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-[[!meta copyright="Copyright © 2011 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
-License|/fdl]]."]]"""]]
-
-For delivering a signal, Mach forwards an `msg_sig_post` message from the
-invoker of `kill` to the target process.  The target process' [[signal_thread]]
-job is it to listen to such messages and to set up signal handler contexts in
-other threads.
-
----
-
-[[!tag open_issue_documentation]]
-
-    <braunr> bugs around signals are very tricky
-    <braunr> signals are actually the most hairy part of the hurd
-    <braunr> and the reason they're aynchronous is that they're handled by a
-      second thread
-    <braunr> (so yes, every process on the hurd has at least two threads)
-    <svante_> braunr: How to solve the asynch problem then if every process has
-      two threads?
-    <braunr> the easiest method would be to align ourselves on what most other
-      Unices do
-    <braunr> establish a "signal protocol" between kernel and userspace
-    <braunr> with a set of signal info in a table, most likely at the top of
-      the stack
-    <braunr> but this is explicitely what the original Mach developers didn't
-      want, and they were right IMO
-    <braunr> having two threads is very clean, but it creates incompatibilites
-      with what POSIX requires
-    <braunr> so there might be a radical choice to make here
-    <braunr> and i doubt we have the resources to make it happen
-    <svante_> What is the advantage of having two threads per process, a per
-      the original design?
-    <braunr> it's clean
-    <braunr> you don't have to define async-signal-safe functions
-    <braunr> it's like using sigwait() yourself in a separate thread, or
-      multiplexing them through signalfd()
-    <svante_> Regardless of the advantages, isn't two threads per process a
-      waste of resources?
-    <braunr> sure it is
-    <braunr> but does it really matter ?
-    <braunr> mach and the hurd were intended to be "hyperthreaded"
-    <braunr> so basically, a thread should consume only a few kernel resources
-    <braunr> in GNU Mach, it doesn't even consume a kernel stack because only
-      continuations are used
-    <braunr> and in userspace, it consumes 2 MiB of virtual memory, a few table
-      entries, and almost no CPU time
-    <svante_> What does "hyperthreaded" mean: Do you have a reference?
-    <braunr> in this context, it just means there are a lot of threads
-    <braunr> even back in the 90s, the expected number of threads could scale
-      up to the thousand
-    <braunr> today, it isn't much impressive any more
-    <braunr> but at the time, most systems didn't have LWPs yet
-    <braunr> and a process was very expensive
-    <svante_> Looks like I have some catching up to do: What is "continuations"
-      and LWP? Maybe I also need a reference to an overview on multi-threading.
-    <ArneBab> Lightweight process?
-      http://en.wikipedia.org/wiki/Light-weight_process
-    <braunr> svante_: that's a whole computer science domain of its own
-    <braunr> yes
-    <braunr> LWPs are another names for kernel threads usually
-    <braunr> continuations are a facility which allows a thread to store its
-      state, yield the processor to another thread, and when it's dispatched
-      again by the scheduler, it can resume with its saved state
-    <braunr> most current kernels support kernel preemption though
-    <braunr> which means their state is saved based on scheduler decisions
-    <braunr> unlike continuations where the thread voluntarily saves its state
-    <braunr> if you only have continuations, you can't have kernel preemption,
-      but you end up with one kernel stack per processor
-    <braunr> while the other model allows kernel preemption and requires one
-      kernel stack per thread
-    <svante_> I know resources are limited, but it looks like kernel preemption
-      would be nice to have. Is that too much for a GSoC student?
-    <braunr> it would require a lot of changes in obscure and sensitive parts
-      of the kernel
-    <braunr> and no, kernel preemption is something we don't actually need
-    <braunr> even current debian linux kernels are built without kernel
-      preemption
-    <braunr> and considering mach has hard limitations on its physical memory
-      management, increasing the amount of memory used for kernel stacks would
-      imply less available memory for the rest of the system
-    <svante_> Are these hard limits in mach difficult to change?
-    <braunr> yes
-    <braunr> consider mach difficult to change
-    <braunr> that's actually one of the goals of my stalled project
-    <braunr> which I hope to resume by the end of the year :/
-    <svante_> Reading Wikipedia it looks like LWP are "kernel treads" and other
-      threads are "user threads" at least in IBM/AIX. LWP in Linux is a thread
-      sharing resources and in SunOS they are "user threads". Which is closest
-      for Hurd?
-    <braunr> i told you
-    <braunr> 14:09 < braunr> LWPs are another names for kernel threads usually
-    <svante_> Similar to to the IBM definition then? Sorry for not remembering
-      what I've  been reading.