Re: [eclipse-incubator-e4-dev] [resources] EFS, ECF and asynchronous

["David Orme" <djo@xxxxxxxxxxxxxxxxxxxxxxxxx>]

Iterators are a special case.  You have to trade off the object construction/GC cost versus the algorithmic complexity of traversing the particular list start-to-finish without an Iterator (ie: O(n^2) for LinkedList; O(n) for ArrayList).

From: http://code.google.com/android/toolbox/performance.html#foreach

"""The enhanced for loop (also sometimes known as "for-each" loop) can be
used for collections that implement the Iterable interface.
With these objects, an iterator is allocated to make interface calls
to hasNext() and next(). With an ArrayList, you're better off walking
through
it directly, but for other collections the enhanced for loop syntax
will be equivalent
to explicit iterator usage."""

I'm not suggesting we should design specifically for the embedded case, but that it would be helpful to be aware of it.


Dave

On Wed, Oct 29, 2008 at 9:39 AM, Ed Merks <ed.merks@xxxxxxxxx> wrote:

David,

So iterators are bad? :-P

Cheers,
Ed


David Orme wrote:

Re: explosions of inner classes

On Android (and probably other embedded VMs) construction is expensive and also leads to more GC pauses.  Google recommends minimising architectures that force a lot of short-lived objects.

Just one more data point for the discussion.

-Dave Orme

On Oct 29, 2008 5:30 AM, "Oberhuber, Martin" <Martin.Oberhuber@xxxxxxxxxxxxx> wrote:

Hi Scott,

to me, Futures and Listeners don't need to be a contradiction.
What's more interesting to me, is how to deal with Progress.
When a Progress Monitor already exists for the client, then
using it makes a lot of sense even if the result is obtained
asynchronously:

final CBFuture<IFileStore[]> childrenF = myFileStore.list(myProgress);
childrenF.chain(new Callback() {
  public void onDone(IStatus result) {
     if (result.isOK()) {
        handleResult(childrenF.get());
     }
  };
});

I'm using class "CBFuture" as an "enhanced Future" that allows
registering Callbacks. Using a Callback style of handling things,
or CBFuture.waitFor() remains up to the client. Note that I'm
using a "chain()" method to indicate that the Framework/Future could
allow chaining multiple callbacks such that one is exeucuted after
the other. Also note how the callback retrieves the result of
computation from the Future, and not from the callback itself.

The problems that I have seen with callbacks in our products
in the past are listed on
http://wiki.eclipse.org/E4/Pervasive_Themes#Becoming_More_Asynchronous

* Much boilerplate code - Closures would be nice to avoid explosion
 of anonymous inner classes, which could cause bloat

* Need clarification on what thread and in what context the
 callback will be called

* When debugging, it is very hard to trace back the flow of
 operation across multiple callback invocations. It can even
 make debuging close to impossible unless some Tracing
 functionality for the callbacks is built into the Framework
 (we ended up doing this in our commercial product).

* Exception handling needs to be clarified. Java6 Future only
 provides Future#isCanceled(), that's not enough since the
 result of an operation might also be an exception. I'm
 introducint "Istatus result" above but that's also not
 optimal.

The synchronous variant needs more verbosity writing it than
one would expect, because cancellation and errors (exceptions)
need to be handled, wrapped and potentially re-wrapped with
Futures:

final CBFuture<IFileStore[]> childrenF = myFileStore.list(myProgress);
try {
  handleResult(childrenF.get());
} catch(CancellationException e) {
  throw new OperationCancelledException(e);
} catch(ExecutionExeption e) {
  throw new CoreException(new Status(/*.blabla*/));
}

although that could perhaps be simplified if we declared some
Eclipse specific implementation of Future which throws the
kinds of Exceptions that we already know (like CoreException
embedding an Istatus) instead of the JRE's ExecutionException
that's really alien to our current code.

Cheers, -- Martin Oberhuber, Senior Member of Technical Staff, Wind River Target Management Project...

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