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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 :/
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