path: root/open_issues/synchronous_ipc.mdwn
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authorThomas Schwinge <>2012-08-07 23:25:26 +0200
committerThomas Schwinge <>2012-08-07 23:25:26 +0200
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+[[!meta copyright="Copyright © 2012 Free Software Foundation, Inc."]]
+[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
+id="license" text="Permission is granted to copy, distribute and/or modify this
+document under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; with no Invariant
+Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license
+is included in the section entitled [[GNU Free Documentation
+[[!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
+ <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 :/