[saga-rg] tasks and jobs...
Graeme Pound
G.E.POUND at soton.ac.uk
Wed Feb 8 07:41:15 CST 2006
Andre,
Sure, no problem. Here are two examples of how asynchronous support
could be provided in the Java bindings. The first completely
encapsulates threading within the SAGA.Task model, the second exposes
more of the threading capabilities.
#1 Encapsulated threading model
Following the syntax of the SAGA.Task model, the task factory will
return objects implementing the Task interface for each of the Directory
methods.
//Example #1
DirectoryTaskFactory dtf = dir.createTaskFactory ();
Task t1 = dtf.ls (dir);
t1.run ();
In this example when the method run() is invoked a thread is created and
started internally by the object t1. Inner classes could also be used to
conceal the gruesome details of this.
All management of the state of the thread must be via the Task
interface. This is fine as this is the purpose of the Task interface,
particularly at the level at which the user programs. However the
problem comes when the more detailed control over the thread is
required, this is may be required by more sophisticated implementations
of TaskContainer.
#2 Exposed threading model
If we need to expose fine-grained control over the threading model in
the Java bindings it would be necessary to change the SAGA.Task namespace.
//Example #2
DirectoryTaskFactory dtf = dir.createTaskFactory ();
java.lang.Thread t1 = dtf.ls (dir);
t1.start (); //Or t1.run();
TaskContainer tc;
tc.addTask (t1);
Through this approach the interface exposed to the user is more complex,
however the TaskContainer may be more powerful. Implementations based
in Java 5.0 can use the high level concurrency utilities to control the
threads efficiently (and safely?).
This approach would require the Task namespace to be specific to
asynchronous method invocation, and preclude any link to the
JobManagement namespace - but, of course, I do not think that this is a
problem.
A similar situation exists in C# in which 'tasks' could be returned as
delegates or threads to facilitate the use of high-level threading
support in the language; include the thread pool.
---
Of course it is a tangential issue about the hazards of invoking these
methods asynchronously. For example, the developer of a Java
implementation should ensure that asynchronous methods synchronize
properly.
Also the example of the asynchronous methods called on a SAGA directory
can easily be used to illustrate the potential issues arising from
calling operations that move and copy the contents of a directory. ie:
// Create Task factories
DirectoryTaskFactory dtf = dir.createTaskFactory ();
// Create Tasks
Task t1 = dtf.ls (result);
Task t2 = dtf.copy (source,target);
Task t3 = dtf.move (source,target);
With no control about the order of these operations a user of the SAGA
API could quickly write very unsafe code.
Graeme
Andre Merzky wrote:
> Hi Graeme,
>
> thanks for the pointer!
>
> I still have some questions if you don't mind...
>
> What I have seen at the URL you sent is how to program
> threads in Java. What I am unsure about is, what would a
> Java-SAGA programmer actually _do_.
>
> For example, if he wants to do a async seek on a file, what
> would he do?
>
> Lets try:
>
> - Create a new class which is 'Runnable', and inherits
> 'saga::file'
> - implement 'run' to perform a seek
>
> Can't be, because run is void (or can you change that?)
>
>
> try again:
>
> - Create a new class which is 'Runnable', and inherits
> 'saga::file'
> - have seek not doing anything, but setting args
> - have run perform the seek async
>
> Hmm, again not nice, as your object needs to be very
> stateful: e.g. store args for many reads, which are later
> run
>
> try again:
>
> - saga::file is already 'Runnable'
> - no, wait, it still has only one 'run' method, so I can't
> do a async seek and then async read...
>
>
> You see, I am really on the wrong track - sorry, this looks
> probably very foolish to you...
>
> I would very much appreciate a code example. That usualoly
> helps me most :-)
>
> I will google somewhat, I am sure there are for example
> async file classes around or so..
>
> Thanks, Andre.
>
>
>
> Quoting [G.E.POUND at soton.ac.uk] (Feb 07 2006):
>> Date: Tue, 7 Feb 2006 14:33:34 +0000
>> From: G.E.POUND at soton.ac.uk
>> To: Andre Merzky <andre at merzky.net>
>> Cc: G.E.POUND at soton.ac.uk, Thilo Kielmann <kielmann at cs.vu.nl>,
>> saga-rg at ggf.org
>> Subject: Re: [saga-rg] tasks and jobs...
>>
>> Andre,
>>
>>
>> The benefits of the application of the TaskContainer semantics to job
>> submission are compelling. Whilst the separation of JobService.submitJob()
>> into JobService.create() and Job.run() methods adds a little complexity,
>> the possibility of optimised bulk operations may justify this.
>>
>> The role of the JobService changes a little, it may be used to create Job
>> objects without submitting them to the resource manager. Will
>> JobService.runJob() invoke the Job object that has been submitted to the
>> resource manager?
>>
>> ---
>>
>> I am yet to be convinced that the 'Task' and 'JobManagement' namespaces
>> should be linked. Whilst the semantics of these namespaces are similar they
>> are designed for different purposes; a simple model of asynchronous method
>> calls in the client, and the submission of batch jobs to a remote resource.
>>
>> The advantage of keeping these two namespaces separate is to avoid an
>> unnecessary dependency between these two different areas of the API. For
>> example; if in the future additional methods are required to support
>> asynchronous method calls these would be reflected in the JobManagement
>> package. [Or if the Task namespace were altered to support language
>> specific features, see below]
>>
>> The advantage of linking the namespaces would be to allow all classes
>> implementing TaskContainer to handle Job objects.
>>
>> ---
>>
>> In Java the basis for multithreading support is sub-classing the
>> java.lang.Thread class (or implementing the java.lang.Runnable interface).
>> In practice SAGA implementations could return inner classes that may be
>> called asynchronously (by either of these approaches) for each API method.
>> This is essentially the whole story (prior to Java 5.0), and the whole
>> approach could be encapsulated within the SAGA.Task model.
>>
>> The problem with encapsulation within the SAGA.Task model is that the
>> fine-grain control over the threads (when sub-classing java.lang.Thread) is
>> lost. Furthermore when using Java 5.0 it would not be possible to leverage
>> the high-level concurrency utilities now available
>> (java.util.concurrent.*). For example the task scheduling framework would
>> be appropriate to control the execution of the threads.
>>
>> For more details see:
>> http://java.sun.com/docs/books/tutorial/essential/threads/index.html
>>
>> In C# the model is nicer than in Java. Methods may be invoked asynchronously
>> by creating a 'delegate' for that method using Thread.ThreadStart.
>> Evaluation of the delegates in different threads can be managed at a
>> high-level via the thread pool.
>>
>> When working with these languages it there will be some functionality that
>> it will be important not to preclude. In Java <5.0 there may be little
>> above and beyond that which is available in SAGA.Task of value to most
>> users, and encapsulation may be the correct approach. However the
>> high-level support for concurrency available in Java 5.0 and C# is
>> certainly important. I am unsure whether access to this functionality is
>> best achieved by by-passing, or altering SAGA.Task namespace in the
>> language bindings.
>>
>>
>> Graeme
>>
>>
>>
>>
>>
>> Quoting Andre Merzky <andre at merzky.net>:
>>
>>> Hi Graeme,
>>>
>>> Quoting [G.E.POUND at soton.ac.uk] (Feb 06 2006):
>>>> Andre,
>>>>
>>>> I have a couple of reservations about this action that you may be able
>>> to
>>>> answer.
>>>>
>>>> I had been hoping to avoid implementing the 'Task' namespace in the
>>> java
>>>> bindings and encourage developers to use the language's support to
>>>> threading to allow asynchronous method calls in the client code.
>>> I understood that much from your last comments, but my
>>> limited knowledge does not allow me to give a qualified
>>> answer I'm afraid. Hmm, maybe we can work this out together
>>> :-)
>>>
>>> Could you post some code examples to demonstrate how a
>>> asynchronous seek (for example) would be coded in a java
>>> application?
>>>
>>> The C++ code would be:
>>>
>>> saga::file f (url);
>>> saga::task t = f.seek <saga::task> (off, whence, &pos);
>>>
>>> t.run ();
>>> t.wait ();
>>>
>>>
>>> Next question is, how would a java application manage many
>>> tasks - i.e. is there something similar to a
>>> saga::task_container ?
>>>
>>> saga::task_container tc;
>>>
>>> tc.add (task_1);
>>> tc.add (task_2);
>>> tc.add (task_3);
>>>
>>> tc.run ();
>>> tc.wait ();
>>>
>>>
>>>> I am therefore concerned about creating a dependency between the
>>>> 'JobManagement' namespace and the 'Task' namespace.
>>> I think, for the java bindings that would mean that job
>>> implements the task interface. Apart from the run you
>>> mention below, the semantic of the task interface is
>>> actually included in job already, more or less, only the
>>> methods and states are differently named (that is one
>>> motivation for the proposal really).
>>>
>>>
>>>> The submission of remote jobs is naturally asynchronous, and there are
>>>> natural semantic parallels to the asynchronous model described by the
>>>> 'Task' namespace. However from my reading of the API I understood these
>>> two
>>>> models to be independent in purpose; creating a dependency could hinder
>>> the
>>>> natural description (and development) of these two areas of the API.
>>> Both models are not really different on purpose. Would you
>>> see an advantage of having them truly separate?
>>>
>>>> The idea of a job container for the management of a large number of
>>> remote
>>>> jobs is useful. However the TaskContainter does not appear to be wholly
>>>> compatible; the run() method to start the asynchronous operations is
>>>> unnecessary for jobs that have been submitted to a remote resource.
>>> You are right. Well, that might be somewhat subtle, but in
>>> the example code below, the submit_job() call is accompanied
>>> by c new reate_job(). That creates a job which needs to be
>>> run(), which would make it compatiple to the task model.
>>>
>>> saga::job j1 = job_server.create_job (jd);
>>> // job state is 'pending'
>>>
>>> j1.run ();
>>> // job state is 'running' or so - 'not pending'
>>>
>>> j1.wait ();
>>> // job state is Done or Failed
>>>
>>>
>>>
>>> saga::job j2 = job_server.submit_job (jd);
>>> // job state is 'running' or so - 'not pending'
>>>
>>> j1.wait ();
>>> // job state is Done or Failed
>>>
>>>
>>> That is very similar to the semantics we have for tasks...
>>>
>>>
>>> One reason for this proposal is additionally that we want to
>>> approach the bulk operations soon. Consider a parameter
>>> sweep, where 100.000 jobs are to be run.
>>>
>>> for ( i = 0; i < 100.000; i++ )
>>> {
>>> jobs[i] = job_server.submit (jd[i]); // SUBMIT
>>> }
>>>
>>> for ( i = 0; i < 100.000; i++ )
>>> {
>>> jobs[i].wait ();
>>> }
>>>
>>> As for each submission, you are very likely to have at least
>>> one remote operation (they are independend), that will take
>>> a lot of time.
>>>
>>> Compare that to:
>>>
>>> task_container tc;
>>>
>>> for ( i = 0; i < 100.000; i++ )
>>> {
>>> tc.add (job_server.create (jd[i])); // CREATE
>>> }
>>>
>>> tc.run ();
>>> tc.wait ();
>>>
>>> It is rather straigh forward to optimize the task container
>>> for bulk job submission, or bulk operations in general (we
>>> hope).
>>>
>>>
>>> What I am not sure is, what would that look like in native
>>> java? Are there similar mechanisms?
>>>
>>>
>>> Looking forward to your comments,
>>>
>>> Andre.
>>>
>>>
>>>> Graeme
>>>>
>>>>
>>>> Quoting Andre Merzky <andre at merzky.net>:
>>>>
>>>>> Hi,
>>>>>
>>>>> I just had a discussion with Thilo about the topic, as he
>>>>> and me obviously talked somewhat orthogonal to each other...
>>>>>
>>>>> Well, now we have the same opinion, kind of, and I have
>>>>> barely any bruises... Anyway, I want to summarize our point
>>>>> here, as I probably was not really clear in my initial post.
>>>>>
>>>>> Sorry if re-iteration of the topic bores you...
>>>>>
>>>>> So, we have tasks, which represent async operations, with a
>>>>> couple of states attached, and the ability to call run(),
>>>>> wait() and cancel() on these. And we can collect them in
>>>>> containers, and wait() on many of these tasks conveniently.
>>>>>
>>>>> And then we have jobs, which represent remote executables,
>>>>> with a couple of states attached, and the ability to call
>>>>> run (== create them), wait() and cancel(). And some more
>>>>> methods. And we can't collect them in containers right now,
>>>>> but would like to.
>>>>>
>>>>> You see the similarities, right? Its even more obvious in
>>>>> code:
>>>>>
>>>>> Tasks:
>>>>> --------------------------------------------
>>>>> task_container tc;
>>>>> task t = file.copy <saga::task> (...);
>>>>> t.run ( );
>>>>> t.wait (1.0);
>>>>>
>>>>> tc.add (t);
>>>>> tc.wait ( );
>>>>> --------------------------------------------
>>>>>
>>>>> Jobs:
>>>>> --------------------------------------------
>>>>> job_container jc;
>>>>> job j = job_server.submit (job_descr);
>>>>> j.wait (1.0);
>>>>>
>>>>> jc.add (j);
>>>>> jc.wait ( );
>>>>> --------------------------------------------
>>>>>
>>>>> slightly changed:
>>>>> --------------------------------------------
>>>>> job_container jc;
>>>>> ! job j = job_server.create (job_descr);
>>>>> + j.run ( );
>>>>> j.wait (1.0);
>>>>>
>>>>> jc.add (j);
>>>>> jc.wait ( );
>>>>> --------------------------------------------
>>>>>
>>>>> The similarities are obvious I think. Now, if job would
>>>>> IMPLEMENT the task interface (or inherit from task), we
>>>>> would unify both classes, and hence:
>>>>>
>>>>> - simplify jobs (leave only those methods which are
>>>>> specific to jobs, like migrate, signal, ...
>>>>>
>>>>> - allow to out jobs into task containers, efficiently
>>>>> handling large amounts of jobs and other tasks
>>>>>
>>>>> - have the API and used paradigms more uniform.
>>>>>
>>>>> Also, if later tasks get suspendable, as Gregor rightly
>>>>> suggested, we can move more methods to tasks, w/o breaking
>>>>> the paradigms.
>>>>>
>>>>> In terms of state, following mappings would be appropriate:
>>>>>
>>>>> job::Pending -> task::Pending
>>>>> job::Done -> task::Done
>>>>> job::Failed -> task::Failed
>>>>> job::??? -> task::Cancelled
>>>>>
>>>>> job::Queued,Running,Pre/Poststaging,... -> task::Running
>>>>>
>>>>> So, no adjustements to the statet models are needed AFAICS,
>>>>> apart from Cancelled
>>>>> (Does it make sense on jobs?
>>>>> Should job::stop be job::cancel?
>>>>> Should tasks::cancel be task::job?)
>>>>>
>>>>> Hope that clearifies things. I think Gregor was on target
>>>>> with his remarks, and Hartmut signalled consent as well.
>>>>> And I think I convinced Thilo (Andre rubs his bruises).
>>>>>
>>>>> Unless there is any opposition, I'll go ahead and document
>>>>> that in the strawman then, ok?
>>>>>
>>>>> Cheers, Andre.
>>>>>
>>>>>
>>>>> Quoting [Thilo Kielmann] (Feb 04 2006):
>>>>>> Date: Sat, 4 Feb 2006 22:29:41 +0100
>>>>>> From: Thilo Kielmann <kielmann at cs.vu.nl>
>>>>>> To: saga-rg at ggf.org
>>>>>> Subject: Re: [saga-rg] tasks and jobs...
>>>>>>
>>>>>>> Wouldn't it be useful to have jobs implementing the task
>>>>>>> interface?
>>>>>> Certainly, no.
>>>>>> Jobs and Tasks are two different things, and they are this on
>>> purpose.
>>>>>> However, Tasks always have been the mechanism for asynchronous
>>>>> operation,
>>>>>> which is kind-of obsoleted by having asynchronous ops directly.
>>>>>>
>>>>>> If you want to work on the "S" of SAGA: why not unify both Tasks
>>> and
>>>>> Jobs
>>>>>> into a better "Job" notion, and do local asynchronous operations
>>> via
>>>>>> async, local calls?
>>>>>>
>>>>>> Thilo
>>>>> --
>>>>> "So much time, so little to do..." -- Garfield
>>>>>
>>>>>
>>>>>
>>>>>
>>>>
>>>
>>>
>>> --
>>> "So much time, so little to do..." -- Garfield
>>>
>>>
>>>
>>>
>>
>
>
>
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