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Re: http/2 prioritization/fairness bug with proxies

From: 陈智昌 <willchan@chromium.org>
Date: Mon, 11 Feb 2013 14:46:31 -0800
Message-ID: <CAA4WUYg+evqEMdiYSv+EZ7668eCq_dwqKiYmA4Lq-xZkoD_9Fw@mail.gmail.com>
To: Martin Thomson <martin.thomson@gmail.com>
Cc: Mike Belshe <mike@belshe.com>, Poul-Henning Kamp <phk@phk.freebsd.dk>, Ben Niven-Jenkins <ben@niven-jenkins.co.uk>, Amos Jeffries <squid3@treenet.co.nz>, HTTP Working Group <ietf-http-wg@w3.org>
Theoretically possible is one thing. But the moment we get into the game of
trying to carve up portions of BDP via per-stream flow control windows for
prioritization purposes in normal operation (as opposed to just trying to
make reasonable progress under excessive load), I think we're in trouble,
and likely to get into performance issues due to poor implementations. As
I've stated before, I hope most implementations (and believe we should add
recommendations for this behavior) only use flow control (if they use it at
all, which hopefully they don't because it's hard) for maintaining
reasonable buffer sizes.


On Mon, Feb 11, 2013 at 2:39 PM, Martin Thomson <martin.thomson@gmail.com>wrote:

> Keep in mind that it is always possible for the intermediary to apply
> flow control to the infinite length stream so that, regardless of
> priority, it doesn't consume more than its fair share.
>
> On 10 February 2013 14:40, William Chan (陈智昌) <willchan@chromium.org>
> wrote:
> > First, I totally agree SPDY/4 prioritization changes are far more
> reaching.
> > Let's not talk about them yet. I share your complexity concern and agree
> we
> > need data before proceeding with many of those features, and I plan on
> > getting data.
> >
> > As far as grouping, I think it's definitely a bug for the case where you
> > have a forward proxy with users sharing the same HTTP/2.0 connection to
> an
> > origin server. You can have many high priority short-lived streams which
> can
> > starve out others, unless we implement the vague notion of "don't starve
> out
> > streams", which is difficult when it might be a sustained rate of high
> > priority short-lived streams. It's easier in your latter case of
> infinite,
> > high-priority streams.
> >
> > You're right that the high priority, infinite stream can starve streams
> > within the same group. I don't think this means that grouping is not
> > required, but that grouping is potentially insufficient. For intragroup
> > starvation, I think it's debatable about whether the server should be
> smart
> > and not allow streams to _completely_ starve other streams within a
> group,
> > or that clients should have a reprioritization facility. I think this is
> a
> > discussion worth having, but I'd personally classify it as separate from
> > whether or not the grouping feature is necessary.
> >
> > On Sun, Feb 10, 2013 at 2:06 PM, Mike Belshe <mike@belshe.com> wrote:
> >>
> >>
> >>
> >>
> >> On Mon, Feb 4, 2013 at 5:47 PM, William Chan (陈智昌) <
> willchan@chromium.org>
> >> wrote:
> >>>
> >>> I'm sorry if I am unclear in any way. Please continue to
> >>> challenge/question my comments/assertions so I can clarify my position
> >>> as appropriate.
> >>>
> >>> Just to be clear here, I stand by that it's a protocol bug currently.
> >>
> >>
> >> +0.5.  :-)
> >>
> >> Mark - I think we could open a issue ticket with the current HTTP/2.0
> >> draft that this is a bug which will present itself for servers that
> >> implement naively.  This isn't strictly a "bug", since the spec says the
> >> server can do whatever it wants with priorities, but this is subtle
> enough
> >> (surfacing primarily in http/2 -> http/2 proxy situations), that many
> server
> >> implementors won't think of it unless we mention it in the spec.
> >>
> >> The bare minimum would be to simply document it and tell servers not to
> >> starve out streams.  However, this is probably a wimpy approach and I
> think
> >> we can do better.
> >>
> >> The SPDY/4 prioritization changes are far more reaching than just fixing
> >> this bug.  While I like grouping as a feature, I don't believe it
> actually
> >> fixes this bug:  a browser could open a high priority, infinite stream,
> >> which is competing across a shared proxy backend with other streams;
> unless
> >> the user manually switches tabs (or does something to force changed
> groups),
> >> the starvation can still occur.
> >>
> >> Mike
> >>
> >>
> >>
> >>
> >>>
> >>> I agree with adding more hooks to convey advisory priority semantics.
> >>> That said, "advisory" is open to interpretation. I agree that the
> >>> sender should ultimately be in control of how it orders responses, and
> >>> indeed there are of course many situations where it's best for the
> >>> sender to ignore the advisory priority. Yet, if the advisory priority
> >>> semantics are generally not respected, then clients will not be able
> >>> to rely on them, and will be forced to implement prioritization at a
> >>> higher layer, which suffers from the link underutilization vs
> >>> contention tradeoff I highlighted earlier.
> >>>
> >>> I appreciate the concern that we're adding complexity by introducing
> >>> new semantics. I am arguing that because the existing mechanisms for
> >>> addressing starvation are suboptimal, we should treat this as a
> >>> protocol bug and thus change the protocol in such a way as to fix this
> >>> problem. My suggestion for doing so was adding new priority "grouping"
> >>> semantics. I am hopeful that these new semantics will not introduce an
> >>> inordinate amount of specification, as the primary idea is that the
> >>> current SPDY priority levels would apply within a "group". I think we
> >>> can come up with a way to define a group that will be relatively easy
> >>> to spec.
> >>>
> >>> SPDY/4 introduces other prioritization semantics beyond just grouping,
> >>> but I wanted to focus on this one first, as I believe this is a bug
> >>> that we *need* to fix. The other SPDY/4 priority changes are of a
> >>> performance optimization nature, and I believe they will need to be
> >>> justified by data. I have no plans to raise them up in this group
> >>> until we have said data.
> >>>
> >>> On Tue, Feb 5, 2013 at 5:34 AM, Poul-Henning Kamp <phk@phk.freebsd.dk>
> >>> wrote:
> >>> > Content-Type: text/plain; charset=ISO-8859-1
> >>> > --------
> >>> > In message <42A54D15-0AA3-4172-94F7-E94C86E84D7F@niven-jenkins.co.uk
> >,
> >>> > Ben Nive
> >>> > n-Jenkins writes:
> >>> >
> >>> >>So the idea is the protocol contains enough 'hooks' to sufficiently
> >>> >>express the different priorities between & within groups that folks
> >>> >>would like to express but isn't prescriptive about how anyone uses or
> >>> >>implements different prioritisation, scheduling, etc schemes.
> >>> >
> >>> > That was clearly not how the original poster presented it:
> >>> >
> >>> >         "I consider all those options as suboptimal, and thus
> >>> >         consider this issue to be a protocol bug. Our SPDY/4
> >>> >         prioritization proposal addresses this by [...]"
> >>> >
> >>> > --
> >>> > Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> >>> > phk@FreeBSD.ORG         | TCP/IP since RFC 956
> >>> > FreeBSD committer       | BSD since 4.3-tahoe
> >>> > Never attribute to malice what can adequately be explained by
> >>> > incompetence.
> >>>
> >>
> >
>
Received on Monday, 11 February 2013 22:46:59 GMT

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