Re: Reprioritization - implementation intent

+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
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'

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 <>:
> (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

Received on Tuesday, 21 July 2020 19:07:07 UTC