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Diffstat (limited to 'glibc/signal')
| -rw-r--r-- | glibc/signal/signal_thread.mdwn | 93 |
1 files changed, 93 insertions, 0 deletions
diff --git a/glibc/signal/signal_thread.mdwn b/glibc/signal/signal_thread.mdwn new file mode 100644 index 00000000..28855dbd --- /dev/null +++ b/glibc/signal/signal_thread.mdwn @@ -0,0 +1,93 @@ +[[!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]]."]]"""]] + +[[!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. |