[webkit-dev] Making WTF::StringImpl and WTF::AtomString thread-safe

[Geoff Garen]

>> 1. It’s not just ref counting. 
>> 
>> To make String thread-safe, you also need to address all other data
>> members. That means all state in m_hashAndFlags, including the
>> 8bit/16bit state.
>> 
>> It appears that your testing strategy did not reveal this point so
>> far; so, you probably need to expand your plan for unit testing
>> concurrent access to a string, with a focus on writing tests that fail
>> with the current implementation.
> 
> I did consider this, I've also made m_hashAndFlags atomic in the
> attached patch. Indeed, tests for concurrent string usage and expected
> behaviour would be desirable, I would take that as a given (but it's
> worth mentioning).

Great!

>> 3. I’m surprised by the premise that thread-safe String is a
>> requirement for FontCache and/or the GPU Process.
>> 
>> It’s certainly a false premise that there’s consensus on this premise,
>> since I do not agree.
>> 
>> Can you share some problem statements regarding FontCache and/or the
>> GPU Process that explain the problem we’re trying to solve?
> 
> I can't talk about GPU Process, this was something that was mentioned to
> me when I was talking about methods of making FontCache thread-safe.
> 
> FontCache makes extensive use of AtomString for look-ups and
> comparisons. I had a few alternative ideas for making FontCache safe to
> use in a Worker, but after discussing them on Slack, it seemed like
> making FontCache safe for concurrent access and making String
> thread-safe were both desirable for future work. I really hope other
> people will chime in here, my personal preference would be to do
> something less invasive.

So the goal is to enable use of fonts (and FontCache) in Workers?

> Other ideas I had for making FontCache Worker-safe;
> - Add a FontCacheProxy object that calls onto the main-thread FontCache
> and blocks (not ideal for performance)

Yeah, that doesn’t sound great.

> - Make FontCache not rely on static data and have a completely separate
> FontCache per-Worker, created on-demand (not ideal for memory usage)

Seems OK. I think the main downside to this proposal is that an app that moves font-related work to a worker as a performance optimization will also experience a performance regression by hitting a cold FontCache.

> - Make FontCache thread-safe, but do it via introducing a completely
> separate thread-safe AtomString type and leave the current one as it is
> (I don't have a good grasp of how difficult this would actually be)

I had to chuckle at this point because the obvious name for this new thread-safe AtomString class would be AtomicString, the prior name of AtomString.

I think it might be worthwhile to prototype either a per-thread FontCache or a FontCache based on a custom string type or both. One reason it might be worthwhile is that it will reveal the non-string work that needs to happen to achieve thread safety (or at least thread isolation) in FontCache. For example, FontDataCache and its associated types look like they might need work. Another reason it might be worthwhile is that I believe that solving this problem just for FontCache will be a smaller project than making Strings generally thread-safe.

I think you’re right that making Strings generally thread-safe would be good for the project overall. I’m just worried that it might set back the FontCache project.

Thanks,
Geoff

> 
> Cheers,
> 
> Chris
> 
>> Thanks,
>> Geoff
>> 
>>> On Dec 1, 2020, at 9:09 AM, Chris Lord via webkit-dev <webkit-dev at lists.webkit.org> wrote:
>>> 
>>> Hi all,
>>> 
>>> As part of the work for making FontCache thread-safe, it's necessary for
>>> there to be a thread-safe AtomString. After discussion, it seems that a
>>> thread-safe StringImpl is generally desirable and GPUProcess also has a
>>> need of it. I've filed a bug to track this work:
>>> https://bugs.webkit.org/show_bug.cgi?id=219285
>>> 
>>> Google have already done this for Blink and there's a nice plan and lots
>>> of discussion to read. Their plan document is linked in the bug. I think
>>> we'd be well-served by taking broadly the same approach, which is to
>>> make ref-counting on StringImpl atomic and to guard AtomStringTable
>>> access with a lock.
>>> 
>>> Making String thread-safe has implications, of course, and I'd like to
>>> open discussion on this - Making ref-counting atomic on StringImpl has a
>>> significant, negative impact on the expense of ref and deref operations.
>>> I'm interested in discussing how we should approach this in terms of
>>> tracking the work in Bugzilla and how to go about landing it. Perhaps
>>> people also have alternative ideas?
>>> 
>>> On the bug is a first-run at implementing the above approach, currently
>>> minus the follow-up of everywhere taking into consideration that
>>> String/AtomString are now thread-safe. The impact on StringImpl
>>> ref/deref performance has it running on my Xeon desktop machine at about
>>> 30-50% of non-atomic ref/deref performance. Speedometer 2.0 takes a 1-8%
>>> hit considering error margins, but I'm fairly certain it's mostly on the
>>> higher end of that and I've not run enough iterations just yet.
>>> Jetstream 1.1 seems practically unaffected, I can't run 2.0 with or
>>> without the patch, it appears to hang the browser on the bomb-workers
>>> test (at least if it completes, it's not in a reasonable time-frame). I
>>> would guess that results may vary wildly depending on platform and
>>> available atomic access primitives. As one might expect, the impact is
>>> far less on a debug build.
>>> 
>>> I think the initial patch of making it thread-safe vs. the follow-up of
>>> altering various areas to take it into account could/should be split,
>>> but I assume we'd want to land them at the same time. This is cumbersome
>>> with how WebKit Bugzilla currently works and I'd like to hear what
>>> people think and how similar changes have been made in the past.
>>> 
>>> Thoughts?
>>> 
>>> Regards,
>>> 
>>> Chris
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