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

Yoav Weiss <yoav@yoav.ws> Mon, 15 June 2020 12:55 UTC

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From: Yoav Weiss <yoav@yoav.ws>
Date: Mon, 15 Jun 2020 14:51:49 +0200
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To: Patrick Meenan <patmeenan@gmail.com>
Cc: =?UTF-8?Q?Bence_B=C3=A9ky?= <bnc@chromium.org>, HTTP Working Group <ietf-http-wg@w3.org>, Kazuho Oku <kazuhooku@gmail.com>, Lucas Pardue <lucaspardue.24.7@gmail.com>, Stefan Eissing <stefan.eissing@greenbytes.de>
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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 2:10 PM Patrick Meenan <patmeenan@gmail.com> wrote:

> Even without a priority tree it is likely that the H3 extensible
> priorities structure would cause not-yet-started responses to need to be
> scheduled ahead of in-flight responses. The urgency value is effectively a
> parent/child relationship.
>

I wouldn't consider the inflight response dependent on the
not-yet-started one here, because the not-yet-started one doesn't need a
specific response to hold for it, it just needs *any* response to finish
(and free up a worker thread in the architecture Stefan outlined) in order
to be sent (with its appropriate priority).
So, it might be somewhat delayed, but won't be held back forever if a
specific response is a long sending one. (assuming the number of worker
thread is not very small)


> It's not as unbounded as H2 but if you churned through a bunch of
> reprioritizations with stalled streams you could cause issues for a server
> that didn't protect against it.
>
> Limiting the reprioritizations to "what stream to pick next" would help
> but wouldn't solve the long download problem.
>
> On Mon, Jun 15, 2020 at 7:44 AM Yoav Weiss <yoav@yoav.ws> wrote:
>
>>
>>
>> On Mon, Jun 15, 2020 at 1:18 PM Stefan Eissing <
>> stefan.eissing@greenbytes.de> wrote:
>>
>>>
>>> Stefan Eissing
>>>
>>> <green/>bytes GmbH
>>> Hafenweg 16
>>> <https://www.google.com/maps/search/Hafenweg+16+%0D%0A48155+M%C3%BCnster?entry=gmail&source=g>
>>> 48155 Münster
>>> <https://www.google.com/maps/search/Hafenweg+16+%0D%0A48155+M%C3%BCnster?entry=gmail&source=g>
>>> www.greenbytes.de
>>>
>>> > Am 15.06.2020 um 12:14 schrieb Yoav Weiss <yoav@yoav.ws>ws>:
>>> >
>>> >
>>> >
>>> > On Mon, Jun 15, 2020 at 11:03 AM Stefan Eissing <
>>> stefan.eissing@greenbytes.de> wrote:
>>> >
>>> > > Am 15.06.2020 um 10:28 schrieb Yoav Weiss <yoav@yoav.ws>ws>:
>>> > >
>>> > >
>>> > >
>>> > > 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"?
>>> >
>>> > It is tricky to obey re-prioritizations to the letter, managing
>>> memory+backend connections and protecting the infrastructure against DoS
>>> attacks. The reality is that there are limited resources and a server is
>>> expected to protect those. It's a (pun intended) top priority.
>>> >
>>> > Another priority topping the streams is the concept of fairness
>>> between connections. In Apache httpd, the resources to process h2 streams
>>> are foremost shared evenly between connections.
>>> >
>>> > That makes sense. Would re-prioritization of specific streams somehow
>>> require to change that?
>>> >
>>> > The share a connection gets is then allocated to streams based on
>>> current h2 priority settings. Any change after that will "only" affect the
>>> downstream DATA allocation.
>>> >
>>> > I *think* this makes sense as well, assuming that by "downstream" you
>>> mean "future". Is that what you meant? Or am I missing something?
>>> >
>>> > Also, the number of "active" streams on a connection is dynamic. It
>>> will start relatively small and grow if the connection is well behaving,
>>> shrink if it is not. That one of the reasons that Apache was only partially
>>> vulnerable to a single issue on the Netflix h2 cve list last year (the
>>> other being nghttp2).
>>> >
>>> > tl;dr
>>> >
>>> > By "suicidal" I mean a server failing the task of process thousands of
>>> connections in a consistent and fair manner.
>>> >
>>> > Apologies if I'm being daft, but I still don't understand how
>>> (internal to a connection) stream reprioritization impacts cross-connection
>>> fairness.
>>>
>>> *fails to imagine Yoav as being daft*
>>>
>> :)
>>
>> Thanks for outlining the server-side processing!
>>
>>
>>> A server with active connections and workers. For simplicity, assume
>>> that each ongoing request allocates a worker.
>>> - all workers are busy
>>> - re-prio arrives and makes a stream A, being processed, depend on a
>>> stream B which has not been assigned a worker yet.
>>>
>>
>> OK, I now understand that this can be concerning.
>> IIUC, this part is solved by with Extensible Priorities (because there's
>> no dependency tree).
>>
>> Lucas, Kazuho - can you confirm?
>>
>>
>>> - ideally, the server would freeze the processing of A and assign the
>>> resources to B.
>>> - however re-allocating the resources is often not possible  (Imagine a
>>> CGI process running or a backend HTTP/1.1 or uWSGI connection.)
>>> - the server can only suspend the worker or continue processing,
>>> ignoring the dependency.
>>> - a suspended worker is very undesirable and a possible victim of a
>>> slow-loris attack
>>> - To make this suspending less sever, the server would need to make
>>> processing of stream B very important. To unblock it quickly again. This is
>>> then where unfairness comes in.
>>>
>>> The safe option therefore is to continue processing stream A and ignore
>>> the dependency on B. Thus, priorities are only relevant:
>>> 1. when the next stream to process on a connection is selected
>>> 2. when size/number of DATA frames to send is allocated on a connection
>>> between all streams that want to send
>>>
>>> (Reality is often not quite as bad as I described: when static
>>> file/cache resources are served for example, a worker often just does the
>>> lookup, producing a file handle very quickly. A connection easily juggles a
>>> number of file handles to stream out according to priorities and stalling
>>> one file on another comes at basically no risk and cost.)
>>>
>>> Now, this is for H2 priorities. I don't know enough about QUIC
>>> priorities to have an opinion on the proposals. Just wanted to point out
>>> that servers see the world a little different than clients. ;)
>>>
>>
>> I checked and it seems like Chromium does indeed change the parent
>> dependency as part of reprioritization. If the scenario you outlined is a
>> problem in practice, we should discuss ways to avoid doing that with H2
>> priorities.
>>
>>
>>>
>>> Cheers, Stefan
>>>
>>>
>>> > >
>>> > >
>>> > > 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)
>>> >
>>>
>>>