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+[[!meta copyright="Copyright © 2011 Free Software Foundation, Inc."]]
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+[[!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.