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Writing benchmarks

Benchmarks are stored in a collection of .py files in the benchmark suite’s benchmark directory (as defined by benchmark_dir in the asv.conf.json file). They may be arbitrarily nested in subdirectories, and all .py files will be used, regardless of their file name.

Within each .py file, each benchmark is a function or method. The name of the function must have a special prefix, depending on the type of benchmark. asv understands how to handle the prefix in either CamelCase or lowercase with underscores. For example, to create a timing benchmark, the following are equivalent:

def time_range():
    for i in range(1000):

def TimeRange():
    for i in range(1000):

Benchmarks may be organized into methods of classes if desired:

class Suite:
    def time_range(self):
        for i in range(1000):

    def time_xrange(self):
        for i in xrange(1000):

Running benchmarks during development

There are some options to asv run that may be useful when writing benchmarks.

You may find that asv run spends a lot of time setting up the environment each time. You can have asv run use an existing Python environment that already has the benchmarked project and all of its dependencies installed. Use the --python argument to specify a Python environment to use:

asv run --python=python

If you don’t care about getting accurate timings, but just want to ensure the code is running, you can add the --quick argument, which will run each benchmark only once:

asv run --quick

In order to display the standard error output (this includes exception tracebacks) that your benchmarks may produce, pass the --show-stderr flag:

asv run --show-stderr

Finally, there is a special command, asv dev, that uses all of these features and is equivalent to:

asv run --python=same --quick --show-stderr --dry-run

Setup and teardown functions

If initialization needs to be performed that should not be included in the timing of the benchmark, include that code in a setup method on the class, or add an attribute called setup to a free function.

For example:

class Suite:
    def setup(self):
        # load data from a file
        with open("/usr/share/words.txt", "r") as fd:
            self.words = fd.readlines()

    def time_upper(self):
        for word in self.words:

# or equivalently...

words = []
def my_setup():
    global words
    with open("/usr/share/words.txt", "r") as fd:
        words = fd.readlines()

def time_upper():
    for word in words:
time_upper.setup = my_setup

You can also include a module-level setup function, which will be run for every benchmark within the module, prior to any setup assigned specifically to each function.

Similarly, benchmarks can also have a teardown function that is run after the benchmark. This is useful if, for example, you need to clean up any changes made to the filesystem.

Note that although different benchmarks run in separate processes, for a given benchmark repeated measurement (cf. repeat attribute) and profiling occur within the same process. For these cases, the setup and teardown routines are run multiple times in the same process.

If setup raises a NotImplementedError, the benchmark is marked as skipped.

The setup method is run multiple times, for each benchmark and for each repeat. If the setup is especially expensive, the setup_cache method may be used instead, which only performs the setup calculation once and then caches the result to disk. It is run only once also for repeated benchmarks and profiling, unlike setup. setup_cache can persist the data for the benchmarks it applies to in two ways:

  • Returning a data structure, which asv pickles to disk, and then loads and passes it as the first argument to each benchmark.
  • Saving files to the current working directory (which is a temporary directory managed by asv) which are then explicitly loaded in each benchmark process. It is probably best to load the data in a setup method so the loading time is not included in the timing of the benchmark.

A separate cache is used for each environment and each commit of the project begin tested and is thrown out between benchmark runs.

For example, caching data in a pickle:

class Suite:
    def setup_cache(self):
        fib = [1, 1]
        for i in range(100):
            fib.append(fib[-2] + fib[-1])
        return fib

    def track_fib(self, fib):
        return fib[-1]

As another example, explicitly saving data in a file:

class Suite:
    def setup_cache(self):
        with open("test.dat", "wb") as fd:
            for i in range(100):

    def setup(self):
        with open("test.dat", "rb") as fd:
   = [int(x) for x in fd.readlines()]

    def track_numbers(self):
        return len(

The setup_cache timeout can be specified by setting the .timeout attribute of the setup_cache function. The default value is the maximum of the timeouts of the benchmarks using it.

Benchmark attributes

Each benchmark can have a number of arbitrary attributes assigned to it. The attributes that asv understands depends on the type of benchmark and are defined below. For free functions, just assign the attribute to the function. For methods, include the attribute at the class level. For example, the following are equivalent:

def time_range():
    for i in range(1000):
time_range.timeout = 120.0

class Suite:
    timeout = 120.0

    def time_range(self):
        for i in range(1000):

The following attributes are applicable to all benchmark types:

  • timeout: The amount of time, in seconds, to give the benchmark to run before forcibly killing it. Defaults to 60 seconds.

  • benchmark_name: If given, used as benchmark function name instead of generated one <module>..

  • pretty_name: If given, used to display the benchmark name instead of the benchmark function name.

  • version: Used to determine when to invalidate old benchmark results. Benchmark results produced with a different value of the version than the current value will be ignored. The value can be any Python string (or other object, str() will be taken).

    Default (if version=None or not given): hash of the source code of the benchmark function and setup and setup_cache methods. If the source code of any of these changes, old results become invalidated.

Parameterized benchmarks

You might want to run a single benchmark for multiple values of some parameter. This can be done by adding a params attribute to the benchmark object:

def time_range(n):
   for i in range(n):
time_range.params = [0, 10, 20, 30]

This will also make the setup and teardown functions parameterized:

class Suite:
    params = [0, 10, 20]

    def setup(self, n):
        self.obj = range(n)

    def teardown(self, n):
        del self.obj

    def time_range_iter(self, n):
        for i in self.obj:

If setup raises a NotImplementedError, the benchmark is marked as skipped for the parameter values in question.

The parameter values can be any Python objects. However, it is often best to use only strings or numbers, because these have simple unambiguous text representations.

When you have multiple parameters, the test is run for all of their combinations:

def time_ranges(n, func_name):
    f = {'range': range, 'arange': numpy.arange}[func_name]
    for i in f(n):

time_ranges.params = ([10, 1000], ['range', 'arange'])

The test will be run for parameters (10, 'range'), (10, 'arange'), (1000, 'range'), (1000, 'arange').

You can also provide informative names for the parameters:

time_ranges.param_names = ['n', 'function']

These will appear in the test output; if not provided you get default names such as “param1”, “param2”.

Note that setup_cache is not parameterized.

Benchmark types


Timing benchmarks have the prefix time.

The timing itself is based on the Python standard library’s timeit module, with some extensions for automatic heuristics shamelessly stolen from IPython’s %timeit magic function. This means that in most cases the benchmark function itself will be run many times to achieve accurate timing.

The default timing function is the POSIX CLOCK_PROCESS_CPUTIME, which measures the CPU time used only by the current process. This is available as time.process_time in Python 3.3 and later, but a backport is included with asv for earlier versions of Python.


One consequence of using CLOCK_PROCESS_CPUTIME is that the time spent in child processes of the benchmark is not included. If your benchmark spawns other processes, you may get more accurate results by setting the timer attribute on the benchmark to timeit.default_timer.

For best results, the benchmark function should contain as little as possible, with as much extraneous setup moved to a setup function:

class Suite:
    def setup(self):
        # load data from a file
        with open("/usr/share/words.txt", "r") as fd:
            self.words = fd.readlines()

    def time_upper(self):
        for word in self.words:

How setup and teardown behave for timing benchmarks is similar to the Python timeit module, and the behavior is controlled by the number and repeat attributes, as explained below.


  • goal_time: asv will automatically select the number of iterations to run the benchmark so that it takes at least goal_time seconds each time. If not specified, goal_time defaults to 0.1 seconds.

  • warmup_time: asv will spend this time (in seconds) in calling the benchmarked function repeatedly, before starting to run the actual benchmark. If not specified, warmup_time defaults to 0.1 seconds (on PyPy, the default is 1.0 sec).

  • number: Manually choose the number of iterations. If number is specified, goal_time is ignored. Note that setup and teardown are not run between iterations: setup runs first, then the timing routine is called number times, and after that teardown runs.

  • repeat: The number of times to repeat the benchmark, with each repetition running the benchmark number of times. The median time from all of these repetitions is used as the final result. When not provided, defaults to 10. Setup and teardown are run on each repeat.

  • timer: The timing function to use, which can be any source of monotonically increasing numbers, such as time.clock, time.time or time.process_time. If it’s not provided, it defaults to time.process_time (or a backported version of it for versions of Python prior to 3.3), but other useful values are timeit.default_timer to use the default timeit behavior on your version of Python.

    On Windows, time.clock has microsecond granularity, but time.time‘s granularity is 1/60th of a second. On Unix, time.clock has 1/100th of a second granularity, and time.time is much more precise. On either platform, timeit.default_timer measures wall clock time, not the CPU time. This means that other processes running on the same computer may interfere with the timing. That’s why the default of time.process_time, which only measures the time used by the current process, is often the best choice.

The goal_time, number, repeat, and timer attributes can be adjusted in the setup() routine, which can be useful for parameterized benchmarks.


Memory benchmarks have the prefix mem.

Memory benchmarks track the size of Python objects. To write a memory benchmark, write a function that returns the object you want to track:

def mem_list():
    return [0] * 256

The asizeof module is used to determine the size of Python objects. Since asizeof includes the memory of all of an object’s dependencies (including the modules in which their classes are defined), a memory benchmark instead calculates the incremental memory of a copy of the object, which in most cases is probably a more useful indicator of how much space each additional object will use. If you need to do something more specific, a generic Tracking (Generic) benchmark can be used instead.


The memory benchmarking feature is still experimental. asizeof may not be the most appropriate metric to use.


The memory benchmarks are not supported on PyPy.

Peak Memory

Peak memory benchmarks have the prefix peakmem.

Peak memory benchmark tracks the maximum resident size (in bytes) of the process in memory. This does not necessarily count memory paged on-disk, or that used by memory-mapped files. To write a peak memory benchmark, write a function that does the operation whose maximum memory usage you want to track:

def peakmem_list():
    [0] * 165536


The peak memory benchmark also counts memory usage during the setup routine, which may confound the benchmark results. One way to avoid this is to use setup_cache instead.

Tracking (Generic)

It is also possible to use asv to track any arbitrary numerical value. “Tracking” benchmarks can be used for this purpose and use the prefix track. These functions simply need to return a numeric value. For example, to track the number of objects known to the garbage collector at a given state:

import gc

def track_num_objects():
    return len(gc.get_objects())
track_num_objects.unit = "objects"


  • unit: The unit of the values returned by the benchmark. Used for display in the web interface.