Re: Extensible Priorities and Reprioritization

Lucas Pardue <> Mon, 06 July 2020 21:54 UTC

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From: Lucas Pardue <>
Date: Mon, 6 Jul 2020 22:50:56 +0100
Message-ID: <>
To: Patrick Meenan <>
Cc: Yoav Weiss <>, Kazuho Oku <>, Kinuko Yasuda <>, =?UTF-8?Q?Bence_B=C3=A9ky?= <>, Eric Kinnear <>, Patrick Meenan <>, HTTP Working Group <>
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Subject: Re: Extensible Priorities and Reprioritization
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Hi Patrick,

Thanks for running this experiment and presenting the data back to the

Also thanks to the Chrome folk for enabling the disabling flag.


On Mon, 6 Jul 2020, 21:19 Patrick Meenan, <> wrote:

> Sorry about the delay, just gathered the results.  The full raw results
> are here
> <>.
> 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:
> - (Filmstrip
> <>) -
> The main background image in the interstitial loads at < 10s vs 90s without
> reprioritization
> - (Filmstrip
> <>)
> - 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.
> - (Filmstrip
> <>)
> - Another hero background image (detecting a theme?) loads at 10s vs 60s
> Looking at a few of the bigger DCL regressions:
> - (Filmstrip
> <>)
> - 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 <>
> 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 <> wrote:
>>> On Wed, Jun 17, 2020 at 9:55 AM Kazuho Oku <> wrote:
>>>> 2020年6月11日(木) 6:46 Kinuko Yasuda <>rg>:
>>>>> (Sorry, sent it too soon...)
>>>>> On Thu, Jun 11, 2020 at 6:12 AM Kinuko Yasuda <>
>>>>> 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
>>> <> to
>>> turn off Chromium's H2 request prioritization. I believe +Pat Meenan
>>> <> 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 <
>>>>>>> wrote:
>>>>>>> On Tue, Jun 9, 2020 at 4:27 PM Patrick Meenan <>
>>>>>>> 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