On 04/11/2014 04:45 PM, roger riggs wrote:
Hi Peter,
Understood, but didn't want ignore another requested feature.
Iterating over children is workable but I'm not clear whether if an
intermediate child
dies/is killed that its children are reparented up the tree.
If a parent dies, all it's immediate children are reparented to pid 1
(init process). init process is responsible for waiting on their exits.
That's why there're no zombies left behind such children when they exit
- the init process collects them.
If for one reason or another a grandchild is reparented to pid 1 then
it would not
be discovered by iteration.
That's right. If a JVM child dies, the tree view of processes, which is
rooted in the JVM process, looses an entire branch. This usually happens
for processes that wish to become background daemons. They fork() a
child and exit(). Their child becomes a daemon. You won't be able to
ever see this daemon PID in the tree if you're not quick enough. The
daemons also change their session and group, so tracking them by group
id is not possible either.
No Zombies please.
No Cranberries ;-)
Regards, Peter
Thanks, Roger
On 4/11/2014 6:47 AM, Peter Levart wrote:
On 04/09/2014 03:20 PM, roger riggs wrote:
Hi Peter,
On a related topic, the request to be able to destroy a Process and
all of its children
might also want to used the group pid to be able to identify all of
the children.
Hi Roger,
This would require each child spawned by Process API to be assigned
it's own process group. The grandchildren would inherit this process
group. You could then send KILL/TERM signal to a process group in
order to destroy the child and all it's descendants (that did not
change the process group in the meanwhile).
But we can only group processes for one purpose, since a process can
only belong to one group at a time. To send signals to a (sub-)tree
of processes, the child-parent relationship is more natural to
follow, I think, since no waiting/blocking is involved in sending the
signals, so enumerating and iterating is appropriate.
Waiting on children is another purpose where process group(s) could
be employed and I think they would be better spent this way.
I think I now have a picture of how this could work. See my reply to
Martin.
Regards, Peter
Roger
On 4/9/2014 2:08 AM, Peter Levart 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.
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?
