Re: Reprioritization - implementation intent

Yoav Weiss <> Tue, 28 July 2020 06:45 UTC

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From: Yoav Weiss <>
Date: Tue, 28 Jul 2020 08:42:06 +0200
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To: Eric Kinnear <>, Ian Swett <>, =?UTF-8?Q?Bence_B=C3=A9ky?= <>, David Schinazi <>
Cc: Mark Nottingham <>, HTTP Working Group <>, Lucas Pardue <>
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Subject: Re: Reprioritization - implementation intent
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I ran the above by +David Schinazi <>om>, who better
represents the Chrome networking team than me. His reply is: "Chrome will
closely follow proposals related to HTTP/3 reprioritization, and is very
likely to implement a proposal that gets consensus in the IETF QUIC Working

On Tue, Jul 21, 2020 at 9:06 PM Yoav Weiss <> wrote:

> +Ian Swett <> +Bence Béky <>
> Google's Chrome and QUIC teams similarly intend to implement and support
> reprioritization, for similar use-cases.
> Upgrading images that are in or approaching the viewport as well as
> downgrading the priority of large downloads both seem like important use
> cases.
> Video streaming use-cases where either quality-tier change or user actions
> result in download changes also seem worthwhile (although I'm not sure if
> cancellation can't handle some of those).
> Another use-case I heard from folks is that of JS reprioritization as a
> result of user-actions: scripts that large apps want to download in
> low-priority can become critical as a result of a user-action that needs
> them. Being able to reprioritize can significantly impact such apps'
> responsiveness.
> On Tue, Jul 21, 2020 at 7:59 PM Eric Kinnear <> wrote:
>> Hi all,
>> We (both in Safari and in URLSession for general HTTP usage on Apple
>> platforms) are quite excited about the new priorities document and the
>> opportunities it brings to simplify and focus on information that’s
>> strictly necessary to communicate between the client, server, and
>> intermediaries.
>> Specific to reprioritization, we have several cases where we use, and so
>> far believe we need to continue to use, reprioritization (although ideas on
>> workarounds are always welcome!).
>> First, the Web download case that’s been discussed (and thanks Patrick
>> for running some related experiments for web traffic!), where we use
>> reprioritization to modify the incremental bit on resources.
>> Second, when streaming HLS video, we prioritize the currently playing
>> tier above the other tiers. We may have several requests outstanding for
>> the next several segments of video, and when we switch up/down we need to
>> be able to reprioritize those alternate tiers. Unfortunately, so far it’s
>> looking like not being able to reprioritize these requests would prevent
>> our implementation of the new priority scheme. For Low-Latency HLS, we
>> certainly will need to use reprioritization if we’re to fit within the
>> currently proposed priority tiers.
>> Finally, a more generalized example. As we work to help customers and
>> clients of the APIs we offer, we’ve found that many of our efforts to guide
>> them towards appropriate prioritization of less important work at lower
>> priorities is only enabled by the ability to raise that priority later when
>> circumstances change.
>> As a very contrived (but unfortunately close to real world) example,
>> consider a case where we ask a client to de-prioritize loading of images in
>> a list view that aren’t close to being scrolled into view by the user. If
>> we can offer higher priority for those images once the user starts
>> scrolling closer to having those items come into view, our clients are
>> generally happy to initially load such images at lower priorities. However,
>> if they’re stuck with that initial priority forever, they end up loading
>> the entire set of images at a high priority *just in case* they might be
>> eventually blocking render. A good bit of the time, that never happens, so
>> we end up having everything at high priority when in reality we would
>> rarely have needed to reprioritize the requests. And once everything’s at
>> high priority, we no longer have the utility of the priority system at all.
>> There are all sorts of ways to dissect that particular example, but the
>> general response we’ve seen remains: folks are much more willing to fully
>> utilize a prioritization system in the real world if they’re able to adjust
>> the priorities that they assigned later on when they have more information
>> or the circumstances change.
>> Thanks,
>> Eric
>> Side note:
>> For the document as a whole, we’ve gotten some feedback internally that
>> it would be really nice if there were some (minimal, recommendation only)
>> guidance as to how to respond to the priority signals when received. This
>> wouldn’t be restrictive, as we’re really excited to experiment here and see
>> what awesome results we can achieve, but having a baseline of “implement
>> this as written and you’ll do *okay” *might be worth considering to
>> increase the likelihood that we have a large group of generally-performant
>> implementations.
>> An example here would be if two requests of the same urgency arrive
>> back-to-back, the first with the incremental bit set and the second
>> without. What gets sent when? What do you do next if a third request
>> arrives with the incremental bit also set before the first is complete?
>> There are lots and lots of permutations, but a general approach of handling
>> new items coming in is something that I think we’ve all been imagining
>> during discussions, but we haven’t really written it down explicitly.
>> Internally, as we discussed with some folks new to the topic, we discovered
>> that our imaginations of what to do in cases like these didn’t actually
>> align as well as we thought.
>> On Jul 9, 2020, at 11:46 PM, Mark Nottingham <> wrote:
>> All,
>> Thanks to everyone for their efforts so far.
>> There's one other aspect that the we think it'd be helpful to get a sense
>> of -- what the implementer intent is regarding reprioritisation.
>> In particular, it'd be very helpful to have an indication from each
>> implementation -- in user agents as well as servers (including
>> intermediaries) -- as to how likely they are to produce/consume
>> reprioritisations if specified.
>> Note that's per-implementation, *not* per-person, so please coordinate if
>> your implementation has multiple participants here.
>> Responding to this e-mail is fine.
>> Cheers,
>> On 7 Jul 2020, at 7:50 am, Lucas Pardue <>
>> wrote:
>> Hi Patrick,
>> Thanks for running this experiment and presenting the data back to the
>> group.
>> Also thanks to the Chrome folk for enabling the disabling flag.
>> Cheers
>> Lucas
>> 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
>> --
>> Mark Nottingham