Re[3]: [Gc] Re: GC v.7.1 DARWIN x86 usage of threads

Ivan Maidanski ivmai at mail.ru
Mon Sep 6 23:23:49 PDT 2010


Hi!

I'm trying to understand the darwin-specific part to implement register_my_thread for darwin (and prevent stopping of marker threads).

Looking into GC_push_all_stacks, its implementation looks to me similar to Win32 DLL-based approach - we use task_threads() (in case of !DARWIN_DONT_PARSE_STACK) instead of iterating over GC_pthread just to scan all thread even if it wasn't registered (neither explicitly nor pthread_create wasn't intercepted).

If I'm right, the solution comes from Win32 - deprecate old behavior (I mean scanning all threads) and scan only threads that are registered, but leave the possibility to use the old behavior (i.e. implement GC_use_DllMain() for darwin which turns on the old one but disables parallel markers).

Hans - any comments?

And the 2nd question: how to implement suspend/resume for only the registered threads (unless switched to old behavior) - use something like HashSet<task_t>?



Fri, 03 Sep 2010 00:48:47 +0400 Ivan Maidanski <ivmai at mail.ru>:

> 
> Mon, 30 Aug 2010 17:02:32 -0700 Jim Hourihan <jimhourihan at gmail.com>:
> 
> > 
> > The patch is against 7.2alpha, I'll get the 7.1 code and create it against that when I get a chance. The main thing is that this function GC_stop_world() in darwin_stop_world.c is stopping *all* task threads (in mach parlance) and scanning their stacks.
> 
> To be precise, if DARWIN_DONT_PARSE_STACK then only registered thread stacks are scanned.
> 
> And, it seems to erroneously stop the parallel marker threads too.
> 
> > GC_stop_world for darwin calls the mach system call task_threads followed by GC_suspend_thread_list which loops over the thread list eventually calling  the mach API thread_suspend on each except the invoking thread. It doesn't use any other information (i.e. registered threads list). I found this out because it was stopping my HAL audio thread which was causing popping.
> > 
> > So basically, AFAIK, on darwin the pthreads interface is moot. The code actually uses the more low-level mach interface which makes the code very simple and easy to follow, but ignores the thread registration part of the gc API. So you don't need to call GC_register_my_thread and doing so will have no effect. Likewise with GC_unregister_my_thread.
> > 
> > You see all of this pretty easily if you put a break point in GC_stop_world().
> > 
> > ...
> 
> AICS, to stop and scan only registered threads on Darwin, we need to maintain HashSet<task_t> (in GC_register/unregister_my_thread) and test this set in threads start/stop/scan. Do I miss something?
>...


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