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[[!tag open_issue_hurd]]


# IRC, freenode, #hurd, 2012-07-20

From [[Genode RPC|microkernel/genode/rpc]].

    <braunr> assuming synchronous ipc is the way to go (it seems so), there is
      still the need for some async ipc (e.g signalling untrusted recipients
      without risking blocking on them)
    <braunr> 1/ do you agree on that and 2/ how would this low-overhead async
      ipc be done ? (and 3/ are there relevant examples ?
    <antrik> if you think about this stuff too much you will end up like marcus
      and neal ;-)
    <braunr> antrik: likely :)
    <antrik> the truth is that there are various possible designs all with
      their own tradeoffs, and nobody can really tell which one is better
    <braunr> the only sensible one i found is qnx :/
    <braunr> but it's still messy
    <braunr> they have what they call pulses, with a strictly defined format
    <braunr> so it's actually fine because it guarantees low overhead, and can
      easily be queued
    <braunr> but i'm not sure about the format
    <antrik> I must say that Neal's half-sync approach in Viengoos still sounds
      most promising to me. it's actually modelled after the needs of a
      Hurd-like system; and he thought about it a lot...
    <braunr> damn i forgot to reread that
    <braunr> stupid me
    <antrik> note that you can't come up with a design that allows both a)
      delivering reliably and b) never blocking the sender -- unless you cache
      in the kernel, which we don't want
    <antrik> but I don't think it's really necessary to fulfill both of these
      requirements
    <antrik> it's up to the receiver to make sure it gets important signals
    <braunr> right
    <braunr> caching in the kernel is ok as long as the limit allows the
      receiver to handle its signals
    <antrik> in the Viengoos approach, the receiver can allocate a number of
      receive buffers; so it's even possible to do some queuing if desired
    <braunr> ah great, limits in the form of resources lent by the receiver
    <braunr> one thing i really don't like in mach is the behaviour on full
      message queues
    <braunr> blocking :/
    <braunr> i bet the libpager deadlock is due to that

[[libpager_deadlock]].

    <braunr> it simply means async ipc doesn't prevent at all from deadlocks
    <antrik> the sender can set a timeout. blocking only happens when setting
      it to infinite...
    <braunr> which is commonly the case
    <antrik> well, if you see places where blocking is done but failing would
      be more appropriate, try changing them I'd say...
    <braunr> it's not that easy :/