Hi Roger,
On 05/14/2015 03:44 PM, Roger Riggs wrote:
Hi Peter,
On 5/14/15 8:19 AM, Peter Levart wrote:
Hi Roger,
The new API using Optional(s) looks fine. In particular for the
ProcessHandle returning methods. They now either return
Stream<ProcessHandle> or Optional<ProcessHandle>.
At some point in the development of this API, the implementation
introduced the AsyncExecutor to execute synchronous continuations of
the onExit() returned CompletableFuture(s). What was the main
motivation for this given that:
- previously, ForkJoinPoll.commonPool() was used instead that by
default possesses some similar characteristics (Innocuous threads
when SecurityManager is active)
The AsyncExecutor also uses InnocuousThreads.
So that's not what is lost or gained by AsyncExecutor when comparing it
with commonPool().
- this AsyncExecutor is only effective for 1st "wave" of synchronous
continuations. Asynchronous continuations and synchronous
continuations following them will still use ForkJoinPoll.commonPool()
Unfortunately, the common ForkJoinPool assumes that tasks queued to it
complete relatively
quickly and free the thread. It does not grow the number of threads
and is not appropriate
for tasks that block for indefinite periods as might be need to wait
for a Process to exit.
Ok, AsyncExecutor might be required for default implementation of
Process.onExit(). But waiting on process exit in ProcessImpl and
ProcessHandle is performed by internal 32K stack-sized reaper threads
and what we did in ProcessImpl.onExit() was this (before the AsyncExecutor):
@Override
public CompletableFuture<Process> onExit() {
return ProcessHandleImpl.completion(pid, false)
.handleAsync((exitStatus, unusedThrowable) -> this);
}
Which means that FJP.commonPool() thread is employed after
ProcessHandleImpl.completion() is completed. Meaning that just user code
is run by commonPool() and that code does not wait for process to exit
because it already did exit. User code might run for extended period of
time, but any use of CompletableFuture is susceptible to that abuse. Are
you afraid that users of Process API are not common CompletableFuture
users and are not aware of commonPool() constraints?
Would an alternative be to define two overloaded onExit() methods in
the style of CompletableFuture itself?
CompletableFuture<ProcessHandle> onExit();
CompletableFuture<ProcessHandle> onExit(Executor executor);
...and give the user a chance to supply it's own Executor if the
default ForkJoinPoll.commonPool() does not fit?
It is only one more method in PH and Process but that function is
available from CompletableFuture
though perhaps not as conveniently.
The onExit method returns a CompletableFuture that has the entire
complement of
synchronous and async methods available to it. The application can
control
where subsequent computations are performed.
That's true for xxxxAsync methods. Synchronous continuations are
executed by whichever thread executed the previous stage. We insert an
asynchronous stage to shield the internal 32K threads from user code.
What we do by executing that stage in AsyncExecutor might be desirable
when the user attaches a synchronous continuation. But when he attaches
an asynchronous one, the thread from AsyncExecutor is used just for
mapping the result into Process instance and triggering the execution of
next stage. Can we eliminate this unnecessary asynchronous stage?
What would be if we stoped pretending that user can execute a
synchronous onExit() continuation when in fact that continuation is
always attached to an asynchronous stage so it is actually asynchronous?
What if CompletableFuture had a public superclass called
AsyncCompletableFuture that only exposed .xxxxAsync() continuation
methods which returned normal CompletableFuture(s)? Such class would by
definition shield the thread that completes an AsyncCompletableFuture
from user code that attaches continuations. And user code would have
exclusive control on what type of thread executes it. Isn't this a
desirable property that jsr166 could provide? It could be useful in
other scenarios too.
The only problem with that approach is that we need a mapping stage that
"attaches" the Proses[Handle] instance to
AsyncCompletableFuture<Integer> (which is generic and returns an exit
status) to get AsyncCompletableFuture<Process[Handle]>. Such operation
could be provided by AsyncCompletableFuture as an "immediate" operation
- not taking any lambda function which needs a thread to be executed,
but a replacement object instead:
public class AsyncCompletableFuture ... {
/**
* Returns a new CompletionStage that, when this stage completes
* normally, completes with the given replacementResult.
*
* @param replacementResult the successful completion value of the
returned
* CompletionStage
* @param <U> the value's type
* @return the new CompletionStage
*/
public <U> AsyncCompletableFuture<U> thenReplace(U
replacementResult) { ...
I can ask on the concurrency-interest list about the feasibility of such
[Async]CompletableFuture split. Would you be interested in using
AsyncCompletableFuture if it was available?
That convenience method could be added later when the use case and
frequency is clearer.
I saw it only as a workaround for needing AsyncExecutor. I don't like it
either.
Regards, Peter
Is there expectation that ForkJoinPoll.commonPool() will not fit in
the common case?
I don't have a sense of how onExit would be used or how often it would
be used.
The common FJP seems like a good start for computations to be performed
after the Process has exited. I'd wait and see what needs can be
articulated.,
hopefully within the JDK 9 timeframe.
Thanks, Roger
Regards, Peter
On 05/13/2015 04:16 PM, Roger Riggs wrote:
Hi,
Are there any comments about the use of java.util.Optional in the
ProcessHandle API?
Or a review of the changes?
Thanks, Roger
On 5/11/2015 11:49 AM, Roger Riggs wrote:
Please review clarifications and updates to the proposed Precess API.
A few loose ends in the ProcessHandle API were identified.
1) The ProcessHandle.parent() method currently returns null if the
parent cannot
be determined and the ProcessHandle.of(PID) method returns null if
the PID does not exist.
It has been suggested to return an Optional<ProcessHandle> to make
these methods more flexible and allow a fluent style and work
better with streams.
2) The behavior of Processhandle.destroy and destroyForcibly are
different
than Process.destroy and destroyForcibly. Those functions always
succeed because
they are children of the spawning process.
In contrast, ProcessHandle.destroy and destroyForcible are requests to
destroy the process and may not succeed due to operating system
restrictions such
as the process not being a child or not having enough privilege.
The description of the methods needs to be clarified that it is a
request to destroy
and it may not succeed, In that case the destroy and
destroyForcibly methods
should indicate that the request was not successful. In
particular, the caller
may not want to wait for the process to terminate (its not going to).
The proposed update is to return an Optional<ProcessHandle> .
It can be streamed and can take advantage of the conditional
operations on the Optional.
3) Using Optional is also attractive for the return values of the
information
about a ProcessHandles, since not all values are available from
every OS.
The returns values of Info.command, arguments, startInstant,
totalDuration, and user
are proposed to be updated to return Optional<x>.
It allows for more compact code and fewer explicit checks for null.
Please review and comment:
Webrev:
http://cr.openjdk.java.net/~rriggs/webrev-ph/
javadoc:
http://cr.openjdk.java.net/~rriggs/ph-apidraft/
Diffs of the spec/javadoc from previous draft:
http://cr.openjdk.java.net/~rriggs/ph-diffs-2015-05-11/overview-summary.html
Thanks, Roger
