PSA : Sequencification and PostTask paradigm update

[Gabriel Charette]

Hello all,

tl;dr; you should now prefer SequencedTaskRunners obtained from base/task_scheduler/post_task.h to run tasks off the UI/IO threads in a thread-safe fashion.

Chromium has evolved from an initial dedicated thread (BrowserThread::FILE, etc.). As such our codebase has many single-thread constructs (SingleThreadTaskRunner, ThreadTaskRunnerHandle, ThreadChecker, etc.) in classes that are merely thread-unsafe but not thread-affine per se.

SequencedTaskRunners provide thread-safety as well and are much prefered to SingleThreadTaskRunners as they provide more flexibility for scheduling.

Over the last few months we made many core APIs sequence-friendly.

We just made a scripted pass over the codebase to migrate away from base::NonThreadSafe (gone gone!) to SequenceChecker. It turned out that < 5% of the code truly needed thread-affinity (ThreadChecker).

As you start using SequencedTaskRunner, either in new components or replacing existing SingleThreadTaskRunners, if you run into DCHECKs in any leaf dependency (outside of your component) per it using ThreadChecker/ThreadTaskRunnerHandle/etc. : you may ideally fix those components (they're not typically thread-affine unless using TLS or third-party APIs that are affine) or, alternatively, you can temporarily use base::CreateSingleThreadTaskRunnerWithTraits(...) with a TODO tagged with a bug # marked as a blocker for https://crbug.com/675631 (one of us will take a look at it and flip your code to use a SequencedTaskRunner when fixed).

PS: We automated much of the migration to TaskScheduler away from old APIs but are getting to the harder cases (requiring manual steps). We will soon reach out to individual owners with precise migration docs to complete this on a component-by-component basis.

Thanks!

Gab on behalf of the TaskScheduler team.

[Gabriel Charette]

Oh and I forgot the most important part... we have brand new documentation for threading/tasks in Chromium :) : https://chromium.googlesource.com/chromium/src/+/master/docs/threading_and_tasks.md

[Marijn Kruisselbrink]

Keep in mind that if you're using mojo in any way (either implementing a mojo interface, or connection/communication to a mojo interface) you still won't be able to use SequencedTaskRunners until http://crbug.com/678155 is fixed. Which I imagine excludes a large part of the code base, especially as more and more code is migrated to mojo...

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[Gabriel Charette]

On Tue, Jun 6, 2017 at 1:33 PM Marijn Kruisselbrink <m...@chromium.org> wrote:

Keep in mind that if you're using mojo in any way (either implementing a mojo interface, or connection/communication to a mojo interface) you still won't be able to use SequencedTaskRunners until http://crbug.com/678155 is fixed. Which I imagine excludes a large part of the code base, especially as more and more code is migrated to mojo...

True but the same rules apply (except in this case you have an existing bug to use to tag your use of base::CreateSingleThreadTaskRunnerWithTraits()'s TODO).

PS: We are in the process of implementing SequenceLocalStorage as of this writing. It's the last requirement to allow Mojo (and most Chromium code that uses TLS) to become sequence-friendly :).

[James Cook]

These docs are awesome!

One request: Could you add a sentence or two about *why* we are doing all this thread / sequence stuff? I presume it's so we can scale the number of threads based on CPU hardware and OS, and so the scheduler can be more clever about keeping all of them busy.

James

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[Gabriel Charette]

On Wed, Jun 7, 2017 at 12:58 PM James Cook <jame...@chromium.org> wrote:

These docs are awesome!

One request: Could you add a sentence or two about *why* we are doing all this thread / sequence stuff? I presume it's so we can scale the number of threads based on CPU hardware and OS, and so the scheduler can be more clever about keeping all of them busy.

Precisely yes. Added a section to elaborate on this @ https://chromium.googlesource.com/chromium/src/+/master/docs/threading_and_tasks.md#Prefer-Sequences-to-Threads