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    <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.