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From Shivaram Venkataraman <shiva...@eecs.berkeley.edu>
Subject Re: "Dynamic variables" in Spark
Date Tue, 22 Jul 2014 21:09:29 GMT
>From reading Neil's first e-mail, I think the motivation is to get some
metrics in ADAM ? --  I've run into a similar use-case with having
user-defined metrics in long-running tasks and I think a nice way to solve
this would be to have user-defined TaskMetrics.

To state my problem more clearly, lets say you have two functions you use
in a map call and want to measure how much time each of them takes. For
example, if you have a code block like the one below and you want to
measure how much time f1 takes as a fraction of the task.

a.map { l =>
   val f = f1(l)
   ... some work here ...
}

It would be really cool if we could do something like

a.map { l =>
   val start = System.nanoTime
   val f = f1(l)
   TaskMetrics.get("f1-time").add(System.nanoTime - start)
}

These task metrics have a different purpose from Accumulators in the sense
that we don't need to track lineage, perform commutative operations etc.
 Further we also have a bunch of code in place to aggregate task metrics
across a stage etc. So it would be great if we could also populate these in
the UI and show median/max etc.
I think counters [1] in Hadoop served a similar purpose.

Thanks
Shivaram

[1]
https://www.inkling.com/read/hadoop-definitive-guide-tom-white-3rd/chapter-8/counters



On Tue, Jul 22, 2014 at 1:43 PM, Neil Ferguson <nferguson@gmail.com> wrote:

> Hi Reynold
>
> Thanks for your reply.
>
> Accumulators are, of course, stored in the Accumulators object as
> thread-local variables. However, the Accumulators object isn't public, so
> when a Task is executing there's no way to get the set of accumulators for
> the current thread -- accumulators still have to be passed to every method
> that needs them.
>
> Additionally, unless an accumulator is explicitly referenced it won't be
> serialized as part of a Task, and won't make it into the Accumulators
> object in the first place.
>
> I should also note that what I'm proposing is not specific to Accumulators
> -- I am proposing that any data can be stored in a thread-local variable. I
> think there are probably many other use cases other than my one.
>
> Neil
>
>
> On Tue, Jul 22, 2014 at 5:39 AM, Reynold Xin <rxin@databricks.com> wrote:
>
> > Thanks for the thoughtful email, Neil and Christopher.
> >
> > If I understand this correctly, it seems like the dynamic variable is
> just
> > a variant of the accumulator (a static one since it is a global object).
> > Accumulators are already implemented using thread-local variables under
> the
> > hood. Am I misunderstanding something?
> >
> >
> >
> > On Mon, Jul 21, 2014 at 5:54 PM, Christopher Nguyen <ctn@adatao.com>
> > wrote:
> >
> > > Hi Neil, first off, I'm generally a sympathetic advocate for making
> > changes
> > > to Spark internals to make it easier/better/faster/more awesome.
> > >
> > > In this case, I'm (a) not clear about what you're trying to accomplish,
> > and
> > > (b) a bit worried about the proposed solution.
> > >
> > > On (a): it is stated that you want to pass some Accumulators around.
> Yet
> > > the proposed solution is for some "shared" variable that may be set and
> > > "mapped out" and possibly "reduced back", but without any accompanying
> > > accumulation semantics. And yet it doesn't seem like you only want just
> > the
> > > broadcast property. Can you clarify the problem statement with some
> > > before/after client code examples?
> > >
> > > On (b): you're right that adding variables to SparkContext should be
> done
> > > with caution, as it may have unintended consequences beyond just serdes
> > > payload size. For example, there is a stated intention of supporting
> > > multiple SparkContexts in the future, and this proposed solution can
> make
> > > it a bigger challenge to do so. Indeed, we had a gut-wrenching call to
> > make
> > > a while back on a subject related to this (see
> > > https://github.com/mesos/spark/pull/779). Furthermore, even in a
> single
> > > SparkContext application, there may be multiple "clients" (of that
> > > application) whose intent to use the proposed "SparkDynamic" would not
> > > necessarily be coordinated.
> > >
> > > So, considering a ratio of a/b (benefit/cost), it's not clear to me
> that
> > > the benefits are significant enough to warrant the costs. Do I
> > > misunderstand that the benefit is to save one explicit parameter (the
> > > "context") in the signature/closure code?
> > >
> > > --
> > > Christopher T. Nguyen
> > > Co-founder & CEO, Adatao <http://adatao.com>
> > > linkedin.com/in/ctnguyen
> > >
> > >
> > >
> > > On Mon, Jul 21, 2014 at 2:10 PM, Neil Ferguson <nferguson@gmail.com>
> > > wrote:
> > >
> > > > Hi all
> > > >
> > > > I have been adding some metrics to the ADAM project
> > > > https://github.com/bigdatagenomics/adam, which runs on Spark, and
> > have a
> > > > proposal for an enhancement to Spark that would make this work
> cleaner
> > > and
> > > > easier.
> > > >
> > > > I need to pass some Accumulators around, which will aggregate metrics
> > > > (timing stats and other metrics) across the cluster. However, it is
> > > > cumbersome to have to explicitly pass some "context" containing these
> > > > accumulators around everywhere that might need them. I can use Scala
> > > > implicits, which help slightly, but I'd still need to modify every
> > method
> > > > in the call stack to take an implicit variable.
> > > >
> > > > So, I'd like to propose that we add the ability to have "dynamic
> > > variables"
> > > > (basically thread-local variables) to Spark. This would avoid having
> to
> > > > pass the Accumulators around explicitly.
> > > >
> > > > My proposed approach is to add a method to the SparkContext class as
> > > > follows:
> > > >
> > > > /**
> > > >  * Sets the value of a "dynamic variable". This value is made
> available
> > > to
> > > > jobs
> > > >  * without having to be passed around explicitly. During execution
> of a
> > > > Spark job
> > > >  * this value can be obtained from the [[SparkDynamic]] object.
> > > >  */
> > > > def setDynamicVariableValue(value: Any)
> > > >
> > > > Then, when a job is executing the SparkDynamic can be accessed to
> > obtain
> > > > the value of the dynamic variable. The implementation of this object
> is
> > > as
> > > > follows:
> > > >
> > > > object SparkDynamic {
> > > >   private val dynamicVariable = new DynamicVariable[Any]()
> > > >   /**
> > > >    * Gets the value of the "dynamic variable" that has been set in
> the
> > > > [[SparkContext]]
> > > >    */
> > > >   def getValue: Option[Any] = {
> > > >     Option(dynamicVariable.value)
> > > >   }
> > > >   private[spark] def withValue[S](threadValue: Option[Any])(thunk: =>
> > > S): S
> > > > = {
> > > >     dynamicVariable.withValue(threadValue.orNull)(thunk)
> > > >   }
> > > > }
> > > >
> > > > The change involves modifying the Task object to serialize the value
> of
> > > the
> > > > dynamic variable, and modifying the TaskRunner class to deserialize
> the
> > > > value and make it available in the thread that is running the task
> > (using
> > > > the SparkDynamic.withValue method).
> > > >
> > > > I have done a quick prototype of this in this commit:
> > > >
> > > >
> > >
> >
> https://github.com/nfergu/spark/commit/8be28d878f43ad6c49f892764011ae7d273dcea6
> > > > and it seems to work fine in my (limited) testing. It needs more
> > testing,
> > > > tidy-up and documentation though.
> > > >
> > > > One drawback is that the dynamic variable will be serialized for
> every
> > > Task
> > > > whether it needs it or not. For my use case this might not be too
> much
> > > of a
> > > > problem, as serializing and deserializing Accumulators looks fairly
> > > > lightweight -- however we should certainly warn users against
> setting a
> > > > dynamic variable containing lots of data. I thought about using
> > broadcast
> > > > tables here, but I don't think it's possible to put Accumulators in a
> > > > broadcast table (as I understand it, they're intended for purely
> > > read-only
> > > > data).
> > > >
> > > > What do people think about this proposal? My use case aside, it seems
> > > like
> > > > it would be a generally useful enhancment to be able to pass certain
> > data
> > > > around without having to explicitly pass it everywhere.
> > > >
> > > > Neil
> > > >
> > >
> >
>

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