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Trybot: fuchsia-binary-size

[TOC]

About

The fuchsia-binary-size trybot exists for two reasons:

  1. To measure and make developers aware of the binary size impact of commits on Chrome-Fuchsia products. Reducing growth on Chrome-Fuchsia products on the limited-size platforms is key to shipping Chrome (and its derivatives) successfully on Fuchsia.
  2. To perform checks that require comparing builds with & without patch.

Measurements and Analysis

The bot provides analysis using:

Checks:

  • The changes are aggregated in the Generate commit size analysis files step of the trybot
  • The difference with & without the patch are compared in the Generate Diffs step. This summarizes how each package grew, shrank, or stayed the same.

***note Note: The tool currently excludes shared-libary contributions to the package size.

Binary Size Increase

  • What: Checks that compressed fuchsia archive size increases by no more than 12kb.
    • These packages are cast_runner and web_engine.
  • Why: Chrome-Fuchsia deploys on platforms with small footprints. As each release rolls, Fuchsia runs its own set of size checks that will reject a release if the Chrome-Fuchsia packages exceed the allocated size budget. This builder tries to mitigate the unending growth of Chromium's impact on Chrome-Fuchsia continuous deployment.

What to do if the Check Fails?

  • The Read diff results stdout will also give a breakdown of growth by package, if any:
{
  "archive_filenames": [],
  "compressed": {
    "cast_runner": 0,
    "chrome_fuchsia": 40960,  # chrome_fuchsia = web_engine + cast_runner
    "web_engine": 40960  # This package grew by 40kB (post-compression)
  },
  "links": [],
  "status_code": 1,
[...]
  "uncompressed": {
    "cast_runner": 0,
    "chrome_fuchsia": 33444,
    "web_engine": 33444  # This package grew by 32.66kB (pre-compression)
  }
}
  • If cast_runner grew in size, you may need assistance from the Chrome-Fuchsia team ([email protected]).
  • If you are writing a new feature or including a new library, consider:
    • If it is a feature only intended for Chrome (a full-browser), Contact [email protected] to help fix this.
    • Whether it belongs on a size-contrained device (think a low-end phone with a <1GB in total disk storage). [email protected] can be helpful:
      • If it does, look into how to reduce the overall size
      • If it does not, look to remove it. See below.

If you find it should be removed from a size-constrained platform, you should guard the code with a BUILDFLAG and disable the associated feature in size-constrained builds by explicitly setting the GN arg in size_optimized_cast_receiver_args.gn.

  • See if any of the generic optimization advice is applicable.
  • See the section below
  • If reduction is not practical, add a rationale for the increase to the commit description, and skip the check. It could include:
    • A list of any optimizations that you attempted (if applicable)
    • If you think that there might not be a consensus that the code your adding is worth the added file size, then add why you think it is.

Skipping the check

Add a footer to the commit description along the lines of:

  • Fuchsia-Binary-Size: Size increase is unavoidable.
  • Fuchsia-Binary-Size: Increase is temporary.
  • Fuchsia-Binary-Size: See commit description. <-- use this if longer than one line.

***note Note: Make sure there are no blank lines between Fuchsia-Binary-Size: and other footers.

Compressed vs Uncompressed

The size metric we care about the most is the compressed size. This is an estimate of how large the Chrome-Fuchsia packages will be when delivered on device (actual compression can vary between devices, so the computed numbers may not be accurate). However, you may see the uncompressed and compressed size grow by different amounts (and sometimes the compressed size is larger than the uncompressed)!

This is due to how sizes are calculated and how the compression is done. The uncompressed size is exactly that: the size of the package before it is compressed.

Compression is done via the blobfs-compression tool, exported from the Fuchsia SDK. This compresses the file into a package that is ready to be deployed to the Fuchsia device. With the current (default) compression-mode, this compresses the package in page-sizes designed for the device and filesystem. Since each page is at least 8K, increments in the compressed size are always multiples of 8K with the current compression mode. So, if your change causes the previous compression page to go over the limit, you may see an 8K increase for an otherwise small change.

Large changes will increase more than a page's work (to at least 16K), which is why we only monitor 12K+ changes (12K isn't possible due to the 8K page size) and not 8K+.

You are responsible only for pre-compression size increases. If your change did not cause a pre-compression size increase, but still failed the builder, please ignore it using the Fuchsia-Binary-Size: footer.

Running a local binary-size check

If you want to check your changes impact to binary size locally (instead of against the trybot), do the following:

0. First time compiling Chrome-Fuchsia

Add the following to your .gclient config:

{
  # ... whatever you have here ...
  "solutions": [
    {
      # ...whatever you have here...
    }
  ],
  "target_os": [
    "fuchsia"
  ],
}

Then run gclient sync to add the fuchsia-sdk to your third_party directory.

1. GN Args

Set up a build directory with the following GN Args:

import("//build/config/fuchsia/size_optimized_cast_receiver_args.gn")
dcheck_always_on = false
is_debug = false
is_official_build = true
target_cpu = "arm64"
target_os = "fuchsia"
use_remoteexec = true  # If appropriate.

2. Build

Build the fuchsia_sizes target:

autoninja -C <path/to/out/dir> fuchsia_sizes

3. Run the size script

Run the size script with the following command:

build/fuchsia/binary_sizes.py --build-out-dir <path/to/out/dir>

The size breakdown by blob and package will be given, followed by a summary at the end, for chrome_fuchsia, web_engine, and cast_runner. The number that is deployed to the device is the compressed version.

How to reduce your binary-size for Fuchsia

TODO(crbug.com/40821616): Fill this out.

Obvious regressions

Many new blobs

(shamelessly stolen from this doc, but looking for any tips on how to improve this for Fuchsia specifically)

Look at blobs and see that there aren't a huge number of blobs added in.

  • What does it mean if blobs are added in?
    • Something changed in the dependencies, causing new files to be pulled in.
  • Is there a way to reverse-engineer a blobs source?
    • Locales are language pack
    • Try searching BUILD files to see if the .so was included from somewhere in particular.

Bloaty

Bloaty can be used to determine the composition of the binary (and can be helpful for determining the cause of the increase).

  1. (first time only) Install Bloaty using these instructions.
  2. Build two copies of the packages (see above) - with and without your changes in two separate output directories.
  3. Generate Bloaty results. You can run Bloaty against the stripped binaries (<out dir>/web_engine_exe and <out dir>/cast_runner_exe). However, if you want more information, you will have to run it against the unstripped binaries (located in <out dir>/exe.unstripped. You only need to run Bloaty against the binary your change affected.
$ bloaty -d compileunits,symbols $OUT_DIR/exe.unstripped/web_engine_exe \
  -n $ROW_LIMIT -s vm

-n $ROW_LIMIT determines the number of rows to show per level before collapsing. Setting to 0 shows all rows. Default is 20.

-s vm indicates to sort by Virtual Memory (VM) size increase. This is the metric that grows somewhat closely to the binary-size bot's size metric.

NOTE: that the sizes reported from Bloaty will not be exactly the same as those reported by the binary_sizes script since Bloaty analyzes the uncompressed (and potentially unstripped) binary, but the reported relative growth can point you in the right direction. The File Size can vary a lot due to debug symbol information. The VM Size is usually a good lead.

If Bloaty reports your change decreased the uncompressed size, use a footer to ignore the check.

You can also directly generate a comparison with the following:

$ bloaty -d compileunits,symbols \
  $OUT_DIR_WITH_CHANGE/exe.unstripped/web_engine_exe -n $ROW_LIMIT -s vm -- \
  $OUT_DIR_WITHOUT_CHANGE/exe.unstripped/web_engine_exe

You can find out more about sections of ELF binaries here.

If All Else Fails

  • For help, email [[email protected]]. They're expert Chrome-Fuchsia developers! See here for more details.
  • Not all checks are perfect and sometimes you want to overrule the trybot (for example if you did your best and are unable to reduce binary size any further).
  • Check out the Chromium binary-size Google group.
  • Adding a “Fuchsia-Binary-Size: $ANY_TEXT_HERE” footer to your cl (next to “Bug:”) will bypass the bot assertions.
    • Most commits that trigger the warnings will also result in Telemetry alerts and be reviewed by a binary size sheriff. Failing to write an adequate justification may lead to the binary size sheriff filing a bug against you to improve your cl.

Code Locations