Re: Priority implementation complexity (was: Re: Extensible Priorities and Reprioritization)

Yoav Weiss <yoav@yoav.ws> Mon, 15 June 2020 08:32 UTC

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In-Reply-To: <2C53D8AF-EFA8-42A3-9666-955A054468DB@greenbytes.de>
From: Yoav Weiss <yoav@yoav.ws>
Date: Mon, 15 Jun 2020 10:28:37 +0200
Message-ID: <CACj=BEic2qzMXEfcsKS9CYnowChc-kMRjH66d3uKs+pqTz9Fug@mail.gmail.com>
To: Stefan Eissing <stefan.eissing@greenbytes.de>
Cc: Kazuho Oku <kazuhooku@gmail.com>, Patrick Meenan <patmeenan@gmail.com>, Lucas Pardue <lucaspardue.24.7@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>, =?UTF-8?Q?Bence_B=C3=A9ky?= <bnc@chromium.org>
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Subject: Re: Priority implementation complexity (was: Re: Extensible Priorities and Reprioritization)
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On Mon, Jun 15, 2020 at 9:55 AM Stefan Eissing <stefan.eissing@greenbytes.de>
wrote:

> > Am 11.06.2020 um 10:41 schrieb Kazuho Oku <kazuhooku@gmail.com>om>:
> >
> > That depends on how much clients would rely on reprioritization. Unlike
> H2 priorities, Extensible Priority does not have inter-stream dependencies.
> Therefore, losing *some* prioritization signals is less of an issue
> compared to H2 priorities.
> >
> > Assuming that reprioritization is used mostly for refining the initial
> priorities of a fraction of all the requests, I think there'd be benefit in
> defining reprioritization as an optional feature. Though I can see some
> might argue for not having reprioritization even as an optional feature
> unless there is proof that it would be useful.
>
>
> > We should decide if reprioritization is good or bad, based on as much
> data as we can pull, and make sure it's implemented only if we see benefits
> for it in some cases, and then make sure it's only used in those cases.
>
> When thinking about priority implementations, I recommend thinking about a
> H3 reverse proxy in front of a legacy H1 server. Assume limited memory,
> disk space and backend connections.
>
> (Re-)prioritization in H2 works well for flow control, among the streams
> that have response data to send. Priorities can play a part in server
> scheduling, but
> it's more tricky. By "scheduling" I mean that the server has to pick one
> among the opened streams for which it wants to compute a response for. This
> is often impossible to re-prioritize afterwards (e.g. suicidal for a server
> implementation).
>

Can you expand on why it is "suicidal"?


>
> If we would do H2 a second time, my idea would be to signal priorities in
> the HTTP request in a connection header and use this in the H2 frame layer
> to allocate DATA space on the downlink. Leave out changing priorities on a
> request already started. Let the client use its window sizes if it feels
> the need.
>
> Cheers, Stefan (lurking)