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Re: Priority straw man

From: 陈智昌 <willchan@chromium.org>
Date: Fri, 7 Feb 2014 18:32:27 -0800
Message-ID: <CAA4WUYgNNcZcuJPkjfQWa_63BZCXvwEUxAi=KVLeOWCF0fLw2A@mail.gmail.com>
To: Jeff Pinner <jpinner@twitter.com>
Cc: Roberto Peon <grmocg@gmail.com>, Martin Thomson <martin.thomson@gmail.com>, Peter Lepeska <bizzbyster@gmail.com>, Tatsuhiro Tsujikawa <tatsuhiro.t@gmail.com>, Michael Sweet <msweet@apple.com>, Osama Mazahir <OSAMAM@microsoft.com>, HTTP Working Group <ietf-http-wg@w3.org>
Jeff, do you mind clarifying your position? I think we can infer it
given your sentence structure, but it'd be nice if you explicitly
said, between the simplified state management and removing the need to
rely on heuristics, which one you valued more. I think everyone can
appreciate the tradeoff we need to make here, and it's important to
identify which one people find most important. I for one think the
richer protocol semantics are worth the implementation burden.

It's somewhat interesting to hear Roberto's description of proxies'
need of stream dependencies, as it conflicts with Microsoft's
description of the situation, and they both operate proxies. I say
only somewhat, because Roberto has previously voiced similar concerns
on many occasions. My understanding of Roberto's concerns is because
proxies take many many many users' flows and multiplex them over the
same connections to a gateway's backends, richer protocol semantics
between the gateway and the backends would help out significantly.
It'd be interesting to hear Microsoft's discussion of the same issues
and why they believe that a simpler protocol with simpler semantics is
overall less complexity. Do they simply have different uses? My
understanding is that they also encounter these gateway concerns (not
to mention forward proxy issues), so it's intriguing to see their
evaluation of these protocol mechanisms. I'm surprised to see so clear
a signal that they're so willing to accept implementation heuristic
complexity in order to have simpler protocol mechanics.

Martin, as far as the video scenario goes, I think it happens,
although not necessarily that often. Seeking is very common, but I
suspect that it wouldn't significantly affect the user experience if,
upon a seek, the user agent simply cancelled previous resource
requests and initiated new ones. That said, I think that video seeking
is just one example of a common trend of more dynamic loading and
reprioritization. I may very possibly be wrong here, but my instinct
is that this is the direction that HTTP usage is moving towards, and
it would be rather short-sighted to assume ignore such dynamic and
real-time request prioritization / reprioritization desires, which in
general would be better suited by direct protocol semantic
equivalents, rather than heuristic translations.

Also, as far as Herve's suggestion for insertion, I don't have a
concrete opinion yet. I personally view that as within the level of
marginal benefit/cost that it's not worth fighting much over. I'm more
interested in discussing the dependencies vs priority levels issue.

Cheers! Happy weekend y'all.

On Fri, Feb 7, 2014 at 4:07 PM, Jeff Pinner <jpinner@twitter.com> wrote:
> I like the simplified state management with Osamas proposal, but I worry
> that removing dependencies causes us to need to fall back to heuristics to
> achieve the same effect, and that those will be much harder to "get right."
> And should there be more stream dependency links than priority levels than
> instead of simplifying the state management you simply move it to the client
> as it has to reprioritize as streams close to achieve the same effect.
>
>
> On Fri, Feb 7, 2014 at 3:42 PM, Roberto Peon <grmocg@gmail.com> wrote:
>>
>> The usecase seems likely to me... ;)
>>
>>
>> Given experience with SPDY, I'm strongly convinced that we need to think
>> of this from the perspective of the proxy, which would be forced to do
>> substantially suboptimal things with any proposal which doesn't offer a
>> 'linkable' ordering (i.e. representable as operations to a list or tree)
>> with multiple roots.
>>
>> This problem can become painful if we stick to "simple" numeric
>> priorities-- Imho, we very much trade off some minor simplicity in the
>> specification for mountains of machinery.
>> I'd be perfectly happy to see some clients ignore whatever they believe to
>> be complicated and stick to a strict-priority scheme (which the proposal
>> from the interim supported) and allow proxies to utilize the mechanism to
>> solve the very real problem there.
>>
>> Remember that this proposal was borne out of the pain we experienced when
>> deploying a proxy: it is not theoretical.
>> -=R
>>
>>
>> On Fri, Feb 7, 2014 at 3:30 PM, Martin Thomson <martin.thomson@gmail.com>
>> wrote:
>>>
>>> Just to clarify:
>>>
>>> what I wrote up (stream dependencies) > osama's simplification
>>> (priority instead of depedencies) >> the crap in -09
>>>
>>> And maybe I can infer also:
>>>
>>> something with better insertion, like Herve's suggestion > what I wrote
>>> up
>>>
>>> ---
>>>
>>> How realistic do you imagine this video scenario to be?  Seeking
>>> hurts, but that's enough of a specialized scenario that I think I'm
>>> willing to suggest that it's going to need application-specific
>>> interactions to get "right" anyway.
>>>
>>>
>>> On 7 February 2014 14:46, William Chan (陈智昌) <willchan@chromium.org>
>>> wrote:
>>> > Thanks for the proposal Osama!
>>> >
>>> > Overall, I like this proposal. It incorporates a lot of our feedback
>>> > about the problems we've identified with the existing prioritization
>>> > mechanism. It is simpler in terms of protocol understanding, although
>>> > it has some other complexity costs, namely in usage, at least on the
>>> > client. I am hesitant to claim that this proposal is strictly better
>>> > or worse than our stream dependencies proposal, because I see some of
>>> > the appeal of keeping it simple. Let me point out some downsides and
>>> > things to consider:
>>> >
>>> > * From a client perspective, when you only have priority levels, it
>>> > becomes harder to decide how to allocate the priority level ranges.
>>> > During dynamic request allocation (for example, during web document
>>> > parsing), it's common to want to insert a request at a priority level
>>> > in between two other requests. document.write() is an obvious example
>>> > of this. And let's be clear, this is extremely common in web
>>> > documents, and even more common with web apps where much more of the
>>> > loading occurs dynamically using scripts. A dependency list makes this
>>> > elegant since you simply need a list insertion operation. With integer
>>> > priority levels, now you have to pre-plan the integer ranges and leave
>>> > spaces in between to allow insertion of stream priorities in between
>>> > different streams. And it's unclear where to insert it...halfway in
>>> > the priority range between two streams? When we get it wrong, we may
>>> > need to blast out a lot of PRIORITY frames to correct this.
>>> > * Also from a client perspective, how do you describe a pipeline of
>>> > ordered resources? Video frames are the obvious example. You want
>>> > frame 1 before frame 2 before etc.
>>> > * Let's now make it more complicated. What happens when you fast
>>> > forward or rewind in the video? What happens when you scroll within a
>>> > loading document? How do you dynamically reprioritize large numbers of
>>> > streams?
>>> > * What's WinInet's API going to look like? Is it going to punt
>>> > knowledge/responsibility of HTTP/2 priority levels out to the
>>> > application, and let the application decide how to heuristically
>>> > assign these priorities?
>>> >
>>> > By keeping the protocol semantics simple, we impose more burden on
>>> > application developers to utilize heuristics to assign priorities. Is
>>> > it the right tradeoff?
>>> >
>>> > I think that HTTP/2 removes the need for many browser/client-side
>>> > heuristics which is a great thing. I'd like to remove more of them. I
>>> > think that this new proposal is a step in the right direction, but it
>>> > doesn't go as far as I'd like. But I wholeheartedly approve of it as
>>> > an improvement over what currently exists in the spec.
>>> >
>>> > Cheers.
>>> >
>>> > On Thu, Feb 6, 2014 at 8:44 AM, Peter Lepeska <bizzbyster@gmail.com>
>>> > wrote:
>>> >> I like the way that the dependencies give us additional information
>>> >> about
>>> >> the way that the page is constructed. This information can be useful
>>> >> for
>>> >> various applications I can think of. However, if this is our goal:
>>> >>
>>> >> "The purpose of prioritization is to allow an endpoint to express how
>>> >> it
>>> >> would prefer its peer allocate resources when managing concurrent
>>> >> streams.
>>> >> Most importantly, priority can be used to select streams for
>>> >> transmitting
>>> >> frames when there is limited capacity for sending."
>>> >>
>>> >> Then I can't think of anything that the dependency-based approaches
>>> >> allows
>>> >> us to do that cannot also be accomplished via Osama's proposal and I
>>> >> agree
>>> >> with the five benefits he outlines.
>>> >>
>>> >> Can anyone else think of a benefit to the dependency-based approach
>>> >> from the
>>> >> perspective of allowing an endpoint to express how it would prefer its
>>> >> peers
>>> >> to allocate network resources?
>>> >>
>>> >> Thanks,
>>> >>
>>> >> Peter
>>> >>
>>> >>
>>> >> On Thu, Feb 6, 2014 at 8:59 AM, Tatsuhiro Tsujikawa
>>> >> <tatsuhiro.t@gmail.com>
>>> >> wrote:
>>> >>>
>>> >>>
>>> >>> On Feb 4, 2014, at 3:16 PM, Osama Mazahir <OSAMAM@microsoft.com>
>>> >>> wrote:
>>> >>>
>>> >>> > Priority is definitely useful when you have more DATA to send than
>>> >>> > network capacity.  However, Microsoft has no interest in
>>> >>> > implementing
>>> >>> > dependencies; neither advertising nor honoring across IE, IIS,
>>> >>> > client stack
>>> >>> > APIs and server stack APIs.
>>> >>> >
>>> >>> > Being able to express an ordering relation is valuable but the
>>> >>> > approach
>>> >>> > should be more decoupled from the stream lifecycle and more
>>> >>> > stateless on the
>>> >>> > server.  Below is our feedback and suggested changes.  The
>>> >>> > one-liner
>>> >>> > explanation of the change is: replace the Stream Dependency field
>>> >>> > with a
>>> >>> > numerical priority field.
>>> >>> >
>>> >>>
>>> >>> +1
>>> >>>
>>> >>> I prefer Osama's proposal. Without relying on "ghost" streams is a
>>> >>> good
>>> >>> property.
>>> >>> The complexity of dependency is only paid by the client that wants it
>>> >>> and
>>> >>> server just uses the numerical value. Also the simple browser which
>>> >>> uses
>>> >>> static priority based on content type works fine without additional
>>> >>> priority
>>> >>> adjustment which described in
>>> >>> http://lists.w3.org/Archives/Public/ietf-http-wg/2014JanMar/0415.html
>>> >>>
>>> >>> Best regards,
>>> >>> Tatsuhiro Tsujikawa
>>> >>>
>>> >>>
>>> >>> On Wed, Feb 5, 2014 at 5:55 AM, Michael Sweet <msweet@apple.com>
>>> >>> wrote:
>>> >>>>
>>> >>>> +1
>>> >>>>
>>> >>>> Seems like this will work for both client->server and
>>> >>>> client(s)->proxy->server(s) without too much effort in the proxy,
>>> >>>> and it
>>> >>>> gives us a simple method of grouping priorities to prevent
>>> >>>> starvation.
>>> >>>>
>>> >>>>
>>> >>>> On Feb 4, 2014, at 3:16 PM, Osama Mazahir <OSAMAM@microsoft.com>
>>> >>>> wrote:
>>> >>>>
>>> >>>> > Priority is definitely useful when you have more DATA to send than
>>> >>>> > network capacity.  However, Microsoft has no interest in
>>> >>>> > implementing
>>> >>>> > dependencies; neither advertising nor honoring across IE, IIS,
>>> >>>> > client stack
>>> >>>> > APIs and server stack APIs.
>>> >>>> >
>>> >>>> > Being able to express an ordering relation is valuable but the
>>> >>>> > approach
>>> >>>> > should be more decoupled from the stream lifecycle and more
>>> >>>> > stateless on the
>>> >>>> > server.  Below is our feedback and suggested changes.  The
>>> >>>> > one-liner
>>> >>>> > explanation of the change is: replace the Stream Dependency field
>>> >>>> > with a
>>> >>>> > numerical priority field.
>>> >>>> >
>>> >>>> > Certain details, such as the bit widths (X, Y and Z), are
>>> >>>> > purposefully
>>> >>>> > omitted to keep the focus on design.
>>> >>>> >
>>> >>>> > ==== OVERVIEW
>>> >>>> >
>>> >>>> > - A stream is assigned to a group
>>> >>>> > - A stream is assigned a priority that represents its importance,
>>> >>>> > with
>>> >>>> > respect to other streams, within its group
>>> >>>> > - A group is identified by a group ID
>>> >>>> > - A group is assigned a weight that represents its importance,
>>> >>>> > with
>>> >>>> > respect to other groups, within its connection
>>> >>>> >
>>> >>>> > The client knows the relative importance of streams and is trying
>>> >>>> > to
>>> >>>> > convey a simple representation of that information to the server
>>> >>>> > so that the
>>> >>>> > server can choose to allocate resources more intelligently.
>>> >>>> > Priorities and
>>> >>>> > weights are merely advisory and do not guarantee any specific
>>> >>>> > server
>>> >>>> > behavior.
>>> >>>> >
>>> >>>> > Group weight defines a proportional definition of importance with
>>> >>>> > respect to other groups.  For example, if Group1, Group2 and
>>> >>>> > Group3 are
>>> >>>> > assigned weights 1, 3 and 6, then ideally they receives 10%, 30%
>>> >>>> > and 60% of
>>> >>>> > the resources, respectively.
>>> >>>> >
>>> >>>> > Within a group, stream priority defines a strict ordering of
>>> >>>> > importance.  If two streams have the same priority then their
>>> >>>> > resource
>>> >>>> > assignment is server implementation specific (e.g. round robin,
>>> >>>> > shortest job
>>> >>>> > next, first in first out, etc).
>>> >>>> >
>>> >>>> > The PRIORITY frame payload (the same payload can also be included
>>> >>>> > as
>>> >>>> > part of the HEADERS frame) contains:
>>> >>>> > - Group ID        (X bits)
>>> >>>> > - Group Weight    (Y bits)
>>> >>>> > - Stream Priority (Z bits)
>>> >>>> >
>>> >>>> > Similar to what it does currently, the payload assigns the given
>>> >>>> > stream
>>> >>>> > (from the encapsulating frame header) to the specified Group ID,
>>> >>>> > sets the
>>> >>>> > given Group Weight and assigns the stream the given Stream
>>> >>>> > Priority within
>>> >>>> > that group.
>>> >>>> >
>>> >>>> >
>>> >>>> > ==== MOTIVATION
>>> >>>> >
>>> >>>> > 1) Robustness
>>> >>>> > This avoids the server from having to track relationship
>>> >>>> > dependencies
>>> >>>> > between streams.  Consequently, the design race where the a stream
>>> >>>> > will
>>> >>>> > close while prioritization information that creates a dependency
>>> >>>> > on that
>>> >>>> > stream is in transit is eliminated.  Which also eliminates having
>>> >>>> > to
>>> >>>> > maintain state about closed streams after closure (and all
>>> >>>> > subsequent timer
>>> >>>> > or load based purging).  Increasing the decoupling from the stream
>>> >>>> > lifecycle
>>> >>>> > is design goodness.
>>> >>>> >
>>> >>>> > 2) Changing a stream priority is clearer
>>> >>>> > In the case where a stream has to be reprioritized, all its
>>> >>>> > children
>>> >>>> > need to be reprioritized to their new parent via new dependency
>>> >>>> > advertisements.  With simple priority numbers, this becomes a
>>> >>>> > simple one
>>> >>>> > priority change operation.  See
>>> >>>> >
>>> >>>> > http://lists.w3.org/Archives/Public/ietf-http-wg/2014JanMar/0395.html
>>> >>>> >
>>> >>>> > 3) Simplicity
>>> >>>> > Simpler to describe.  Simpler for basic server and proxies to
>>> >>>> > implement.  Don't pay for complexity in the base cases.  The
>>> >>>> > client is free
>>> >>>> > to have arbitrarily complex prioritization determination schemes
>>> >>>> > (e.g. not
>>> >>>> > necessarily tied to rendering tree) and can afford to spend
>>> >>>> > CPU/memory/etc
>>> >>>> > on this.
>>> >>>> >
>>> >>>> > 4) Equivalency
>>> >>>> > The tree examples used in the current issue #364 proposal can be
>>> >>>> > represented using this weight+priority scheme.  Having Z bits
>>> >>>> > (i.e. the
>>> >>>> > Stream Priority width) will allow us to represent a tree of depth
>>> >>>> > 2^Z.
>>> >>>> >
>>> >>>> > 5) Simple Priority Collapsing
>>> >>>> > Proxies, and perhaps servers, may want to limit the amount of
>>> >>>> > concurrent priorities/depth they track.  Using numerical
>>> >>>> > priorities opens
>>> >>>> > the ability to have a stateless reduction/mapping.  For example, a
>>> >>>> > proxy/server might do something like: internal_priority =
>>> >>>> > wire_priority >>
>>> >>>> > 3.
>>> >>>> >
>>> >>>> >
>>> >>>> > -----Original Message-----
>>> >>>> > From: Martin Thomson [mailto:martin.thomson@gmail.com]
>>> >>>> > Sent: Monday, February 3, 2014 9:45 AM
>>> >>>> > To: HTTP Working Group
>>> >>>> > Subject: Re: Priority straw man
>>> >>>> >
>>> >>>> > I'm starting to get ready for -10, which I would very much like to
>>> >>>> > push
>>> >>>> > soon.
>>> >>>> >
>>> >>>> > More feedback would be nice, but absent stronger feedback, I'm
>>> >>>> > going to
>>> >>>> > push the button on this for -10.
>>> >>>> >
>>> >>>> > p.s., The suggestion for PRIORITY on stream zero requires
>>> >>>> > additional
>>> >>>> > changes and a little more support, so I'll ask for that to be
>>> >>>> > tracked
>>> >>>> > separately.
>>> >>>> >
>>> >>>> > On 26 January 2014 13:34, Martin Thomson
>>> >>>> > <martin.thomson@gmail.com>
>>> >>>> > wrote:
>>> >>>> >> As requested, a writeup on prioritization.
>>> >>>> >>
>>> >>>> >> As a pull request: https://github.com/http2/http2-spec/pull/364
>>> >>>> >>
>>> >>>> >> In HTML form:
>>> >>>> >>
>>> >>>> >> http://martinthomson.github.io/drafts/priority.html#StreamPriority
>>> >>>> >> http://martinthomson.github.io/drafts/priority.html#PRIORITY
>>> >>>> >>
>>> >>>> >> Comments or pull requests happily accepted.
>>> >>>> >
>>> >>>>
>>> >>>> _________________________________________________________
>>> >>>> Michael Sweet, Senior Printing System Engineer, PWG Chair
>>> >>>>
>>> >>>
>>> >>
>>>
>>
>
Received on Saturday, 8 February 2014 02:32:55 UTC

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