ProcessReaper: single thread reaper

roger riggs roger.riggs at
Mon Apr 14 14:37:10 UTC 2014


Jtreg, for example, needs a reliable way to cleanup after tests.
We've had a variety of problems with stray processes left over because
there is no visibility nor reliable way to identify and kill them.


On 4/14/2014 10:31 AM, David M. Lloyd wrote:
> Where does the requirement to manage grandchild processes actually 
> come from?  I'd hate to see the ability to "nicely" terminate 
> immediate child processes lost just because it was difficult to 
> implement some grander scheme.
> On 04/14/2014 08:49 AM, roger riggs wrote:
>> Hi Martin,
>> A new API is needed, overloading the current Process API is not a good
>> option.
>> Even within Process a new method will be needed to destroy the
>> subprocess and all
>> of its children maintain backward compatibility.
>> Are there specific OS features that need to be exposed to applications?
>> Is the destroy-process-and-all-children abstraction too coarse.
>> Roger
>> On 4/11/2014 7:37 PM, Martin Buchholz wrote:
>>> Let's step back again and try to check our goals...
>>> We could try to optimize the one-reaper-thread-per-subprocess thing.
>>>  But that is risky, and the cost of what we're doing today is not that
>>> high.
>>> We could try to implement the feature of killing off an entire
>>> subprocess tree.  But historically, any kind of behavior change like
>>> that has been vetoed.  I have tried and failed to make less
>>> incompatible changes.  We would have to add a new API.
>>> The reality is that Java does not give you real access to the
>>> underlying OS, and unless there's a seriously heterodox attempt to
>>> provide OS-specific extensions, people will have to continue to either
>>> write native code or delegate to an OS-savvy subprocess like a perl
>>> script.
>>> On Fri, Apr 11, 2014 at 7:52 AM, Peter Levart <peter.levart at
>>> <mailto:peter.levart at>> wrote:
>>>     On 04/09/2014 07:02 PM, Martin Buchholz wrote:
>>>>     On Tue, Apr 8, 2014 at 11:08 PM, Peter Levart
>>>>     <peter.levart at <mailto:peter.levart at>> wrote:
>>>>         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.
>>>>     This is a clever idea, and it's arguably better to design
>>>>     subprocesses so they live in separate process groups (emacs does
>>>>     that), but:
>>>>     Every time you create a process group, you change the effect of a
>>>>     user signal like Ctrl-C, since it's sent to only one group.
>>>>     Maybe propagate signals to the subprocess group?  It's starting
>>>>     to get complicated...
>>>     Hi Martin,
>>>     Yes, shells send Ctrl-C (SIGINT) and other signals initiated by
>>>     terminal to a (foreground) process group. A process group is
>>>     formed from a pipeline of interconnected processes. Each pipeline
>>>     is considered to be a separate "job", hence shells call this
>>>     feature "job-control". Child processes by default inherit process
>>>     group from it's parent, so children born with Process API (and
>>>     their children) inherit the process group from the JVM process.
>>>     Considering the intentions of shell job-controll, is propagating
>>>     SIGTERM/SIGINT/SIGTSTP/SIGCONT signals to children spawned by
>>>     Process API desirable? If so, then yes, handling those signals in
>>>     JVM and propagating them to current process group that contains
>>>     all children spawned by Process API and their descendants would
>>>     have to be performed by JVM. That problem would certainly have to
>>>     be addressed. But let's first see what I found out about
>>>     sigaction(SIGCHLD, ...), setpgid(pid, pgid), waitpid(-pgid, ...),
>>>     etc...
>>>     waitpid(-pgid, ...) alone seems to not be enough for our task.
>>>     Mainly because a process can re-assign it's group and join some
>>>     other group. I don't know if this is a situation that occurs in
>>>     real world, but imagine if we have one live child process in a
>>>     process group pgid1 and no unwaited exited children. If we issue:
>>>         waitpid(-pgid1, &status, 0);
>>>     Then this call blocks, because at the time it was given, there
>>>     were >0 child processes in the pgid1 group and none of them has
>>>     exited yet. Now if this one child process changes it's process
>>>     group with:
>>>         setpgid(0, pgid2);
>>>     Then the waitpid call in the parent does not return (maybe this is
>>>     a bug in Linux?) although there are no more live child processes
>>>     in the pgid1 group any more. Even when this child exits, the call
>>>     to waitpid does not return, since this child is not in the group
>>>     we are waiting for when it exits. If all our children "escape" the
>>>     group in such way, the tread doing waiting will never unblock. To
>>>     solve this, we can employ signal handlers. In a signal handler for
>>>     SIGCHLD signal we can invoke:
>>>         waitpid(-pgid1, &status, WNOHANG); // non-blocking call
>>> loop until it either returns (0) which means that there're
>>>     no more unwaited exited children in the group at the momen or (-1)
>>>     with errno == ECHILD, which means that there're no more children
>>>     in the queried group any more - the group does not exist any more.
>>>     Since signal handler is invoked whith SIGCHLD being masked and
>>>     there is one bit of pending signal state in the kernel, no child
>>>     exit can be "skipped" this way. Unless the child "escapes" by
>>>     changing it's group. I don't know of a plausible reason for a
>>>     program to change it's process group. If a program executing as
>>>     JVM child wants to become a background daemon it usually behaves
>>>     as follows:
>>>     - fork()s a grand-child and then exit()s (so we get notified via
>>>     signal and waitpid(-pgid, ...) successfully for it's exitstatus)
>>>     - the grand-child then changes it's session and group (becomes
>>>     session and group leader), closes file descriptors, etc. The
>>>     responsibility for waiting on the grand-child daemon is
>>>     transferred to the init process (pid=1) since the grand-child
>>>     becomes an orphan (has no parent).
>>>     Ignoring this still unsolved problem of possible ill-behaved child
>>>     program that changes it's process group, I started constructing a
>>>     proof-of-concept prototype. What I will do in the prototype is
>>>     start throwing IllegalStateException from the methods of the
>>>     Process API that pertain to such children. I think this is
>>> reasonable.
>>>     Stay tuned,
>>>     Peter

More information about the core-libs-dev mailing list