Thanks Richard, more replies below...
Quoting Richard Eckart de Castilho <[email protected]>:
Hi Nick,
On 02.04.2015, at 01:37, Nick Hill <[email protected]> wrote:
From my point of view, it would be nice if it was possible to
configure the UIMA framework to produce either this new kind of
CAS or the old one without having to exchange a JAR - doing so
statically at initialization time or even dynamically at runtime.
E.g. to allow easily running test cases against both
implementations.
When you say "produce", there shouldn't be any visible difference
in anything output or persisted, the impl is just how the CAS is
stored internally in memory while processing is happening.
It won't be possible to switch the impl being used at runtime.
There are classes for example with the same names but different
impls (e.g. CASImpl). I know this isn't ideal for tests/comparisons
between the two impls but quite a lot of things are currently
tightly-coupled to the heap internals and so switching a jar
doesn't seem too big a price to pay given no other code changes are
needed.
What do you plan to be the ultimate goal of this experiment? Is it
to support different CAS implementations or is it to replace the
existing CAS implementation with a totally different one?
Most things in UIMA are created through factories (not the CAS so
far). So theoretically, one could replace most classes by custom
classes by reconfiguring the framework to use different factory
classes or having the factories produce different implementations.
Can you imagine that as well for the CAS?
For users the implementation shouldn't matter. They shouldn't observe
any functional difference and therefore shouldn't really care if the
impl changes underneath. All consuming code should work as-is, with
the exception of code which accesses 'internals' directly - but I'd
see this as analogous to accessing private fields in some java SDK
class, which breaks when those fields change in a newer SDK version.
As such I don't think it would make sense (or be very practical from a
maintenance pov) to support two implementations concurrently or to
have a factory.
Does it mean that the UIMA-C++ implementation is going to be
discontinued officially?
No, just to clarify no agreements or plans have been made. I just
wanted to initiate a discussion around this as a possible idea.
If we were to pursue this alternate implementation, I don't know of
any reason why the C++ impl would be discontinued. I had just listed
C++ AEs as one of the things which don't yet work with my current
prototype.
Having to recompile the JCas classes is a bit of a blocker to me -
but I remember that Marshall was contemplating about a way to
generate JCas classes at runtime, so this might just be a
temporary blocker.
When I say recompile, I don't mean regenerate using JCasGen, just
recompile .class files from the existing jcas .java files. I would
expect that you would typically only be using one version (other
than for comparison purposes - to validate functional equivalence
and/or compare performance), and so this isn't something that would
need to be done often.
Compiled JCas classes tend to be shipped as part of frameworks. This
means that it will not be possible to switch to a new CAS impl just
by replacing a JAR. It will also mean that components from different
UIMA-based frameworks cannot be mixed and matched anymore unless
some broker like UIMA-AS is used.
The current JCas cover class format is quite old and tightly-coupled
to the heap-based CAS internals. Saying that all new versions of UIMA
must be binary-compatible with these therefore imposes a (somewhat
crippling) restriction on possible internal improvements. You might
say that the current JCas classes break standard
abstraction/encapsulation principles if the expectation is they will
be forever forwards binary-compatible.
It would not be hard on the UIMA side to move to a simpler and more
abstract JCas cover class format that should avoid this problem in
future, but the actual move to such a format would be even more
disruptive than requiring a recompilation (would require a
re-JCasGen), and would have the same issues you mention above.
I managed to make this object-based impl at least source-compatible
with existing jcas cover classes, by 'converting' the impl of methods
called that were intended to make CAS heap changes to actually be
manipulating the FS objects directly.
In one context, we also rely heavily on CAS addresses serving as
unique identifiers of feature structures in the CAS. Does your
implementation provide any stable feature structure IDs,
preferably ones that are part of the system and not actually
declared as features?
Yes, there are various cases where an 'equivalent' of an FS address
is required (for example if the LL API is being used). In this case
the id gets allocated on the fly and will subsequently be unique to
that FS within the CAS. In many cases an FS might never have such
an ID allocated (it's not really part of the non-LL "public" APIs),
but you can always 'request' one.
I imagine that IDs would be necessary to implement stuff like
delta-CAS later on too.
Are any of the changes so far in any way related to potentially
allowing additions to the type system at runtime?
Not directly related; my goal was just to make the implementation
functionally equivalent but threadsafe (and simpler, faster).
But it's possible (not certain) this new impl may impose fewer
barriers to enabling such capability.
What would be the incentive/benefit for the developer of a
UIMA-based framework/applications or for the users of such
frameworks/applications to switch to the new implementation?
That was the "summary of advantages" I had in the original email, I've
included it again below. The primary "external" benefits I think are
the CAS being thread-safe and faster to manipulate. I understand that
many users/developers might not care about these things, just as they
likely wouldn't care about the code footprint or complexity of the
internals, but it also shouldn't adversely impact them to "upgrade" to
a new UIMA version based on this implementation.
I feel that not being able to have more than one thread work on a CAS
at the same time is a major limitation, especially given modern
systems typically have many CPU cores.
- Drastic simplification of code - most proprietary data structure
impls removed, many other classes removed, index/index repo impls are
about 25% of the size of the heap versions (good for future
enhancements/maintainability)
- Thread safety - multiple logically independent annotators can work
on the same CAS concurrently - reading, writing and iterating over
feature structures. Opens up a lot of parallelism possibilities
- No need for heap resizing or wasted space in fixed size CAS backing
arrays, no large up-front memory cost for CASes - pooling them should
no longer be necessary
- Unlike the current heap impl, when a FS is removed from CAS indices
it's space is actually freed (can be GC'd)
- Unification of CAS and JCas - cover class instance (if it exists)
"is" the feature structure
- Significantly better performance (speed) for many use-cases,
especially where there is heavy access of CAS data
- Usage of standard Java data structure classes means it can benefit
more "for free" from ongoing improvements in the java SDK and from
hardware optimizations targeted at these classes
Cheers,
-- Richard
