Hi, On 12/21/2017 11:02 AM, Philipp Kern wrote: > At work our packages and package lists are served by a web service that > has relatively high time to first byte latency. Once it starts > transmitting the bytes are pushed in a fast way but fetching the bytes > in the first place takes a "long" time. While we are somewhat fine with > the latency on "apt update" a lot of packages, it imposes a very high > penalty on systems that need to fetch a lot of small packages (e.g. > build dependencies). > > The only solution apt offers today is pipelining. While it allows to > have a fast start for congestion control purposes, pipelining always > requires to send the answers in order down the pipe. Unless you set a > depth that equals the amount of packages to fetch, it will only > replenish the queue one by one as packages are completed, requiring a > full RTT to insert a new item. Furthermore it does impact the server > negatively if you consider the first hop to be a load balancer that fans > out to other backends. It needs to cache all answers to return them in > order. > > An easy workaround (few lines changed) would be to just spawn multiple > transports for a given host target, to make use of multiple connections. > In this case load balancing the requests onto them speeds up the > transaction essentially to line speed. There is still the drawback that > naive load balancing (essentially adding n queues for a host) happens at > the beginning of the transaction rather than through-out. This is not a > concern in our particular case, though, as the main issue is to enqueue > enough requests on the server side. > > It has been raised that this would cause violated assumptions by mirror > operators though, in case they approximate per-client limits using > per-connection rate limiting (because bucketing is hard). I'd argue that > an optional configuration setting that is not enabled by default should > still be fair to offer. > > Another solution to solve this problem would be to implement HTTP/2 > support, which allows to answer the requests non-linearly. In this case > a single connection would very likely be enough, as the server can just > answer what's available and the pipeline will be replenished > asynchronously. In our case the load balancer would also offer HTTP/2 > server-side[1]. However I'd argue that such an implementation should > then not be hand-rolled like the HTTP(S) transport and would require > depending on another library like nghttp2. So it would likely need to > live in its own apt transport. > > Happy to hear your thoughts on how to solve this. And please keep up the > great work. :) > > Kind regards and thanks > Philipp Kern > > [1] Note that HTTP/2 makes encryption mandatory. gentle post-vacational ping. I'm happy to look at whatever is needed here, however I'd prefer to work towards a consensus. ;-) Kind regards and thanks Philipp Kern
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