ProcessReaper: single thread reaper
peter.levart at gmail.com
Wed Apr 9 06:08:08 UTC 2014
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.
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
> 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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