ProcessReaper: single thread reaper

Peter Levart peter.levart at
Wed Apr 9 06:08:08 UTC 2014

Hi Martin,

As you might have seen in my later reply to Roger, there's still hope on 
that front: setpgid() + wait(-pgid, ...) might be the answer. I'm 
exploring in that direction. Shells are doing it, so why can't JDK?

It's a little trickier for Process API, since I imagine that shells form 
a group of processes from a pipeline which is known in-advance while 
Process API will have to add processes to the live group dynamically. So 
some races will have to be resolved, but I think it's doable.

Stay tuned.

Regards, Peter

On 04/08/2014 07:48 PM, Martin Buchholz wrote:
> Peter, thank you very much for your deep analysis.
> TIL and am horrified: signals on Unix are not queued, not even if you 
> specify SA_SIGINFO.  Providing siginfo turns signals into proper 
> "messages" each with unique content, and it is unacceptable to simply 
> drop some (Especially when proper queueing seems required for 
> so-called real-time signals), but at least the Linux kernel does so 
> very deliberately.   45 years later, we are still fighting with 
> unreliable Unix signals...
> We can't call waitpid(WAIT_ANY, ) because we can only wait for 
> processes owned by the j.l.Process subsystem.  We can't override libc 
> functions like waitpid because the JVM may be a "guest" in some other 
> process.
> I don't know of any public examples, but it is reasonable to add a JVM 
> to a previously pure native code application, similarly to the way tcl 
> or lua is often used to provide a higher-level safer programming api 
> to native code, and some programs at Google use this strategy.
> What problem are we actually trying to solve?  The army of reaper 
> threads is ugly, but the inefficiency is greatly mitigated by the use 
> of small explicit stack sizes.  Redoing the process code is always 
> risky, as we have already seen in this thread.
> Maintaining a single child helper process which spawns all the 
> (grand)child processes seems reasonable, although it would create a 
> permanent intermediate entry in the process table (pstree?) which 
> might confuse some sysadmin scripts.  Is it worth it?

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