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

[Darin Adler via webkit-dev]

> On Dec 9, 2020, at 1:02 PM, Geoff Garen via webkit-dev 
>  wrote:
> 
>> - 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.

If we make a separate thread-safe code path, I’m not sure we need to create a 
variant of AtomString at all.

AtomString optimizes string comparisons (by paying up front every time you 
construct one with a hash table lookup) and memory use (by sharing the same 
memory for all equal strings), but there’s no reason we can’t compare two 
strings without using AtomString. I’m doubting that once we figure out what 
we’re trying to do that we’ll need AtomString.

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

[Geoff Garen via webkit-dev]

>> 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 
>>>  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 

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

[Geoff Garen via webkit-dev]

>> Because it’s so expensive, and because we have a no regression policy for 
>> performance, I don’t think there’s a way to land this change in pieces. It 
>> has to be a mega-patch, so we can test its performance as a whole.
> 
> Were you able to quantify anything additional about the performance 
> regression on Intel, for instance, if there were any types of String usage 
> that were particularly hard hit? If not (or you haven’t tried), do you think 
> there are any metrics we should look into gathering that might help pin point 
> where things could maybe be optimized? Ideally we would be able to identify 
> places where we unnecessarily ref-deref StringImpls where either a move or 
> judicious use of StringView would work just as well. 

I didn’t debug deeply. Maybe we’ll be able to reduce refcount changes. Maybe 
not.

> The Blink document (linked in the bugzilla bug) suggests they identified some 
> mitigations, but I am not clear they will be representative for us today.

It’s not obvious to me that what we’re doing and what Blink are doing are 
analogous. One difference is that we use the String class in our JavaScript 
engine. Another difference is that we have a no regression policy on benchmarks.

Thanks,
Geoff

> 
> Putting on my old bindings hat, one area that might be able to get some wins 
> here is how Strings (and other ref counted things, but let’s stay focused) 
> are passed from the JS bindings layer to the DOM. In most cases Strings 
> should be able to be moved into the DOM without refcount churn, but since 
> many of our functions take `const String&` or `const AtomString&` we lose out.
> 
> - Sam
> 
>> 
>> 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?
>> 
>> Thanks,
>> Geoff
>> 
>>> On Dec 1, 2020, at 9:09 AM, Chris Lord via webkit-dev 
>>>  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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>> 
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Re: [webkit-dev] Making sure the behavior of updating duration of HTMLMediaElement and MediaSource

[Kimoto, Yousuke (SIE) via webkit-dev]

Hi Peng,

Thank you for your reply.

Let me confirm what I understand about the webkit behavior of "duration update":

https://www.w3.org/TR/html52/semantics-embedded-content.html#offsets-into-the-media-resource:
>When the length of the media resource changes to a known value (e.g., from 
>being unknown to known,
>or from a previously established length to a new length) the user agent must 
>queue a task to fire
>a simple event named durationchange at the media element. (The event is not 
>fired when the duration
>is reset as part of loading a new media resource.) If the duration is changed 
>such that the current
>playback position ends up being greater than the time of the end of the media 
>resource,
>then the user agent must also seek to the time of the end of the media 
>resource.

In this part, I understand that "the media data" means data which a MediaSource 
object has.

"duration" is set to HTMLMediaElemnt if media data is available. In my example, 
MediaSource and HTMLMediaElement are just initialized; No data is set. -- No 
media data is available. So MediaSource.duration is updated with "a new 
duration" and the new duration is set to HTMLMediaElement.duration but 
HTMLMediaElement.duration is not updated with the value.
Is my understanding correct?

Best Regards,
Yousuke Kimoto


-Original Message-
From: Peng (WebKit) Liu  
Sent: Tuesday, December 8, 2020 4:18 AM
To: Kimoto, Yousuke (SIE) 
Cc: webkit-dev@lists.webkit.org
Subject: Re: [webkit-dev] Making sure the behavior of updating duration of 
HTMLMediaElement and MediaSource

Hi Yousuke,

I think WebKit’s behavior matches the spec.
The HTML5 spec says: “If no media data is available, then the attributes must 
return the Not-a-Number (NaN) value.” On the test page, there is no data 
appended to the media source, so there is no media data available for the video 
element.


Best regards
Peng

> On Dec 6, 2020, at 10:48 PM, Kimoto, Yousuke (SIE) via webkit-dev 
>  wrote:
> 
> Hi all,
> 
> I have a question about MediaSource's duration.
> 
> What is an expected result of "duration" after processing the following steps?
> 1) Creating HTMLMediaElement object.
> 2) Creating MediaSource object, which is set to the HTMLMediaElement of step 
> 1)
> 3) Updating MediaSource.duration with some numbers. (e.g. 10, 100 etc)
> 4) What does "duration" return?
> 
> The table below shows "duration" of HTMLMediaElement and MediaSource on web 
> browsers with the attached sample, and the results are different.
> (NOTE: the sample at the bottom of this mail is made as a part of LayoutTest 
> media-source.)
> 
>   |  HTMLMediaElement |  MediaSource |
> Chrome:   updated |updated   |
> Safari:NaN|updated   |
>GTK:NaN|updated   | *) MiniBrowser (GTK)
> Firefox: - |   -  | *) no durationchange event 
> happened.
> 
> Points:
> - Safari and GTK are the same results, it's natural because they use the same 
> implementation of "duration".
>   It doesn't update HMLMediaElement.duration because the HTMLMediaElement 
> object's readyState is HAVE_NOTHING.
> - Chrome updates HTMLMediaElement.duration and MediaSource.duration.
> - Firefox doesn't fire a "durationchange" event.
> 
> Could anyone can explain which behavior matches the W3C standards?
> 
> 
> 
> Reference:
> https://www.w3.org/TR/media-source/#duration-change-algorithm
> https://www.w3.org/TR/html51/semantics-embedded-content.html#durationChange
> 
> 
> Sample:
> 
> 
> 
>media-source-set-duration-before-append.html
>
>
>var source;
>function runTest() {
>findMediaElement();
>source = new MediaSource();
>waitForEventOn(source, 'sourceopen', sourceOpen);
>run('video.src = URL.createObjectURL(source)');
>}
>function sourceOpen() {
>waitForEventOn(video, 'durationchange', durationChange);
>run('source.duration = 10.0');
>}
> 
>function durationChange() {
>testExpected('source.duration', 10.0);
>testExpected('video.duration', 10.0);
>endTest();
>}
>
> 
> 
>
> 
> 
> 
> 
> Best Regards,
> 
> --
> Yousuke Kimoto
> ___
> webkit-dev mailing list
> webkit-dev@lists.webkit.org
> https://lists.webkit.org/mailman/listinfo/webkit-dev

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