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From Till Rohrmann <trohrm...@apache.org>
Subject Re: Guarantees for object reuse modes and documentation
Date Thu, 18 Feb 2016 09:50:06 GMT
Judging from our chaining condition

ds.getPushChainDriverClass() != null &&
!(pred instanceof NAryUnionPlanNode) &&    // first op after union is
stand-alone, because union is merged
!(pred instanceof BulkPartialSolutionPlanNode) &&    // partial
solution merges anyways
!(pred instanceof WorksetPlanNode) &&    // workset merges anyways
!(pred instanceof IterationPlanNode) && // cannot chain with iteration
heads currently
inConn.getShipStrategy() == ShipStrategyType.FORWARD &&
inConn.getLocalStrategy() == LocalStrategy.NONE &&
pred.getOutgoingChannels().size() == 1 &&
node.getParallelism() == pred.getParallelism() &&
node.getBroadcastInputs().isEmpty();

it is indeed hard for a newcomer to completely grasp when something is
chained and when not. Also having exceptions to the rule like in the case
of disableObjectReuse and sources which don’t respect that, makes it even
harder to reason about. What I’m wondering about is the actual performance
impact of enforcing our current guarantees in disableObjectReuse mode.
Furthermore, I’m not sure whether we have to tune the disableObjectReuse
mode for performance considering that this is the idea of the
enableObjectReuse mode.

Cheers,
Till
​

On Thu, Feb 18, 2016 at 9:58 AM, Fabian Hueske <fhueske@gmail.com> wrote:

> Thanks Matthias.
> Maybe I should clarify, that I do not want to change the guarantees for the
> enableObjectReuse mode, but for the disableObjectReuse mode.
> The rules for the enableObjectReuse mode should remain the same.
>
>
> 2016-02-18 9:37 GMT+01:00 Matthias J. Sax <mjsax@apache.org>:
>
> > Hi,
> >
> > I like Fabian's proposal. The idea of object reuse is performance gain,
> > and we should not sacrifice this. Even more important is that the rules
> > are easy to understand!
> >
> > -Matthias
> >
> >
> > On 02/17/2016 06:17 PM, Fabian Hueske wrote:
> > > Hi,
> > >
> > >
> > >
> > > Flink's DataSet API features a configuration parameter called
> > > enableObjectReuse(). If activated, Flink's runtime will create fewer
> > > objects which results in better performance and lower garbage
> collection
> > > overhead. Depending on whether the configuration switch is enabled or
> > not,
> > > user functions may or may not perform certain operations on objects
> they
> > > receive from Flink or emit to Flink.
> > >
> > >
> > >
> > > At the moment, there are quite a few open issues and discussions going
> on
> > > about the object reuse mode, including the JIRA issues FLINK-3333,
> > > FLINK-1521, FLINK-3335, FLINK-3340, FLINK-3394, and FLINK-3291.
> > >
> > >
> > >
> > > IMO, the most important issue is FLINK-3333 which is about improving
> the
> > > documentation of the object reuse mode. The current version [1] is
> > > ambiguous and includes details about operator chaining which are hard
> to
> > > understand and to reason about for users. Hence it is not very clear
> > which
> > > guarantees Flink gives for objects in user functions under which
> > > conditions. This documentation needs to be improved and I think this
> > should
> > > happen together with the 1.0 release.
> > >
> > >
> > >
> > > Greg and Gabor proposed two new versions:
> > >
> > > 1. Greg's version [2]  improves and clarifies the current documentation
> > > without significantly changing the semantics. It also discusses
> operator
> > > chaining, but gives more details.
> > > 2. Gabor's proposal [3] aims to make the discussion of object reuse
> > > independent of operator chaining which I think is a very good idea
> > because
> > > it is not transparent to the user when function chaining happens. Gabor
> > > formulated four questions to answer what users can do with and expect
> > from
> > > objects that they received or emitted from a function. In order to make
> > the
> > > answers to these questions independent of function chaining and still
> > keep
> > > the contracts as defined by the current documentation, we have to
> default
> > > to rather restrictive rules. For instance, functions must always emit
> new
> > > object instances in case of disabled object reuse mode. These strict
> > rules
> > > would for example also require DataSourceFunctions to copy all records
> > > which they receive from an InputFormat (see FLINK-3335). IMO, the
> strict
> > > guarantees make the disableObjectReuse mode harder to use and reason
> > about
> > > than the enableObjectReuse mode whereas the opposite should be the
> case.
> > >
> > >
> > >
> > > I would like to suggest a third option. Similar as Gabor, I think the
> > rules
> > > should be independent of function chaining and I would like to break it
> > > down into a handful of easy rules. However, I think we should loosen up
> > the
> > > guarantees for user functions under disableObjectReuse mode a bit.
> > >
> > > Right now, the documentation states that under enableObjectReuse mode,
> > > input objects are not changed across functions calls. Hence users can
> > > remember these objects across functions calls and their value will not
> > > change. I propose to give this guarantee only within functions calls
> and
> > > only for objects which are not emitted. Hence, this rule only applies
> for
> > > functions that can consume multiple values through an iterator such as
> > > GroupReduce, CoGroup, or MapPartition. In object disableObjectReuse
> mode,
> > > these functions are allowed to remember the values e.g., in a
> collection,
> > > and their value will not change when the iterator is forwarded. Once
> the
> > > function call returns, the values might change. Since  functions with
> > > iterators cannot be directly chained, it will be safe to emit the same
> > > object instance several times (hence FLINK-3335 would become invalid).
> > >
> > >
> > >
> > > The difference to the current guarantees is that input objects become
> > > invalid after the function call returned. Since, the disableObjectReuse
> > > mode was mainly introduced to allow for caching objects across iterator
> > > calls within a GroupReduceFunction or CoGroupFunction (not across
> > function
> > > calls), I think this is a reasonable restriction.
> > >
> > >
> > >
> > > tl;dr;
> > >
> > > If we want to make the documentation of object reuse independent of
> > > chaining we have to
> > >
> > > - EITHER, give tighter guarantees / be more restrictive than now and
> > update
> > > internals which might lead to performance regression. This would be
> > in-line
> > > with the current documentation but somewhat defeat the purpose of the
> > > disabledObjectReuse mode, IMO.
> > >
> > > - OR, give weaker guarantees, which breaks with the current
> > documentation,
> > > but would not affect performance or be easier to follow for users, IMO.
> > >
> > >
> > > Greg and Gabor, please correct me if I did not get your points right or
> > > missed something.
> > >
> > > What do others think?
> > >
> > >
> > > Fabian
> > >
> > >
> > >
> > > [1]
> > >
> >
> https://ci.apache.org/projects/flink/flink-docs-master/apis/batch/index.html#object-reuse-behavior
> > >
> > > [2]
> > >
> >
> https://issues.apache.org/jira/browse/FLINK-3333?focusedCommentId=15139151
> > >
> > > [3]
> > >
> >
> https://docs.google.com/document/d/1cgkuttvmj4jUonG7E2RdFVjKlfQDm_hE6gvFcgAfzXg
> > >
> >
> >
>

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