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Re: per stream (initial) flow window size

From: Ilya Grigorik <ilya@igvita.com>
Date: Wed, 9 Oct 2013 23:26:26 -0700
Message-ID: <CAKRe7JGGpmXjGwPo7sG1c1wYqXA1Qn_vpRAcx-BDWzmD=CiW3w@mail.gmail.com>
To: Patrick McManus <mcmanus@ducksong.com>
Cc: "ietf-http-wg@w3.org Group" <ietf-http-wg@w3.org>
Interesting, thanks Patrick. Curious, when you say "small initial window",
what is the value that you're currently using? Looking at
https://hg.mozilla.org/try/rev/f77c2a53735e, it looks like its 64KB (aka,
kDefaultRwin)? Great to hear that [HEADERS, WINDOW_UPDATE] combo is working
out so far. Agreed, 8 bytes may not be much, but that's still 8 extra bytes
per stream. And if all the popular clients end up going with a strategy
like the one you've described, perhaps it's worth entertaining...

On Wed, Oct 9, 2013 at 10:21 PM, Patrick McManus <mcmanus@ducksong.com>wrote:

> Hey Ilya,
> So Firefox has been taking exactly this approach with both spdy/3.[+] and
> http/2 already. We use a small initial window setting that is meant to
> apply to pushes - and we reusme them in much the same manner as you
> describe. For pulls we want to use large windows so we simply pipeline a
> window update in the same SSL data record that contains the HEADER. While
> there is a theoretical race there it in effect can't actually be delayed
> because of the way SSL frames everything atomically; it hasn't ever been a
> problem and no servers have complained to me (yet) about the extra frame
> callback overhead. I actually expected to have some interop problems with
> this strategy in the early days - but it hasn't been a problem at all.
> So imo the use case is already well served by the draft as is.. what's
> left is whether saving 8 extra bytes for the pat/ilya use case is
> interesting and worth the complexity.. I'd be happy with it of course, but
> I don't realistically think it is necessary. ymmv.
> -P
> On Tue, Oct 8, 2013 at 4:14 PM, Ilya Grigorik <ilya@igvita.com> wrote:
>> SETTINGS allows us to set a default starting window for all streams +
>> update this default later. However, I think there are compelling cases
>> where it would be beneficial to allow different streams to start with a
>> different initial window size (similar to optional priority in headers
>> frame). Given that this would effectively be a new feature... let me try
>> to motivate why the extra complexity is worth it based on some of the
>> recent work I've been involved with:
>> We want to make browser pre{fetch,rendering} smarter [1,2]. Meaning, we
>> want to be able to discover critical page resources as early as possible
>> and kick off fetches for those assets also. However, since some of these
>> assets are large (and there can be many of them), we need to control how
>> much data we fetch (e.g. if we are too aggressive we may slow down the host
>> page, and/or incur high overhead for the user).
>> A concrete example of this is the work we've been experimenting with in
>> PageSpeed [3], where we are aggressively inlining CSS/JS and even doing
>> "page splits": the rewriter effectively creates two pages, the first is the
>> "prefetch friendly" version which fits in < 15KB (one RTT), and second is
>> the diff that fills in the rest once the navigation is triggered via an XHR
>> -- if that sounds crazy, then that's not too far from the truth. That said,
>> we have a working prototype, and it's showing impressive results.. For
>> example, we can take a mobile wikipedia page, which currently takes 9s
>> (!!!) to first render (on a 3G profile), down to 1.6s [3] by using the
>> combination of prefetch + page split strategies.
>> What does flow control have to do this with all this? When issuing a
>> prefetch, we would like to be able to open a request with a lower initial
>> window (e.g. 15KB), and fetch just the head of the document - in most
>> cases, this will allow us to discover the critical resources, initiate
>> requests for them, etc. Then, if the navigation is triggered, we would just
>> increment the window and "resume" the stream to get the rest of the page...
>> This eliminates the need for server rewriting / page splitting, which has a
>> lot of gnarly edge conditions. Handling this at the transport layer would
>> make it much, much simpler and more powerful.
>> (Yes, we could lower the initial window for all streams, but that's
>> counter productive in majority of cases... we'd just end up sending a lot
>> more WINDOW_UPDATE frames for non-prefetch request.)
>> Further, the ability to set a custom window size also allows us to adjust
>> this logic based on type of asset, prior knowledge of the site, or other
>> signals. For example, the other use case is progressive rendering of images
>> - i.e. using flow control to fetch image layers and having control over
>> where and when to stop. Concretely, we could fetch first X KB, which may
>> provide a reasonable low-res preview of the asset, and then continue
>> fetching subsequent layers until some condition is met (end of file, or max
>> resolution of device is reached -- if you're a 2x device, don't fetch the
>> 3x layers). I'm intentionally skipping over the image container
>> discussions here, since that's a separate conversation.. But I'll note that
>> many sites are already trying to provide this sort of experience, except
>> through rather poor implementation: inline low-res asset, render that, then
>> initiate a separate XHR to fetch image and replace with high-res version.
>> Needless to say, we can do much better, and there is strong interest in
>> providing this sort of functionality natively...
>> --
>> In short, there are interesting cases where the initiator of the stream
>> may want to control the initial window size: to decrease it, and perhaps
>> even to increase it in some cases. There are multiple ways to achieve this,
>> but one plausible strategy would be to allow an optional SETTINGS_INITIAL_WINDOW_SIZE
>> payload in the headers frame, not unlike the optional priority field --
>> minimal overhead, no races between initiating the request and
>> Thoughts?
>> Ilya
>> [1]
>> https://docs.google.com/document/d/1wck0tFTiibKzZDuBeyK0lrKTKsZ5pqfvcR__1r2m834/edit
>> [2] http://lists.w3.org/Archives/Public/public-web-perf/2013Aug/0010.html
>> [3] https://developers.google.com/speed/pagespeed/optimization
>> [4]
>> http://www.webpagetest.org/result/130715_PZ_08063384bd76cd2206a1b39e8678e438/3/details/
Received on Thursday, 10 October 2013 06:27:49 UTC

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