CPU of this machine,
   including its speed and class.  For example, 'Intel(R) Core(TM)
   i5-2520M CPU @ 2.50GHz (4 cores)'.
cpu [Intel(R) Core(TM) i5-2520M CPU @ 2.50GHz]:
5. ram: The amount of physical RAM on this machine.  For example,
ram [8055476]:


If you ever need to update the machine information later, you can run asv machine.


By default, the name of the machine is determined from your hostname. If you have a hostname that frequently changes, and your ~/.asv-machine.json file contains more than one entry, you will need to use the --machine argument to asv run and similar commands.


Next, the Python environments to run the benchmarks are set up. asv always runs its benchmarks in an environment that it creates, in order to not change any of your existing Python environments. One environment will be set up for each of the combinations of Python versions and the matrix of project dependencies, if any. The first time this is run, this may take some time, as many files are copied over and dependencies are installed into the environment. The environments are stored in the env directory so that the next time the benchmarks are run, things will start much faster.

Environments can be created using different tools. By default, asv ships with support for anaconda, mamba, and virtualenv, though plugins may be installed to support other environment tools. The environment_type key in asv.conf.json is used to select the tool used to create environments.

When using virtualenv, asv does not build Python interpreters for you, but it expects to find the Python versions specified in the asv.conf.json file available on the PATH. For example, if the asv.conf.json file has:

"pythons": ["3.7", "3.12"]

then it will use the executables named python3.7 and python3.12 on the path. There are many ways to get multiple versions of Python installed – your package manager, apt-get, yum, MacPorts or homebrew probably has them, or you can also use pyenv.

The virtualenv environment also supports PyPy. You can specify "pypy" or "pypy3" as a Python version number in the "pythons" list. Note that PyPy must also be installed and available on your PATH.


Finally, the benchmarks are run:

$ asv run
· Cloning project.
· Fetching recent changes
· Creating environments......
· Discovering benchmarks
·· Uninstalling from virtualenv-py3.7
·· Building 4238c44d <main> for virtualenv-py3.7
·· Installing into virtualenv-py3.7.
· Running 10 total benchmarks (1 commits * 2 environments * 5 benchmarks)
[  0.00%] · For project commit 4238c44d <main>:
[  0.00%] ·· Building for virtualenv-py3.7.
[  0.00%] ·· Benchmarking virtualenv-py3.7
[ 10.00%] ··· Running (benchmarks.TimeSuite.time_iterkeys--)....
[ 30.00%] ··· benchmarks.MemSuite.mem_list                                 2.42k
[ 35.00%] ··· benchmarks.TimeSuite.time_iterkeys                     11.1±0.01μs
[ 40.00%] ··· benchmarks.TimeSuite.time_keys                         11.2±0.01μs
[ 45.00%] ··· benchmarks.TimeSuite.time_range                        32.9±0.01μs
[ 50.00%] ··· benchmarks.TimeSuite.time_xrange                       30.3±0.01μs
[ 50.00%] ·· Building for virtualenv-py3.12..
[ 50.00%] ·· Benchmarking virtualenv-py3.12
[ 60.00%] ··· Running (benchmarks.TimeSuite.time_iterkeys--)....
[ 80.00%] ··· benchmarks.MemSuite.mem_list                                 2.11k
[ 85.00%] ··· benchmarks.TimeSuite.time_iterkeys                          failed
[ 90.00%] ··· benchmarks.TimeSuite.time_keys                          9.07±0.5μs
[ 95.00%] ··· benchmarks.TimeSuite.time_range                        35.5±0.01μs
[100.00%] ··· benchmarks.TimeSuite.time_xrange                            failed

To improve reproducibility, each benchmark is run in its own process.

The results of each benchmark are displayed in the output and also recorded on disk. For timing benchmarks, the median and interquartile range of collected measurements are displayed. Note that the results may vary on slow time scales due to CPU frequency scaling, heat management, and system load, and this variability is not necessarily captured by a single run. How to deal with this is discussed in Tuning timing measurements.

The killer feature of airspeed velocity is that it can track the benchmark performance of your project over time. The range argument to asv run specifies a range of commits that should be benchmarked. The value of this argument is passed directly to either git log or to the Mercurial log command to get the set of commits, so it actually has a very powerful syntax defined in the gitrevisions manpage, or the revsets help section for Mercurial.

For example, in a Git repository, one can test a range of commits on a particular branch since branching off main:

asv run main..mybranch

Or, to benchmark all of the commits since a particular tag (v0.1):

asv run v0.1..main

To benchmark a single commit, or tag, use ^! (git):

asv run v0.1^!

Corresponding examples for Mercurial using the revsets specification are also possible.

In many cases, this may result in more commits than you are able to benchmark in a reasonable amount of time. In that case, the --steps argument is helpful. It specifies the maximum number of commits you want to test, and it will evenly space them over the specified range.

You can benchmark all commits in the repository by using:

asv run ALL

You may also want to benchmark every commit that has already been benchmarked on all the other machines. For that, use:

asv run EXISTING

You can benchmark all commits since the last one that was benchmarked on this machine. This is useful for running in nightly cron jobs:

asv run NEW

You can also benchmark a specific set of commits listed explicitly in a file (one commit hash per line):

asv run HASHFILE:hashestobenchmark.txt

Finally, you can also benchmark all commits that have not yet been benchmarked for this machine:

asv run --skip-existing-commits ALL


You can also do a validity check for the benchmark suite without running benchmarks, using asv check.

The results are stored as JSON files in the directory results/$MACHINE, where $MACHINE is the unique machine name that was set up in your ~/.asv-machine.json file. In order to combine results from multiple machines, you can for example store the results in separate repositories, for example git submodules, alongside the results from other machines. These results are then collated and “published” altogether into a single interactive website for viewing (see Viewing the results).

You can also continue to generate benchmark results for other commits, or for new benchmarks and continue to throw them in the results directory. airspeed velocity is designed from the ground up to handle missing data where certain benchmarks have yet to be performed – it’s entirely up to you how often you want to generate results, and on which commits and in which configurations.

Viewing the results

You can use the asv show command to display results from previous runs on the command line:

$ asv show main
Commit: 4238c44d <main>

benchmarks.MemSuite.mem_list [mymachine/virtualenv-py3.7]
  started: 2018-08-19 18:46:47, duration: 1.00s

benchmarks.TimeSuite.time_iterkeys [mymachine/virtualenv-py3.7]
  started: 2018-08-19 18:46:47, duration: 1.00s


To collate a set of results into a viewable website, run:

asv publish

This will put a tree of files in the html directory. This website can not be viewed directly from the local filesystem, since web browsers do not support AJAX requests to the local filesystem. Instead, airspeed velocity provides a simple static webserver that can be used to preview the website. Just run:

asv preview

and open the URL that is displayed at the console. Press Ctrl+C to stop serving.

screenshot screenshot2

To share the website on the open internet, simply put the files in the html directory on any webserver that can serve static content. Github Pages works quite well, for example. For using Github Pages, asv includes the convenience command asv gh-pages to put the results to the gh-pages branch and push them to Github. See asv gh-pages –help for details.

Managing the results database

The asv rm command can be used to remove benchmarks from the database. The command takes an arbitrary number of key=value entries that are “and”ed together to determine which benchmarks to remove.

The keys may be one of:

  • benchmark: A benchmark name

  • python: The version of python

  • commit_hash: The commit hash

  • machine-related: machine, arch, cpu, os, ram

  • environment-related: a name of a dependency, e.g. numpy

The values are glob patterns, as supported by the Python standard library module fnmatch. So, for example, to remove all benchmarks in the time_units module:

asv rm "benchmark=time_units.*"

Note the double quotes around the entry to prevent the shell from expanding the * itself.

The asv rm command will prompt before performing any operations. Passing the -y option will skip the prompt.

Here is a more complex example, to remove all of the benchmarks on Python 3.7 and the machine named giraffe:

asv rm python=3.7 machine=giraffe

Finding a commit that produces a large regression

airspeed velocity detects statistically significant decreases of performance automatically based on the available data when you run asv publish. The results can be inspected via the web interface, clicking the “Regressions” tab on the web site. The results include links to each benchmark graph deemed to contain a decrease in performance, the commits where the regressions were estimated to occur, and other potentially useful information.


However, since benchmarking can be rather time consuming, it’s likely that you’re only benchmarking a subset of all commits in the repository. When you discover from the graph that the runtime between commit A and commit B suddenly doubles, you don’t necessarily know which particular commit in that range is the likely culprit. asv find can be used to help find a commit within that range that produced a large regression using a binary search. You can select a range of commits easily from the web interface by dragging a box around the commits in question. The commit hashes associated with that range is then displayed in the “commits” section of the sidebar. We’ll copy and paste this commit range into the commandline arguments of the asv find command, along with the name of a single benchmark to use. The output below is truncated to show how the search progresses:

$ asv find 05d4f83d..b96fcc53 time_coordinates.time_latitude
- Running approximately 10 benchmarks within 1156 commits
- Testing <----------------------------O----------------------------->
- Testing <-------------O-------------->------------------------------
- Testing --------------<-------O------>------------------------------
- Testing --------------<---O--->-------------------------------------
- Testing --------------<-O->-----------------------------------------
- Testing --------------<O>-------------------------------------------
- Testing --------------<>--------------------------------------------
- Greatest regression found: 2918f61e

The result, 2918f61e is the commit found with the largest regression, using the binary search.


The binary search used by asv find will only be effective when the runtimes over the range are more-or-less monotonic. If there is a lot of variation within that range, it may find only a local maximum, rather than the global maximum. For best results, use a reasonably small commit range.

Running a benchmark in the profiler

airspeed velocity can oftentimes tell you if something got slower, but it can’t really tell you why it got slower. That’s where a profiler comes in. airspeed velocity has features to easily run a given benchmark in the Python standard library’s cProfile profiler, and then open the profiling data in the tool of your choice.

The asv profile command profiles a given benchmark on a given revision of the project.


You can also pass the --profile option to asv run. In addition to running the benchmarks as usual, it also runs them again in the cProfile profiler and save the results. asv profile will use this data, if found, rather than needing to profile the benchmark each time. However, it’s important to note that profiler data contains absolute paths to the source code, so they are generally not portable between machines.

asv profile takes as arguments the name of the benchmark and the hash, tag or branch of the project to run it in. Below is a real world example of testing the astropy project. By default, a simple table summary of profiling results is displayed:

> asv profile time_units.time_very_simple_unit_parse 10fc29cb

     8700042 function calls in 6.844 seconds

 Ordered by: cumulative time

 ncalls  tottime  percall  cumtime  percall filename:lineno(function)
      1    0.000    0.000    6.844    6.844 asv/
      1    0.000    0.000    6.844    6.844 asv/
      1    0.000    0.000    6.844    6.844 /usr/lib64/python3.7/
      3    0.000    0.000    6.844    2.281 /usr/lib64/python3.7/
      3    0.104    0.035    6.844    2.281 /usr/lib64/python3.7/
 300000    0.398    0.000    6.740    0.000 benchmarks/
 300000    1.550    0.000    6.342    0.000 astropy/units/
 300000    0.495    0.000    2.416    0.000 astropy/units/format/
 300000    1.023    0.000    1.841    0.000 astropy/units/format/
 300000    0.168    0.000    1.283    0.000 astropy/units/format/
 300000    0.986    0.000    1.115    0.000 astropy/units/format/
3000002    0.735    0.000    0.735    0.000 {isinstance}
 300000    0.403    0.000    0.403    0.000 {method 'decode' of 'str' objects}
 300000    0.216    0.000    0.216    0.000 astropy/units/format/
 300000    0.152    0.000    0.188    0.000 /usr/lib64/python3.7/
 900000    0.170    0.000    0.170    0.000 {method 'lower' of 'unicode' objects}
 300000    0.133    0.000    0.133    0.000 {method 'count' of 'unicode' objects}
 300000    0.078    0.000    0.078    0.000 astropy/units/
 300000    0.076    0.000    0.076    0.000 {issubclass}
 300000    0.052    0.000    0.052    0.000 astropy/units/
 300000    0.038    0.000    0.038    0.000 {method 'strip' of 'str' objects}
 300003    0.037    0.000    0.037    0.000 {globals}
 300000    0.033    0.000    0.033    0.000 {len}
      3    0.000    0.000    0.000    0.000 /usr/lib64/python3.7/
      1    0.000    0.000    0.000    0.000 /usr/lib64/python3.7/
      6    0.000    0.000    0.000    0.000 {time.time}
      1    0.000    0.000    0.000    0.000 {min}
      1    0.000    0.000    0.000    0.000 {range}
      1    0.000    0.000    0.000    0.000 {hasattr}
      1    0.000    0.000    0.000    0.000 /usr/lib64/python3.7/
      3    0.000    0.000    0.000    0.000 {gc.enable}
      3    0.000    0.000    0.000    0.000 {method 'append' of 'list' objects}
      3    0.000    0.000    0.000    0.000 {gc.disable}
      1    0.000    0.000    0.000    0.000 {method 'disable' of '_lsprof.Profiler' objects}
      3    0.000    0.000    0.000    0.000 {gc.isenabled}
      1    0.000    0.000    0.000    0.000 <string>:1(<module>)

Navigating these sorts of results can be tricky, and generally you want to open the results in a GUI tool, such as RunSnakeRun or snakeviz. For example, by passing the --gui=runsnake to asv profile, the profile is collected (or extracted) and opened in the RunSnakeRun tool.


To make sure the line numbers in the profiling data correctly match the source files being viewed, the correct revision of the project is checked out before opening it in the external GUI tool.

You can also get the raw profiling data by using the --output argument to asv profile.


Since the method name is passed as a regex, parenthesis need to be escaped, e.g. asv profile 'benchmarks.MyBench.time_sort\(500\)' HEAD --gui snakeviz

See asv profile for more options.

To extract information from --profile runs of asv:

$ python
import asv
results_asv = asv.results.iter_results(".asv")
res_objects = list(results_asv)
prof_data = res_objects[0].get_profile_stats('benchmarks.MyBench.time_sort')
prof_data.strip_dirs() # Remove machine specific info

Where different benchmarks may be used. A specific json may also be loaded directly with asv.results.Results.load(<json_path>), after which get_profile_stats can be used.

Comparing the benchmarking results for two revisions

In some cases, you may want to directly compare the results for two specific revisions of the project. You can do so with the compare command:

$ asv compare v0.1 v0.2
All benchmarks:

       before           after         ratio
     [3bfda9c6]       [bf719488]
     <v0.1>           <v0.2>
            40.4m            40.4m     1.00  benchmarks.MemSuite.mem_list [amulet.localdomain/virtualenv-py3.7-numpy]
           failed            35.2m      n/a  benchmarks.MemSuite.mem_list [amulet.localdomain/virtualenv-py3.12-numpy]
      11.5±0.08μs         11.0±0μs     0.96  benchmarks.TimeSuite.time_iterkeys [amulet.localdomain/virtualenv-py3.7-numpy]
           failed           failed      n/a  benchmarks.TimeSuite.time_iterkeys [amulet.localdomain/virtualenv-py3.12-numpy]
         11.5±1μs      11.2±0.02μs     0.97  benchmarks.TimeSuite.time_keys [amulet.localdomain/virtualenv-py3.7-numpy]
           failed      8.40±0.02μs      n/a  benchmarks.TimeSuite.time_keys [amulet.localdomain/virtualenv-py3.12-numpy]
      34.6±0.09μs      32.9±0.01μs     0.95  benchmarks.TimeSuite.time_range [amulet.localdomain/virtualenv-py3.7-numpy]
           failed      35.6±0.05μs      n/a  benchmarks.TimeSuite.time_range [amulet.localdomain/virtualenv-py3.12-numpy]
       31.6±0.1μs      30.2±0.02μs     0.95  benchmarks.TimeSuite.time_xrange [amulet.localdomain/virtualenv-py3.7-numpy]
           failed           failed      n/a  benchmarks.TimeSuite.time_xrange [amulet.localdomain/virtualenv-py3.12-numpy]

This will show the times for each benchmark for the first and second revision, and the ratio of the second to the first. In addition, the benchmarks will be color coded green and red if the benchmark improves or worsens more than a certain threshold factor, which defaults to 1.1 (that is, benchmarks that improve by more than 10% or worsen by 10% are color coded). The threshold can be set with the --factor=value option. Finally, the benchmarks can be split into ones that have improved, stayed the same, and worsened, using the same threshold using the --split option. See asv compare for more.