Re: Proposal - Reduce HTTP2 frame length from 16 to 12 bits

Roberto Peon <grmocg@gmail.com> Tue, 28 May 2013 18:43 UTC

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Date: Tue, 28 May 2013 11:41:52 -0700
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From: Roberto Peon <grmocg@gmail.com>
To: Will Chan <willchan@chromium.org>
Cc: HTTP Working Group <ietf-http-wg@w3.org>, Patrick McManus <mcmanus@ducksong.com>, James M Snell <jasnell@gmail.com>
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Subject: Re: Proposal - Reduce HTTP2 frame length from 16 to 12 bits
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As a reverse proxy, I've seen properties for which 4k writes/reads were too
small and induced latency increases.

Admittedly, frame size doesn't have to be the same as read/write size, but
it certainly does encourage that implementation (which is, I think, the
point of smaller max frame size that you proposed).

I propose we keep the 16 bit frame size and instead allow the (now
negotiated setting of) max frame size to default to 12 bits worth, with
that going upwards out downwards when a settings frame arrives from the
other side indicating it's max receive size. HK

This would give the best chance that the code would be written in such a
way as to adapt with the times as they change.
-=R
On May 28, 2013 10:01 AM, "William Chan (陈智昌)" <willchan@chromium.org>
wrote:

> Can you clarify what you mean by a documented performance metric for
> non-browser use cases? I don't think Patrick said anything browser
> specific. He provided some serialization latency numbers and noted that
> they are high enough to impact responsiveness. And then he provided numbers
> on overhead.
>
> I, for one, find the responsiveness argument compelling for browsers. I'm
> not completely sure 0.2% is low enough overhead for everyone, but I
> wouldn't complain about it. And in absence of complaints, I guess I'd
> support moving forward with only 12 bits for length.
>
>
> On Tue, May 28, 2013 at 9:22 AM, James M Snell <jasnell@gmail.com> wrote:
>
>> Currently, my only challenge with this is that, so far, we have not
>> seen any documented performance metrics for non-browser based uses.
>> .That said, I don't really have the time currently to put together a
>> comprehensive set of such metrics so it wouldn't be polite of me to
>> insist on them ;-) ... perhaps for now we ought to keep the 16-bit
>> size but include a recommendation about not exceeding 12-bits, then
>> see what more implementation experience does for us.
>>
>> On Tue, May 28, 2013 at 7:20 AM, Patrick McManus <mcmanus@ducksong.com>
>> wrote:
>> > Hi All,
>> >
>> > I've been looking at a lot of spdy frames lately, and I've noticed what
>> I
>> > consider a common implementation problem that I think a good http/2 spec
>> > could help with. I'm commonly seeing frames large enough to interfere
>> with
>> > effective prioritization. I've seen this from at least 3 different
>> servers.
>> >
>> > The HTTP/2 draft has a max frame size of 16 bits, which is a huge
>> > improvement from spdy's 24. I propose we reduce it further to 12. (i.e.
>> 4096
>> > bytes).
>> >
>> > The muxxed approach of multiple streams onto one connection done in
>> HTTP/2
>> > has great advantages, but the one downside of it is that it creates
>> head of
>> > line blocking problems between those streams dictated by frame
>> granularity.
>> > With small frames this is pretty manageable, with extremely large ones
>> we've
>> > recreated the same head of line problems that HTTP/1 pipelines have. The
>> > server needs to  be able to respond quickly to higher priority events
>> > (including cancellations) and once it has written a frame header to the
>> wire
>> > it is committed to the entire frame for how ever long it takes to
>> serialize
>> > it. IMO the shorter that time, the better.
>> >
>> > Our spec can help implementations do the right thing here by limiting
>> the
>> > max frame size to 12 bits.
>> >
>> > It takes 500msec to serialize 64KB at 1Mbit/sec... 125msec at 4Mbit/sec.
>> > Those are some pretty notable task-switch times. Dropping the frame to
>> 4096
>> > cuts them to 32msec and 8 msec.. that's much more responsive, at the
>> cost of
>> > 120 extra bytes of transfer (< 1msec at 1Mbit/sec).
>> >
>> > In general - the smaller the better as long as the overhead doesn't get
>> to
>> > be too large. At 8 in 4096 (~.2%) I think that's acceptable. Its
>> roughly the
>> > same overhead as a VLAN tag.
>> >
>> > Obviously this makes a continuation bit for control frames absolutely
>> > mandatory, but I think we're already in that spot with 16 bit frame
>> lengths.
>> >
>> > -Patrick
>> >
>> >
>>
>>
>