Home liberachat/#xmonad: Logs Calendar

Logs on 2022-06-17 (liberachat/#xmonad)

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08:55:33 <liskin> geekosaur: I don't think we have modules which fork a *thread*, because xmonad isn't built with -threaded unless you build it yourself, which can't be relied upon
08:55:47 <liskin> geekosaur: we do have modules that fork a *process*, but that's okay
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10:06:18 <Ether17> so i came across, this issue using the window Frills Deco. When using it scrolling with the mouse wheel on the deco casue the window enter a kind of floating mode. Is there a way to disable this?
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12:36:27 <geekosaur> liskin, the rts uses green threads without -threaded
12:37:05 <geekosaur> although the one case I thought used forkIO instead uses xfork, so I guess you're right about that
13:03:27 <liskin> green threads are useless when you're blocked in a XNextEvent FFI call
13:03:51 <liskin> so any code that wants to actually do something needs to fork process
13:04:05 <liskin> (it's a different story in xmobar though)
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13:11:29 <geekosaur[m]> pretty sure that's only true of "unsafe" FFI calls, "safe" calls go through the IO manager (which is still threaded even in the "non-threaded" runtime)
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13:24:52 <liskin> wouldn't that break the boundedness of the main thread?
13:25:15 <geekosaur> bound threads have bound FFI threads
13:26:44 <geekosaur> it's fairly complex but if you think about it this is the only way things can reasonably work: any given thread has an associated FFI thread, which comes from a pool for forkIO threads or is dedicated for forkOS threads or the main thread
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13:27:04 <geekosaur> otherwise some other Haskell thread will be blocked
13:27:47 <Ether17> how silly of me i had "$ windowArrange"  in "myLayoutHook" sorry..
13:27:50 <liskin> yeah and I thought other Haskell threads being blocked is precisely what happens
13:28:08 <liskin> (I still think that, because why would there be all those forks in xmonad and ifdefs in xmobar)
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13:28:57 <geekosaur> xmonad forks because it manages processes :) and double forking is to not have to worry about zombies, it's an old technique
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13:29:45 <geekosaur> some of the reason you might come to a different conclusion is there are a lot of FFI calls that are incorrectly marked unsafe. notably in network
13:29:46 <liskin> yeah but if what you're saying is true, it could just use threads in many cases
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13:32:11 <geekosaur> I think in most cases inside xmonad, sjanssen was simply following standard unix procedure and not trying to get clever with threads etc.
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15:08:42 <liskin> hm, so I just did strace -f -k xmonadpropread, which I compile without -threaded and which does nextEvent on the main bound thread, nextEvent being a "foreign import ccall safe", and it doesn't spawn a separate thread and the blocking poll calls it does are from X11/xcb, not from GHC RTS
15:09:06 <liskin> so I think I'm going to maintain the illusion I might be right for a little longer :-))
15:09:36 <geekosaur> it shouldn't be spawning a new thread. the IO manager has iirc 4 threads in a pool
15:09:59 <geekosaur> even in the "non-threaded" runtime, which trips people up sometimes
15:10:30 <liskin> strace would tell me about any clone calls where those threads are being created, and it would also start prefixing every line of its output with a task id had there been more than one
15:11:30 <liskin> I'd also see those threads in "ps axm", or in /proc/1384190/task/…
15:11:48 <liskin> there really are no threads unless I forgot how linux/unix works
15:13:00 <liskin> (not doing this to prove you wrong or prove myself right, just wanted to make sure my brain's idea of how the world works matches reality)
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15:17:18 <geekosaur> for one, I don't think clone is used any more since LinuxThreads died, it proved impossible to emulate POSIX threads properly that way
15:25:40 <liskin> clone(child_stack=0x7f973e051eb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTIDstrace: Process 1395254 attached
15:25:42 <liskin> , parent_tid=[1395254], tls=0x7f973e052640, child_tidptr=0x7f973e052910) = 1395254
15:25:44 <liskin> [pid 1395254] set_robust_list(0x7f973e052920, 24 <unfinished ...>
15:25:47 <liskin> this is xmobar compiled with -threaded
15:26:36 <liskin> which other syscall did you think was used to create a thread on x86_64?
15:27:04 <liskin> (actually I'm surprised there's a fork syscall at all; I'm fairly sure glibc does fork via clone as well)
15:27:14 <geekosaur> mm, I thought pthread_create mapped to a new syscall. I recall their trying to make clone work for threads multiple times and failing
15:28:12 <geekosaur> LinuxThreads was a disaster
15:28:26 <geekosaur> (back around kernel 2.4)
15:28:41 <liskin> I remember there being a thing called linuxthreads but I was too young/stupid at the time to understand what that was about
15:30:43 <geekosaur> okay, seems like they decided to redo clone as the new-thread syscall and make it behave properly for that, which meant putting new-process back on fork
15:33:21 <liskin> clone is just a new-task syscall with a gazillion flags about what should be shared with the calling task
15:34:09 <geekosaur> that was the promise of LinuxThreads. it proved not that simple
15:34:43 <geekosaur> they put a *lot* of effort into trying to make that work
15:35:13 <lyiriyah[m]> btw, I've just gotten titles working by rewriting `getName` based on geekosaur's code
15:35:56 <geekosaur> mm, the code there should have done basically the same thing as what I gave you for ghci
15:36:09 <liskin> wikipedia says: "NPTL uses a similar approach to LinuxThreads, in that the primary abstraction known by the kernel is still a process, and new threads are created with the clone() system call (called from the NPTL library). However, NPTL requires specialized kernel support to implement (for example) the contended case of synchronisation primitives which might require threads to sleep and wake
15:36:11 <liskin> again. The primitive used for this is known as a futex.
15:36:13 <liskin> "
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All times are in UTC on 2022-06-17.