Re: Extensible Priorities and Reprioritization

Patrick Meenan <patmeenan@gmail.com> Mon, 06 July 2020 20:22 UTC

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From: Patrick Meenan <patmeenan@gmail.com>
Date: Mon, 6 Jul 2020 16:19:15 -0400
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To: Yoav Weiss <yoav@yoav.ws>
Cc: Kazuho Oku <kazuhooku@gmail.com>, Kinuko Yasuda <kinuko@chromium.org>, Lucas Pardue <lucaspardue.24.7@gmail.com>, =?UTF-8?Q?Bence_B=C3=A9ky?= <bnc@chromium.org>, Eric Kinnear <ekinnear@apple.com>, Patrick Meenan <pmeenan@webpagetest.org>, HTTP Working Group <ietf-http-wg@w3.org>
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Subject: Re: Extensible Priorities and Reprioritization
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Sorry about the delay, just gathered the results.  The full raw results are
here
<https://docs.google.com/spreadsheets/d/14iyeO--I-K-l7er1kGuW-iTKogsgXFz4Z3M-aowSUdI/edit?usp=sharing>.
It looks like the impact dropped quite a bit across the full 25k URLs but
looking at individual tests the impact is quite dramatic when it does
impact (and it does exactly what we'd expect it to do for those outlier
cases).

The 95th percentile numbers tend to be the more interesting ones and in the
data set, reprioritization enabled is the control and disabled is the
experiment so positive changes means disabling reprioritization is that
much slower.

Largest Contentful Paint: 4% slower without reprioritization
Speed Index: 2.75% slower without reprioritization
Dom Content Loaded: 1.3% faster without reprioritization

This is pretty much (directionally) what we'd expect since reprioritization
boosts the priority of visible images (LPC/Speed Index) above late-body
scripts (DCL). It's particularly dramatic for pages that use background
images for any part of the page because they are discovered after all other
resources and would normally be scheduled after all other scripts and
inline images but if they are visible in the viewport the reprioritization
helps them load much sooner.

Looking at a few examples of the extreme cases:

https://www.thehelm.co/ - (Filmstrip
<https://www.webpagetest.org/video/compare.php?tests=200625_MT_32af039543326a2bdb5d87e2adb95fe7-r%3A3-l%3AStock%2C200701_HY_6bb4d26adff32186e991c5b96ffaecea-r%3A1-l%3ADisabled%2C&thumbSize=200&ival=5000&end=full>)
-
The main background image in the interstitial loads at < 10s vs 90s without
reprioritization
https://blog.nerdfactory.ai/ - (Filmstrip
<https://www.webpagetest.org/video/compare.php?tests=200616_KZ_496553703935231e5725c252844918db-r%3A1-l%3AStock%2C200616_BJ_798e2417374c03dfa3995586b01444a3-r%3A3-l%3ADisabled%2C&thumbSize=200&ival=5000&end=full>)
- The background image for the main content loads at <5s vs 70s without
reprioritization. No cost to DCL, just prioritized ahead of not-visible
images.
https://events.nuix.com/ - (Filmstrip
<https://www.webpagetest.org/video/compare.php?tests=200628_1A_0e61aa9f59e08f3bbb8e5d9690fe64fb-r%3A3-l%3AStock%2C200628_Q2_1793a50022566cedd6ab48dd871d5c7e-r%3A3-l%3ADisabled%2C&thumbSize=200&ival=5000&end=full>)
- Another hero background image (detecting a theme?) loads at 10s vs 60s

Looking at a few of the bigger DCL regressions:

https://oaklandcitychurch.org/ - (Filmstrip
<https://www.webpagetest.org/video/compare.php?tests=200627_XR_fae3bd7aa57238591cb122c9fe634cb7-r%3A2-l%3AStock%2C200627_R9_26ad61b65965e7bf89a8aa27a7d78ff1-r%3A2-l%3ADisabled%2C&thumbSize=200&ival=5000&end=full>)
- DCL got much slower (11s -> 33s) as a direct result of the background
image moving from 30s to 10s (the pop-up interstitial was delayed along
with the scripts that control it).

For the specific case that most of these tests exposed (background image
discovered late by CSS) it is theoretically possible for Chrome to detect
the position before making the initial request (since it is only discovered
at layout anyway) but that wouldn't help any of the more dynamic cases like
when a user scrolls a page or a carousel rotates and what is on screen
changes dynamically.

I'm still of the pretty strong opinion that we need reprioritization but
the web won't necessarily break without it and sites (and browsers) may be
able to minimize the impact of not being able to reprioritize (though that
might involve holding back requests and prioritizing locally like Chrome
does for slow HTTP/2 connections).


On Sat, Jun 20, 2020 at 10:17 AM Patrick Meenan <patmeenan@gmail.com> wrote:

> An early read on Yoav's Canary test is that most metrics are neutral but
> Largest Contentful Paint degrades ~6.8% on average and 12% at the 95th
> percentile without reprioritization and Speed Index degrades 2.6% on
> average and 5.4% at the 95th percentile. This is not entirely unexpected
> because the main use case for reprioritization in Chrome right now is
> boosting the priority of visible images after layout is done.
>
> We'll see if it holds after the full test is complete. The early read is
> from 3,000 of the 25,000 URLs that we are testing (all https hosted on
> Fastly for simplicity since we know it handles HTTP/2 reprioritization
> correctly).  The tests are all run at "3G Fast" speeds with desktop pages
> to maximize the liklihood that there will be time for reprioritization to
> happen.  I'll provide the full raw data as well as summary results when the
> test is complete (at least another week, maybe 2).
>
> On Wed, Jun 17, 2020 at 5:43 AM Yoav Weiss <yoav@yoav.ws> wrote:
>
>>
>>
>> On Wed, Jun 17, 2020 at 9:55 AM Kazuho Oku <kazuhooku@gmail.com> wrote:
>>
>>>
>>>
>>> 2020年6月11日(木) 6:46 Kinuko Yasuda <kinuko@chromium.org>rg>:
>>>
>>>> (Sorry, sent it too soon...)
>>>>
>>>> On Thu, Jun 11, 2020 at 6:12 AM Kinuko Yasuda <kinuko@chromium.org>
>>>> wrote:
>>>>
>>>>> Hi all,
>>>>>
>>>>> Reg: reprioritization benefit I can share some recent data for
>>>>> Chrome.  For the two cases that are currently discussed I'm actually not
>>>>> fully sure about its benefit.
>>>>>
>>>>> For the renderer-triggered image reprioritization cases: this is a bit
>>>>> interesting one, we recently found two things:
>>>>> - Delaying to start low-prio requests could often work better (partly
>>>>> because of server-side handling) than re-prioritizing while inflight
>>>>> - In-lab measurements (tested with top 10k real sites, both on Mobile
>>>>> and Desktop) showed that removing in-flight re-prioritization doesn't
>>>>> impact page load performance a lot
>>>>>
>>>>
>>>> Let me stress though that testing this with servers that can properly
>>>> handle reprioritization could change the landscape, and again this isn't
>>>> really capturing how it affects long-lived request cases, or cases where
>>>> tabs go foreground & background while loading, so for now I'm not very
>>>> motivated to remove the reprioritization feature either.
>>>>
>>>
>>> Hi Kinuko,
>>>
>>> Thank you for sharing your data. I feel a bit sad that reprioritization
>>> isn't showing much benefit at the moment. I tend to agree that we are
>>> likely to see different results between server implementations and HTTP
>>> versions being used. The effectiveness of reprioritization depends on the
>>> depth of the send buffer (after prioritization decision is made), at least
>>> to certain extent.
>>>
>>
>> FWIW, I added a flag
>> <https://chromium-review.googlesource.com/c/chromium/src/+/2232923> to
>> turn off Chromium's H2 request prioritization. I believe +Pat Meenan
>> <patmeenan@gmail.com> is currently running tests with and without this
>> flag a list of servers we estimate is likely to handle them well.
>>
>>
>>>
>>>
>>>>
>>>>
>>>>> I suspect this is maybe because server-side handling is not always
>>>>> perfect and most of requests on the web are short-lived, and this may not
>>>>> be true for the cases where long-running requests matter.  I don't have
>>>>> data for whether may impact background / foreground cases (e.g. Chrome
>>>>> tries to lower priorities when tabs become background)
>>>>>
>>>>> For download cases, Chrome always starts a new download with a low
>>>>> priority (even if it has started as a navigation), so reprioritization
>>>>> doesn't happen.
>>>>>
>>>>> Kinuko
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Wed, Jun 10, 2020 at 1:21 AM Lucas Pardue <
>>>>> lucaspardue.24.7@gmail.com> wrote:
>>>>>
>>>>>> On Tue, Jun 9, 2020 at 4:27 PM Patrick Meenan <patmeenan@gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Eric's download example is a great one for exposing the risks that
>>>>>>> would come for an implementation that supported prioritization but not
>>>>>>> reprioritization.
>>>>>>>
>>>>>>> Take the trivial example of an anchor link that links to a download
>>>>>>> (say, a 200MB installer of some kind):
>>>>>>> - When the user clicks on the link, the browser assumes it is doing
>>>>>>> a navigation and issues the request with the "HTML" priority (relatively
>>>>>>> high, possibly non-incremental
>>>>>>> - When the response starts coming back, it has the
>>>>>>> content-disposition to download to a file.
>>>>>>> - At this point, the 200MB download will block every other
>>>>>>> lower-priority request on the same connection (or possibly navigation if it
>>>>>>> is non-incremental)
>>>>>>> - The user clicks on another page on the same site and gets nothing
>>>>>>> or a broken experience until the 200MB download completes
>>>>>>>
>>>>>>> Without reprioritization the browser will effectively have to burn
>>>>>>> the existing QUIC connection and issue any requests on a new connection
>>>>>>> (and repeat for each new download).
>>>>>>>
>>>>>>> Implementing prioritization without reprioritization in this case is
>>>>>>> worse than having no prioritization support at all.
>>>>>>>
>>>>>>
>>>>>> Thanks Eric for presenting this case, and Patrick for breaking it
>>>>>> down. That does seem like a pretty bad outcome.
>>>>>>
>>>>>> Is this a good candidate for a test case? IIUC correctly the problem
>>>>>> might occur today with HTTP/2 depending on how exclusive priorities are
>>>>>> used. I'm curious if browsers can share any more information about what
>>>>>> they do already. How does Firefox manage such a resource with it's priority
>>>>>> groups?
>>>>>>
>>>>>> Cheers
>>>>>> Lucas
>>>>>>
>>>>>>
>>>
>>> --
>>> Kazuho Oku
>>>
>>