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Statistically rigorous benchmark runner for the web

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# tachometer [![Build Status](https://github.com/Polymer/tachometer/actions/workflows/tests.yaml/badge.svg?branch=main)](https://github.com/Polymer/tachometer/actions/workflows/tests.yaml?query=branch%3Amain) [![NPM  package](https://img.shields.io/npm/v/tachometer.svg)](https://npmjs.org/package/tachometer)

> tachometer is a tool for running benchmarks in web browsers. It uses repeated
> sampling and statistics to reliably identify even tiny differences in runtime.

## Install

```sh
npm i tachometer
```

## Usage

```sh
npx tachometer bench1.html [bench2.html ...]
```

## Why?

Even if you run the same JavaScript, on the same browser, on the same machine,
on the same day, you'll still get a different result every time. But if you take
enough _repeated samples_ and apply the right statistics, you can reliably
identify even tiny differences in runtime.

## Example

Let's test two approaches for adding elements to a page. First create two HTML
files:

`inner.html`

```html
<script type="module">
  import * as bench from '/bench.js';
  bench.start();
  for (let i = 0; i < 100; i++) {
    document.body.innerHTML += '<button></button>';
  }
  bench.stop();
</script>
```

`append.html`

```html
<script type="module">
  import * as bench from '/bench.js';
  bench.start();
  for (let i = 0; i < 100; i++) {
    document.body.append(document.createElement('button'));
  }
  bench.stop();
</script>
```

Now run tachometer:

```sh
npx tachometer append.html inner.html
```

Tachometer opens Chrome and loads each HTML file, measuring the time between
`bench.start()` and `bench.stop()`. It round-robins between the two files,
running each at least 50 times.

```
[==============================================------------] 79/100 chrome append.html
```

After a few seconds, the results are ready:

```
┌─────────────┬─────────────────┬─────────────────┬─────────────────┐
│ Benchmark   │        Avg time │   vs inner.html │  vs append.html │
├─────────────┼─────────────────┼─────────────────┼─────────────────┤
│ inner.html  │ 7.23ms - 8.54ms │                 │          slower │
│             │                 │        -        │    851% - 1091% │
│             │                 │                 │ 6.49ms - 7.80ms │
├─────────────┼─────────────────┼─────────────────┼─────────────────┤
│ append.html │ 0.68ms - 0.79ms │          faster │                 │
│             │                 │       90% - 92% │        -        │
│             │                 │ 6.49ms - 7.80ms │                 │
└─────────────┴─────────────────┴─────────────────┴─────────────────┘
```

This tells us that using the `document.body.append` approach instead of the
`innerHTML` approach would be between 90% and 92% faster on average. The ranges
tachometer reports are 95% confidence intervals for the percent change from one
benchmark to another. See _[Interpreting results](#interpreting-results)_ for more
information.

## Features

- Measure your own [specific timings](#callback) with the `/bench.js` module, by
  setting the [window.tachometerResult](#global-result) global (or by polling an
  arbitrary JS expression), or measure [First Contentful Paint](#first-contentful-paint-fcp) on any local or remote
  URL.

- [_Compare benchmarks_](#multiple-benchmarks) by round-robin between two or
  more files, URLs, URL query string parameters, or browsers, to measure which
  is faster or slower, and by how much, with statistical significance.

- [_Swap dependency versions_](#swap-npm-dependencies) of any NPM package you
  depend on, to compare published versions, remote GitHub branches, or local git
  repos.

- [_Automatically sample_](#auto-sample) until we have enough precision to
  answer the question you are asking.

- [_Remote control_](#remote-control) browsers running on different machines
  using remote WebDriver.

## Sampling

### Minimum sample size

By default, a **minimum of 50 samples** are taken from **each** benchmark. You
can change the minimum sample size with the `--sample-size` flag or the
`sampleSize` JSON config option.

### Auto sample

After the initial 50 samples, tachometer will continue taking samples until
there is a clear statistically significant difference between all benchmarks,
for **up to 3 minutes**.

You can change this duration with the `--timeout` flag or the `timeout` JSON
config option, measured in minutes. Set `--timeout=0` to disable auto sampling
entirely. Set `--timeout=60` to sample for up to an hour.

### Auto sample conditions

You can also configure which statistical conditions tachometer should check for
when deciding when to stop auto sampling by configuring _auto sample
conditions_.

To set auto sample conditions from the command-line, use the
`--auto-sample-conditions` flag with a comma-delimited list:

```sh
--auto-sample-conditions=0%,10%
```

To set auto sample conditions from a JSON config file, use the
`autoSampleConditions` property with an array of strings (including if there is
only one condition):

```json
{
  "autoSampleConditions": ["0%", "10%"]
}
```

An auto sample condition can be thought of as a point of interest on the
number-line of either absolute milliseconds, or relative percent change. By
setting a condition, you are asking tachometer to try to shrink the confidence
interval until it is unambiguously placed on one side or the other of that
condition.

| Example condition   | Question                                                   |
| ------------------- | ---------------------------------------------------------- |
| `0%`                | Is A faster or slower than B _at all_? (The **default**)   |
| `10%`               | Is A faster or slower than B by at least 10%?              |
| `+10%`              | Is A slower than B by at least 10%?                        |
| `-10%`              | Is A faster than B by at least 10%?                        |
| `-10%`, `+10%`      | (Same as `10%`)                                            |
| `0%`, `10%`, `100%` | Is A at all, a little, or a lot slower or faster than B?   |
| `0.5ms`             | Is A faster or slower than B by at least 0.5 milliseconds? |

In the following example, we have set `--auto-sample-conditions=10%`, meaning we
are interested in knowing whether A differs from B by at least 10% in either
direction. The sample size automatically increases until the confidence interval
is narrow enough to place the estimated difference squarely on one side or the
other of both conditions.

```
      <------------------------------->     n=50  X -10% X +10%
                <------------------>        n=100 ✔️ -10% X +10%
                    <----->                 n=200 ✔️ -10% ✔️ +10%

|---------|---------|---------|---------| difference in runtime
-20%      -10%        0       +10%      +20%

n    = sample size
<--> = confidence interval for percent difference of mean runtimes
✔️    = resolved condition
X    = unresolved condition
```

In this example, by `n=50` we are not sure whether A is faster or slower than B
by more than 10%. By `n=100` we have ruled out that B is _faster_ than A by more
than 10%, but we're still not sure if it's _slower_ by more than 10%. By `n=200`
we have also ruled out that B is slower than A by more than 10%, so we stop
sampling. Note that we still don't know which is _absolutely_ faster, we just
know that whatever the difference is, it is neither faster nor slower than 10%
(and if we did want to know, we could add `0` to our conditions).

Note that, if the _actual_ difference is very close to a condition, then it is
likely that the condition will never be met, and the timeout will expire.

## Measurement modes

Tachometer supports four modes of time interval measurements, controlled with
the `measurement` config file property, or the `--measure` flag.

If `measurement` is an array, then all of the given measurements will be
retrieved from each page load. Each measurement from a page is treated as its
own benchmark.

A measurement can specify a `name` property that will be used to display its
results.

#### Performance API

Retrieve a measure, mark, or paint timing from the
[`performance.getEntriesByName`](https://developer.mozilla.org/en-US/docs/Web/API/Performance/getEntriesByName)
API. Note this mode can only be used with a config file.

For example, in your benchmark:

```javascript
performance.mark('foo-start');
// Do some work ...
performance.mark('foo-stop');
performance.measure('foo', 'foo-start', 'foo-stop');
```

And in your config file:

```json
"benchmarks": [
  {
    "measurement": {
      "mode": "performance",
      "entryName": "foo"
    }
  }
]
```

The following performance entry types are supported:

- [`measure`](https://developer.mozilla.org/en-US/docs/Web/API/PerformanceMeasure):
  Retrieve the `duration` of a user-defined interval between two marks. Use for
  measuring the timing of a specific chunk of your code.
- [`mark`](https://developer.mozilla.org/en-US/docs/Web/API/User_Timing_API/Using_the_User_Timing_API#Performance_measures):
  Retrieve the `startTime` of a user-defined instant. Use for measuring the time
  between initial page navigation and a specific point in your code.
- [`paint`](https://developer.mozilla.org/en-US/docs/Web/API/PerformancePaintTiming):
  Retrieve the `startTime` of a built-in paint measurement (e.g.
  `first-contentful-paint`).

#### Callback

By default with local (non-URL) benchmarks, or when the `--measure` flag is set
to **`callback`**, your page is responsible for calling the `start()` and
`stop()` functions from the `/bench.js` module. This mode is appropriate for
micro benchmarks, or any other kind of situation where you want full control
over the beginning and end times.

#### Global result

When the `--measure` flag is set to `global`, then you can assign an arbitrary
millisecond result to the `window.tachometerResult` global. In this mode,
tachometer will poll until it finds a result assigned here.

```javascript
const start = performance.now();
for (const i = 0; i < 1000; i++) {}
window.tachometerResult = performance.now() - start;
```

This mode is appropriate when you need full control of the measured time, or
when you can't use callback mode because you are not using tachometer's built-in
server.

Alternatively, to poll an arbitrary JS expression in `global` measurement mode
(rather than `window.tachometerResult`), set `--measurement-expression` to the
JS expression to poll. This option is useful for scenarios where you cannot
easily modify the code under test to assign to `window.tachometerResult` but
are otherwise able to extract a measurement from the page using JavaScript.

#### First Contentful Paint (FCP)

When the `--measure` flag is set to **`fcp`**, or when the benchmark is an
external URL, then the [First Contentful Paint
(FCP)](https://developers.google.com/web/tools/lighthouse/audits/first-contentful-paint)
time will be automatically extracted from your page using the [Performance
Timeline
API](https://developer.mozilla.org/en-US/docs/Web/API/Performance_Timeline).
This interval begins at initial navigation, and ends when the browser first
renders any DOM content. Currently, only Chrome supports the
[`first-contentful-paint`](https://www.w3.org/TR/paint-timing/#first-contentful-paint)
performance timeline entry. In this mode, calling the `start()` and `stop()`
functions is not required, and has no effect.

## Interpreting results

### Average runtime

The first column of output is the **_average runtime_** of the benchmark. This
is a _95% confidence interval_ for the number of milliseconds that elapsed
during the benchmark. When you run only one benchmark, this is the only output.

### Difference table

When you run multiple benchmarks together, you'll get an NxN table summarizing
all of the _differences_ in runtimes, both in _absolute_ and _relative_ terms
(percent-change).

In this example screenshot we're comparing `for` loops, each running with a
different number of iterations (1, 1000, 1001, and 3000):

This table tells us:

- 1 iteration was between 65% and 73% _faster_ than 1000 iterations.

- 1000 iterations was between 179% and 263% _slower_ than 1 iteration. Note that
  the difference between _1-vs-1000_ and _1000-vs-1_ is the choice of which
  runtime is used as the _reference_ in the percent-change calculation, where
  the reference runtime comes from the _column_ labeled _"vs X"_.

- The difference between 1000 and 1001 iterations was ambiguous. We can't tell
  which is faster, because the difference was too small. 1000 iterations could
  be as much as 13% faster, or as much as 21% slower, than 1001 iterations.

## Confidence intervals

Loosely speaking, a confidence interval is a range of plausible values for a
parameter like runtime, and the _confidence level_ (which tachometer always
fixes to _95%_) corresponds to the degree of confidence we have that the interval
contains the _true value_ of that parameter. See
[Wikipedia](https://en.wikipedia.org/wiki/Confidence_interval#Meaning_and_interpretation)
for more information about confidence intervals.

```
    <------------->   Wider confidence interval
                      High variance and/or low sample size

<--->   Narrower confidence interval
                 Low variance and/or high sample size

|---------|---------|---------|---------|
-1%      -0.5%       0%      +0.5%      +1%
```

The way tachometer shrinks confidence intervals is by **increasing the sample
size**. The [central limit
theorem](https://en.wikipedia.org/wiki/Central_limit_theorem) means that, even
when we have high variance data, and even when that data is not normally
distributed, as we take more and more samples, we'll be able to calculate a more
and more precise estimate of the true mean of the data.

## Swap NPM dependencies

Tachometer has specialized support for swapping in custom versions of any NPM
dependency in your `package.json`. This can be used to compare the same
benchmark against one or more versions of a library it depends on.

Use the `benchmarks.packageVersions` JSON config property to specify the version
to swap in, like this:

```json
{
  "benchmarks": [
    {
      "name": "my-benchmark",
      "url": "my-benchmark.html",
      "packageVersions": {
        "label": "my-label",
        "dependencies": {
          "my-package": "github:MyOrg/my-repo#my-branch"
        }
      }
    }
  ]
}
```

The version for a dependency can be any of the following:

- Any version range supported by NPM, including semver ranges, git repos, and
  local paths. See the [NPM
  documentation](https://docs.npmjs.com/configuring-npm/package-json.html#dependencies)
  for more details.

- For monorepos, or other git repos where the `package.json` is not located at
  the root of the repository (which is required for NPM's git install function),
  you can use an advanced git configuration object
  ([schema](https://github.com/Polymer/tachometer/blob/master/config.schema.json#:~:text=GitDependency))
  in place of the NPM version string, e.g.:

```json
  {
    "benchmarks": [
      {
        "name": "my-benchmark",
        "url": "my-benchmark.html",
        "packageVersions": {
          "label": "my-label",
          "dependencies": {
            "my-package": {
              "kind": "git",
              "repo": "git@github.com:MyOrg/my-repo.git",
              "ref": "my-branch",
              "subdir": "packages/my-package",
              "setupCommands": ["npm install", "npm run build"]
            }
          }
        }
      }
    ]
  }
  ```

You can also use the `--package-version` flag to specify a version to swap in
from the command-line, with format `[label=]package@version`. Note that the
advanced git install configuration is not supported from the command line:

```
tach mybench.html \
  --package-version=my-package@1.0.0 \
  --package-version=my-label=my-package@github:MyOrg/my-repo#my-branch
```

When you specify a dependency to swap, the following happens:

1. The `package.json` file closest to your benchmark HTML file is found.

2. A copy of this `package.json`, with the new dependency version swapped in, is
   written to the system's temp directory (use `--npm-install-dir` to change
   this location), and `npm install` is run in that directory.

3. A separate server is started for each custom NPM installation, where any
   request for the benchmark's `node_modules/` directory is served from that
   location.

> **NOTE**: Tachometer will _re-use NPM install directories_ as long as the
> dependencies you specified haven't changed, and the version of tachometer used
> to install it is the same. To _always_ do a fresh `npm install`, set the
> `--force-clean-npm-install` flag.

## JavaScript module imports

JavaScript module imports with _bare module specifiers_ (e.g. `import {foo} from 'mylib';`) will be automatically transformed to browser-compatible _path_
imports using Node-style module resolution (e.g.`import {foo} from './node_modules/mylib/index.js';`).

This feature can be disabled with the `--resolve-bare-modules=false` flag, or
the `resolveBareModules: false` JSON config file property.

## Browsers

| Browser | Headless | [FCP](#first-contentful-paint-fcp) |
| ------- | -------- | ---------------------------------- |
| chrome  | yes      | yes                                |
| firefox | yes      | no                                 |
| safari  | no       | no                                 |
| edge    | no       | no                                 |
| ie      | no       | no                                 |

### Webdriver Plugins

Tachometer comes with WebDriver plugins for Chrome, Safari, Firefox, and
Internet Explorer.

For Edge, follow the [Microsoft WebDriver
installation](https://developer.microsoft.com/en-us/microsoft-edge/tools/webdriver/)
documentation.

If you encounter errors while driving IE, see the [Required
Configuration](https://github.com/SeleniumHQ/selenium/wiki/InternetExplorerDriver#required-configuration)
section of the WebDriver IE plugin documentation. In particular, setting "Enable
Protected Mode" so that it is consistently either enabled or disabled across all
security zones appears to resolve `NoSuchSessionError` errors.

#### On-demand dependencies

Tachometer will install WebDriver plugins for Chrome, Firefox and IE on-demand.
The first time that Tachometer runs a benchmark in any of these browsers, it
will install the appropriate plug-in via NPM or Yarn if it is not already
installed.

If you wish to avoid on-demand installations like this, you can install the
related packages (`chromedriver`, `geckodriver` and `iedriver`, respectively)
ahead of time with `npm install`, for example:

```
npm install tachometer chromedriver
```

In the example above, Tachometer will detect the manually installed `chromedriver`
package and will skip any attempt to install it on-demand later.

### Headless

If supported by the browser, you can launch in headless mode by adding
`"headless": true` to the browser JSON config, or by appending `-headless` to
the browser name when using the CLI flag (e.g. `--browser=chrome-headless`).

### Binary path and arguments

WebDriver automatically finds the location of the browser binary, and launches
it with a default set of arguments.

To customize the binary path (Chrome and Firefox only), use the `binary`
property in the browser JSON config. For example, to launch Chrome Canary from
its standard location on macOS:

```json
{
  "name": "chrome",
  "binary": "/Applications/Google Chrome Canary.app/Contents/MacOS/Google Chrome Canary"
}
```

To pass additional arguments to the binary (Chrome and Firefox only), use the
`addArguments` property in the browser JSON config. To remove one of the
arguments that WebDriver sets by default (Chrome only), use `removeArguments`
(see example in next section).

To configure Firefox preferences that are usually set from the `about:config`
page, use the `preferences` property in the browser JSON config.

### Profiles

It is normally recommended to use the default behavior whereby a new, empty
browser profile is created when the browser is launched, so that state from your
personal profile (cookies, extensions, cache etc.) do not influence benchmark
results.

However, in some cases it may be useful to use an existing browser profile, for
example if the webpage you are benchmarking requires being signed into an
account.

In Chrome and Firefox, use the `profile` JSON config option to specify an
existing profile to use. Other browsers do not yet support this option.

#### Chrome

To find your current profile location in Chrome, visit `chrome://version` and
look for "Profile Path".

If there is an existing Chrome process using this profile, you must first
terminate it. You also need to close all open tabs, or disable the "Continue
where you left off" startup setting, because tachometer does not expect to find
any existing tabs.

You may also need to remove the `use-mock-keychain` default argument if you
encounter authentication problems.

For example, using the standard location of the default user profile on macOS:

```json
{
  "benchmarks": [
    {
      "url": "mybench.html",
      "browser": {
        "name": "chrome",
        "profile": "/Users/<username>/Library/Application Support/Google/Chrome",
        "removeArguments": ["use-mock-keychain"]
      }
    }
  ]
}
```

#### Firefox

To find your current profile location in Firefox, visit `about:support` and look
for "Profile Folder" or "Profile Directory".

Note when using the `profile` option in Firefox, the profile directory is copied
to a temporary location.

You may encounter a `no such file or directory, stat '.../lock'` error, due to a
bug in `selenium-webdriver`. Deleting this `lock` file should resolve the error.

For example, using the standard location of user profiles on macOS:

```json
{
  "benchmarks": [
    {
      "url": "mybench.html",
      "browser": {
        "name": "firefox",
        "profile": "/Users/<username>/Library/Application Support/Firefox/Profiles/<profile-name>"
      }
    }
  ]
}
```

## Performance traces

Once you determine that something is slower or faster in comparison to something
else, investigating why is the natural next step. To assist in determining why,
consider collecting performance traces. These traces can be used to determine
what the browser is doing differently between two versions of code.

When the `trace` option is turned on in Chromium-based browsers, each tachometer
sample will produce a JSON file that can be viewed in Chromium's `about:tracing`
tool. Enter `about:tracing` in the URL bar of Chromium, click load, and select
the `json` file you want to view. Check out the [about:tracing doc
page](https://www.chromium.org/developers/how-tos/trace-event-profiling-tool) to
learn more about using the trace event profiling tool.

To turn on tracing with the default configuration, add `trace: true` to a
Chromium browser's config object. This config turns on tracing with some
[default categories enabled](./src/defaults.ts#L29) and puts the JSON files into
a directory called `logs` in your current working directory.

For example:

```json
{
  "benchmarks": [
    {
      "name": "my-benchmark",
      "url": "my-benchmark.html",
      "browser": {
        "name": "chrome",
        "trace": true
      }
    }
  ]
}
```

To customize where the logs files are placed or what categories of events are
traced, pass an object to the `trace` config as demonstrated below. The
`categories` property is a list of trace categories to collect. The `logDir` is
the directory to store the log files to. If it is relative, it is resolved
relative to the current working directory.

```json
{
  "benchmarks": [
    {
      "name": "my-benchmark",
      "url": "my-benchmark.html",
      "browser": {
        "name": "chrome",
        "trace": {
          "categories": ["blink", "cc", "netlog", "toplevel", "v8"],
          "logDir": "results/trace-logs"
        }
      }
    }
  ]
}
```

Available trace categories can be found by going to `about:tracing` in a
Chromium browser by entering `about:tracing` in the URL bar. Press "Record" in
the top right (1), then expand the "Edit categories" section (2). There, all the
categories available for tracing are listed. Note, for the "Disabled by Default
Categories", preface the name with the string `disabled-by-default-` when adding
it to your tachometer config. For example, to enable the disabled by default
`audio` category shown below (3), specify `disabled-by-default-audio` in your
`browser.trace.categories` tachometer config.

![about:tracing app demonstrating the steps above](https://raw.githubusercontent.com/google/tachometer/main/images/about-tracing-small.png)

Tracing can also be enabled via command line flags. See the table at the end of
the file for details.

## Remote control

Tachometer can control and benchmark browsers running on remote machines by
using the [Standalone Selenium
Server](https://seleniumhq.github.io/docs/remote.html), which supports macOS,
Windows, and Linux.

This may be useful if you want to develop on one platform but benchmark on
another, or if you want to use a dedicated benchmarking computer for better
performance isolation.

> Note you will need to know the IP address of both your local and remote
> machine for the setup steps below. You can typically use `ipconfig` on
> Windows, `ifconfig` on macOS, and `ip` on Linux to find these addresses.
> You'll need to be able to initiate connections between these machines in both
> directions, so if you encounter problems, it's possible that there is a
> firewall or NAT preventing the connection.

#### On the _remote_ machine:

1. Install a [Java Development Kit
   (JDK)](https://www.oracle.com/technetwork/java/javase/downloads/index.html)
   if you don't already have one.

2. Download the latest Standalone Selenium Server `.jar` file from
   [seleniumhq.org](https://www.seleniumhq.org/download/).

3. Download the driver plugins for the browsers you intend to remote control
   from [seleniumhq.org](https://www.seleniumhq.org/download/). Note that if you
   download a plugin archive file, the archive contents must be extracted and
   placed either in the current working directory for the next command, or in a
   directory that is included in your `$PATH` environment variable.

4. Launch the Standalone Selenium Server.

```bash
   java -jar selenium-server-standalone-<version>.jar
   ```

#### On the _local_ machine:

1.  Use the `--browser` flag or the `browser` config file property with syntax
    `<browser>@<remote-url>` to tell tachometer the IP address or hostname of
    the remote Standalone Selenium Server to launch the browser from. Note that
    `4444` is the default port, and the `/wd/hub` URL suffix is required.

```bash
    --browser=chrome@http://my-remote-machine:4444/wd/hub
    ```

2.  Use the `--host` flag to configure the network interface address that
    tachometer's built-in static server will listen on (unless you are only
    benchmarking external URLs that do not require the static server). By
    default, for security, tachometer listens on `127.0.0.1` and will not be
    accessible from the remote machine unless you change this to an IP address
    or hostname that will be accessible from the remote machine.

3.  If needed, use the `--remote-accessible-host` flag to configure the URL that
    the remote browser will use when making requests to your local tachometer
    static server. By default this will match `--host`, but in some network
    configurations it may need to be different (e.g. if the machines are
    separated by a NAT).

## Config file

Use the `--config` flag to control tachometer with a JSON configuration file.
Defaults are the same as the corresponding command-line flags.

All paths in a config file are relative to the path of the config file itself.

You will typically want to set `root` to the directory that contains your
package's `node_modules/` folder, so that the web server will be able to resolve
bare-module imports.

For example, a file called `benchmarks/foo/tachometer.json` might look like
this:

```json
{
  "root": "../..",
  "sampleSize": 50,
  "timeout": 3,
  "autoSampleConditions": ["0%", "1%"],
  "benchmarks": [
    {
      "name": "foo",
      "url": "foo/bar.html?baz=123",
      "browser": {
        "name": "chrome",
        "headless": true,
        "windowSize": {
          "width": 800,
          "height": 600
        }
      },
      "measure": "fcp",
      "packageVersions": {
        "label": "my-branch",
        "dependencies": {
          "mylib": "github:Polymer/mylib#my-branch"
        }
      }
    }
  ]
}
```

Use the `expand` property in a benchmark object to recursively generate multiple
variations of the same benchmark configuration. For example, to test the same
benchmark file with two different browsers, you can use `expand` instead of
duplicating the entire benchmark configuration:

```json
{
  "benchmarks": [
    {
      "url": "foo/bar.html",
      "expand": [
        {
          "browser": "chrome"
        },
        {
          "browser": "firefox"
        }
      ]
    }
  ]
}
```

Which is equivalent to:

```json
{
  "benchmarks": [
    {
      "url": "foo/bar.html",
      "browser": "chrome"
    },
    {
      "url": "foo/bar.html",
      "browser": "firefox"
    }
  ]
}
```

## CLI usage

Run a benchmark from a local file:

```sh
tach foo.html
```

Compare a benchmark with different URL parameters:

```sh
tach foo.html?i=1 foo.html?i=2
```

Benchmark `index.html` in a directory:

```sh
tach foo/bar
```

Benchmark First Contentful Paint time of a remote URL:

```sh
tach http://example.com
```

| Flag -                      | Default                                 | Description                                                                                                                                                        |
| --------------------------- | --------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ |
| `--help`                    | `false`                                 | Show documentation                                                                                                                                                 |
| `--root`                    | `./`                                    | Root directory to search for benchmarks                                                                                                                            |
| `--host`                    | `127.0.0.1`                             | Which host to run on                                                                                                                                               |
| `--port`                    | `8080, 8081, ..., 0`                    | Which port to run on (comma-delimited preference list, `0` for random)                                                                                             |
| `--config`                  | _(none)_                                | Path to JSON config file ([details](#config-file))                                                                                                                 |
| `--package-version` / `-p`  | _(none)_                                | Specify an NPM package version to swap in ([details](#swap-npm-dependencies))                                                                                      |
| `--browser` / `-b`          | `chrome`                                | Which browsers to launch in automatic mode, comma-delimited (chrome, firefox, safari, edge, ie) ([details](#browsers))                                             |
| `--window-size`             | `1024,768`                              | "width,height" in pixels of the browser windows that will be created                                                                                               |
| `--sample-size` / `-n`      | `50`                                    | Minimum number of times to run each benchmark ([details](#minimum-sample-size))                                                                                    |
| `--auto-sample-conditions`  | `0%`                                    | The degrees of difference to try and resolve when auto-sampling ("N%" or "Nms", comma-delimited) ([details](#auto-sample-conditions))                              |
| `--timeout`                 | `3`                                     | The maximum number of minutes to spend auto-sampling ([details](#auto-sample))                                                                                     |
| `--measure`                 | `callback`                              | Which time interval to measure (`callback`, `global`, `fcp`) ([details](#measurement-modes))                                                                       |
| `--measurement-expression`  | `window.tachometerResult`               | JS expression to poll for on page to retrieve measurement result when `measure` setting is set to `global`                                                         |
| `--remote-accessible-host`  | matches `--host`                        | When using a browser over a remote WebDriver connection, the URL that those browsers should use to access the local tachometer server ([details](#remote-control)) |
| `--npm-install-dir`         | system temp dir                         | Where to install custom package versions. ([details](#swap-npm-dependencies))                                                                                      |
| `--force-clean-npm-install` | `false`                                 | Always do a from-scratch NPM install when using custom package versions. ([details](#swap-npm-dependencies))                                                       |
| `--csv-file`                | _none_                                  | Save statistical summary to this CSV file.                                                                                                                         |
| `--csv-file-raw`            | _none_                                  | Save raw sample measurements to this CSV file.                                                                                                                     |
| `--json-file`               | _none_                                  | Save results to this JSON file.                                                                                                                                    |
| `--manual`                  | `false`                                 | Don't run automatically, just show URLs and collect results                                                                                                        |
| `--trace`                   | `false`                                 | Enable performance tracing ([details](#performance-traces))                                                                                                        |
| `--trace-log-dir`           | `${cwd}/logs`                           | The directory to put tracing log files. Defaults to `${cwd}/logs`.                                                                                                 |
| `--trace-cat`               | [default categories](./src/defaults.ts) | The tracing categories to record. Should be a string of comma-separated category names                                                                             |

tachometer

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