GPU Bots & Pixel Wrangling

(December 2017: presentation on GPU bots and pixel wrangling: see slides.)

GPU Pixel Wrangling is the process of keeping various GPU bots green. On the GPU bots, tests run on physical hardware with real GPUs, not in VMs like the majority of the bots on the Chromium waterfall.

Fleet Status

The following links (sorry, Google employees only) show the status of various GPU bots in the fleet.

Primary configurations:

Secondary configurations:

GPU Bots' Waterfalls

The waterfalls work much like any other; see the Tour of the Chromium Buildbot Waterfall for a more detailed explanation of how this is laid out. We have more subtle configurations because the GPU matters, not just the OS and release v. debug. Hence we have Windows Nvidia Release bots, Mac Intel Debug bots, and so on. The waterfalls we’re interested in are:

  • Chromium GPU
    • Various operating systems, configurations, GPUs, etc.
  • Chromium GPU FYI
    • These bots run less-standard configurations like Windows with AMD GPUs, Linux with Intel GPUs, etc.
    • These bots build with top of tree ANGLE rather than the DEPS version.
    • The ANGLE tryservers help ensure that these bots stay green. However, it is possible that due to ANGLE changes these bots may be red while the chromium.gpu bots are green.
    • The ANGLE Wrangler is on-call to help resolve ANGLE-related breakage on this watefall.
    • To determine if a different ANGLE revision was used between two builds, compare the got_angle_revision buildbot property on the GPU builders or parent_got_angle_revision on the testers. This revision can be used to do a git log in the third_party/angle repository.

Test Suites

The bots run several test suites. The majority of them have been migrated to the Telemetry harness, and are run within the full browser, in order to better test the code that is actually shipped. As of this writing, the tests included:

  • Tests using the Telemetry harness:
    • The WebGL conformance tests:
    • A Google Maps test:
    • Context loss tests:
    • Depth capture tests:
    • GPU process launch tests:
    • Hardware acceleration validation tests:
    • Pixel tests validating the end-to-end rendering pipeline:
    • Stress tests of the screenshot functionality other tests use:
  • angle_unittests: see src/gpu/gpu.gyp
  • drawElements tests (on the waterfall): see src/third_party/angle/src/tests/
  • gles2_conform_test (requires internal sources): see src/gpu/gles2_conform_support/gles2_conform_test.gyp
  • gl_tests: see src/gpu/
  • gl_unittests: see src/ui/gl/

And more. See src/content/test/gpu/ for the complete description of bots and tests.

Additionally, the Release bots run:

  • tab_capture_end2end_tests: see src/chrome/browser/extensions/api/tab_capture/ and src/chrome/browser/extensions/api/cast_streaming/

More Details

More details about the bots' setup can be found on the GPU Testing page.



  1. Ideally a wrangler should be a Chromium committer. If you're on the GPU pixel wrangling rotation, there will be an email notifying you of the upcoming shift, and a calendar appointment.
    • If you aren‘t a committer, don’t panic. It's still best for everyone on the team to become acquainted with the procedures of maintaining the GPU bots.
    • In this case you'll upload CLs to Gerrit to perform reverts (optionally using the new “Revert” button in the UI), and might consider using TBR= to speed through trivial and urgent CLs. In general, try to send all CLs through the commit queue.
    • Contact bajones, kainino, kbr, vmiura, zmo, or another member of the Chrome GPU team who's already a committer for help landing patches or reverts during your shift.
  2. Apply for access to the bots.

How to Keep the Bots Green

  1. Watch for redness on the tree.
    1. Sheriff-O-Matic now has support for the waterfall!
    2. The chromium.gpu bots are covered under Sheriff-O-Matic's Chromium tab. As pixel wrangler, ignore any non-GPU test failures in this tab.
    3. The bots are expected to be green all the time. Flakiness on these bots is neither expected nor acceptable.
    4. If a bot goes consistently red, it‘s necessary to figure out whether a recent CL caused it, or whether it’s a problem with the bot or infrastructure.
    5. If it looks like a problem with the bot (deep problems like failing to check out the sources, the isolate server failing, etc.) notify the Chromium troopers and file a P1 bug with labels: Infra>Labs, Infra>Troopers and Internals>GPU>Testing. See the general tree sheriffing page for more details.
    6. Otherwise, examine the builds just before and after the redness was introduced. Look at the revisions in the builds before and after the failure was introduced.
    7. File a bug capturing the regression range and excerpts of any associated logs. Regressions should be marked P1. CC engineers who you think may be able to help triage the issue. Keep in mind that the logs on the bots expire after a few days, so make sure to add copies of relevant logs to the bug report.
    8. Use the Hotlist=PixelWrangler label to mark bugs that require the pixel wrangler‘s attention, so it’s easy to find relevant bugs when handing off shifts.
    9. Study the regression range carefully. Use drover to revert any CLs which break the chromium.gpu bots. Use your judgment about, since not all bots are covered by trybots. In the revert message, provide a clear description of what broke, links to failing builds, and excerpts of the failure logs, because the build logs expire after a few days.
  2. Make sure the bots are running jobs.
    1. Keep an eye on the console views of the various bots.
    2. Make sure the bots are all actively processing jobs. If they go offline for a long period of time, the “summary bubble” at the top may still be green, but the column in the console view will be gray.
    3. Email the Chromium troopers if you find a bot that's not processing jobs.
  3. Make sure the GPU try servers are in good health.
    1. The GPU try servers are no longer distinct bots on a separate waterfall, but instead run as part of the regular tryjobs on the Chromium waterfalls. The GPU tests run as part of the following tryservers' jobs:
      1. linux_chromium_rel_ng on the luci.chromium.try waterfall
    1.  <code>[mac_chromium_rel_ng]</code> on the [tryserver.chromium.mac]
    1.  <code>[win7_chromium_rel_ng]</code> on the []
1.  The best tool to use to quickly find flakiness on the tryservers is the
    new [Chromium Try Flakes] tool. Look for the names of GPU tests (like
    maps_pixel_test) as well as the test machines (e.g.
    mac_chromium_rel_ng). If you see a flaky test, file a bug like [this
    one]( Also look for compile flakes that may
    indicate that a bot needs to be clobbered. Contact the Chromium
    sheriffs or troopers if so.
1.  Glance at these trybots from time to time and see if any GPU tests are
    failing frequently. **Note** that test failures are **expected** on
    these bots: individuals' patches may fail to apply, fail to compile, or
    break various tests. Look specifically for patterns in the failures. It
    isn't necessary to spend a lot of time investigating each individual
    failure. (Use the "Show: 200" link at the bottom of the page to see
    more history.)
1.  If the same set of tests are failing repeatedly, look at the individual
    runs. Examine the swarming results and see whether they're all running
    on the same machine. (This is the "Bot assigned to task" when clicking
    any of the test's shards in the build logs.) If they are, something
    might be wrong with the hardware. Use the [Swarming Server Stats] tool
    to drill down into the specific builder.
1.  If you see the same test failing in a flaky manner across multiple
    machines and multiple CLs, it's crucial to investigate why it's
    happening. []( was one example
    of an innocent-looking Blink change which made it through the commit
    queue and introduced widespread flakiness in a range of GPU tests. The
    failures were also most visible on the try servers as opposed to the
    main waterfalls.
  1. Check if any pixel test failures are actual failures or need to be rebaselined.
    1. For a given build failing the pixel tests, click the “stdio” link of the “pixel” step.
    2. The output will contain a link of the form
    3. Visit the link to see whether the generated or reference images look incorrect.
    4. All of the reference images for all of the bots are stored in cloud storage under chromium-gpu-archive/reference-images. They are indexed by version number, OS, GPU vendor, GPU device, and whether or not antialiasing is enabled in that configuration. You can download the reference images individually to examine them in detail.
  2. Rebaseline pixel test reference images if necessary.
    1. Follow the instructions on the GPU testing page.
    2. Alternatively, if absolutely necessary, you can use the Chrome Internal GPU Pixel Wrangling Instructions to delete just the broken reference images for a particular configuration.
  3. Update Telemetry-based test expectations if necessary.
    1. Most of the GPU tests are run inside a full Chromium browser, launched by Telemetry, rather than a Gtest harness. The tests and their expectations are contained in src/content/test/gpu/gpu_tests/ . See for example, and
    2. See the header of the file a list of modifiers to specify a bot configuration. It is possible to specify OS (down to a specific version, say, Windows 7 or Mountain Lion), GPU vendor (NVIDIA/AMD/Intel), and a specific GPU device.
    3. The key is to maintain the highest coverage: if you have to disable a test, disable it only on the specific configurations it's failing. Note that it is not possible to discern between Debug and Release configurations.
    4. Mark tests failing or skipped, which will suppress flaky failures, only as a last resort. It is only really necessary to suppress failures that are showing up on the GPU tryservers, since failing tests no longer close the Chromium tree.
    5. Please read the section on stamping out flakiness for motivation on how important it is to eliminate flakiness rather than hiding it.
  4. For the remaining Gtest-style tests, use the DISABLED_ modifier to suppress any failures if necessary.

When Bots Misbehave (SSHing into a bot)

  1. See the Chrome Internal GPU Pixel Wrangling Instructions for information on ssh'ing in to the GPU bots.

Reproducing WebGL conformance test failures locally

  1. From the buildbot build output page, click on the failed shard to get to the swarming task page. Scroll to the bottom of the left panel for a command to run the task locally. This will automatically download the build and any other inputs needed.
  2. Alternatively, to run the test on a local build, pass the arguments --browser=exact --browser-executable=/path/to/binary to content/test/gpu/ Also see the telemetry documentation.

Extending the GPU Pixel Wrangling Rotation

See the Chrome Internal GPU Pixel Wrangling Instructions for information on extending the rotation.