Re: Priority implementation complexity (was: Re: Extensible Priorities and Reprioritization)
Patrick Meenan <patmeenan@gmail.com> Mon, 15 June 2020 12:13 UTC
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From: Patrick Meenan <patmeenan@gmail.com>
Date: Mon, 15 Jun 2020 08:10:13 -0400
Message-ID: <CAJV+MGzesbFRZFGK5SUM70HJrx3fdJ8AAGGqmwqDQhrL3eFmUw@mail.gmail.com>
To: Yoav Weiss <yoav@yoav.ws>
Cc: Bence Béky <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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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. 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>: >> > >> > >> > >> > 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>: >> > > >> > > >> > > >> > > 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>: >> > > > >> > > > 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) >> > >> >>
- Extensible Priorities and Reprioritization Lucas Pardue
- Re: Extensible Priorities and Reprioritization Yoav Weiss
- Re: Extensible Priorities and Reprioritization Lucas Pardue
- Re: Extensible Priorities and Reprioritization Patrick Meenan
- Re: Extensible Priorities and Reprioritization Lucas Pardue
- Re: Extensible Priorities and Reprioritization Yoav Weiss
- Re: Extensible Priorities and Reprioritization Eric Kinnear
- Re: Extensible Priorities and Reprioritization Kazuho Oku
- Re: Extensible Priorities and Reprioritization Martin Thomson
- Re: Extensible Priorities and Reprioritization Bence Béky
- Priority implementation complexity (was: Re: Exte… Lucas Pardue
- Re: Priority implementation complexity (was: Re: … Dmitri Tikhonov
- Re: Priority implementation complexity (was: Re: … Patrick Meenan
- Re: Extensible Priorities and Reprioritization Patrick Meenan
- Re: Extensible Priorities and Reprioritization Lucas Pardue
- Re: Priority implementation complexity (was: Re: … Lucas Pardue
- Re: Priority implementation complexity (was: Re: … Dmitri Tikhonov
- Re: Priority implementation complexity (was: Re: … Roy T. Fielding
- Re: Extensible Priorities and Reprioritization Kinuko Yasuda
- Re: Extensible Priorities and Reprioritization Kinuko Yasuda
- Re: Priority implementation complexity (was: Re: … Kazuho Oku
- Re: Priority implementation complexity (was: Re: … Yoav Weiss
- Re: Priority implementation complexity (was: Re: … Martin Thomson
- Re: Priority implementation complexity (was: Re: … Kazuho Oku
- Re: Priority implementation complexity (was: Re: … Stefan Eissing
- Re: Priority implementation complexity (was: Re: … Yoav Weiss
- Re: Priority implementation complexity (was: Re: … Stefan Eissing
- Re: Priority implementation complexity (was: Re: … Yoav Weiss
- Re: Priority implementation complexity (was: Re: … Stefan Eissing
- Re: Priority implementation complexity (was: Re: … Yoav Weiss
- Re: Priority implementation complexity (was: Re: … Patrick Meenan
- Re: Priority implementation complexity (was: Re: … Yoav Weiss
- Re: Priority implementation complexity (was: Re: … Patrick Meenan
- Re: Priority implementation complexity (was: Re: … Kazuho Oku
- Re: Priority implementation complexity (was: Re: … Yoav Weiss
- Re: Priority implementation complexity (was: Re: … Barry Pollard
- Re: Priority implementation complexity (was: Re: … Barry Pollard
- Re: Extensible Priorities and Reprioritization Kazuho Oku
- Re: Extensible Priorities and Reprioritization Yoav Weiss
- Re: Priority implementation complexity (was: Re: … Lucas Pardue
- Re: Priority implementation complexity (was: Re: … Tom Bergan
- Re: Priority implementation complexity (was: Re: … Lucas Pardue
- Re: Priority implementation complexity (was: Re: … Tom Bergan
- Re: Priority implementation complexity (was: Re: … Lucas Pardue
- Re: Extensible Priorities and Reprioritization Patrick Meenan
- Re: Extensible Priorities and Reprioritization Patrick Meenan
- Re: Extensible Priorities and Reprioritization Lucas Pardue
- Reprioritization - implementation intent Mark Nottingham
- Re: Reprioritization - implementation intent Eric Kinnear
- Nice to have guidance (was: Re: Reprioritization … Lucas Pardue
- Re: Nice to have guidance (was: Re: Reprioritizat… Eric Kinnear
- Re: Reprioritization - implementation intent Yoav Weiss
- Re: Reprioritization - implementation intent Yoav Weiss
- Re: Reprioritization - implementation intent Yoav Weiss