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  • YURI: Hi, everyone.

  • My name is Yuri.

  • And today, I'm going to be talking

  • to you about tf.data, which is TensorFlow's input pipeline.

  • As a disclaimer, this presentation

  • assumes familiarity with basic TensorFlow concepts

  • such as ops and kernels.

  • And it contains a lot of code examples

  • that are not necessarily 100% accurate.

  • There may be some details that have been removed

  • because they're either unnecessary or distracting

  • for the purpose of presentation.

  • So with that, let's get started.

  • In this talk, we're going to cover a couple of topics.

  • We're going to peel the two main layers

  • of tf.data's implementation one by one,

  • first focusing on Python view and then on the C++ view

  • of tf.data.

  • And then I'm going to cover three areas of tf.data that

  • might be of interest to the broader audience,

  • support for non-tensor types, and both static and dynamic

  • optimizations in tf.data.

  • So let's get started with the Python view.

  • Throughout the course of the presentation,

  • I'm going to be using the following example, which

  • is a pretty standard example of an input pipeline.

  • What this input pipeline does, it's

  • reading files that are in TFRecord formats--

  • so this contains records--

  • then shuffling those records, applying a map transformation

  • that allows you to transform the records and parse them,

  • pre-process them, and finally, batching the pre-processed data

  • so that it's amenable to machine learning computation.

  • And the idiomatic way to iterate through elements

  • of an input pipeline in TF 2.0 is by a simple for loop.

  • And that's because in TF 2.0, data sets are Python iterables.

  • Besides this approach, you can also

  • use the explicit iter or next keywords.

  • Now, as the comment at the bottom mentions,

  • the user-defined function that you

  • can pass into the map transformation

  • can be both graph or non-graph computation

  • where the non-graph computation is enabled by AutoGraph.

  • And I'll talk a little more about that later on.

  • Just to contrast the simplifications that happened

  • in the transition between 1.x and 2.0,

  • let's take a look at what an input pipeline--

  • or idiomatic iteration of an input pipeline-- in 1.x

  • would look like.

  • And you can see that the definition of the input

  • pipeline that is the top part of the data set remains the same.

  • But the iteration is much more verbose.

  • So hopefully, this kind of illustrates that the simplest

  • way to iterate through a data set

  • has been made much more simple in the 2.0 release

  • of TensorFlow.

  • So let's talk a bit more about what's actually going on when

  • you run the Python program.

  • And what we're going to do is we're

  • going to go through different lines of the Python program

  • and talk about what actually happens under the hood in terms

  • of what types of TensorFlow ops these invocations correspond

  • to.

  • And I'm using a diagram to visualize

  • the different types of ops.

  • The gray box is the actual op-- so

  • in this case, TFRecordDataset-- while the yellow boxes

  • are the different inputs for the op,

  • while the blue box is the output of the op.

  • So in the case of the TFRecordDataset,

  • we have a couple of inputs-- file names, compression types,

  • buffer sizes.

  • And an important thing that I want to highlight here

  • is that this op produces a variant tensor, which

  • is a representation of the data set object

  • that can be passed between different ops.

  • And we will see how that's used right away when we're looking

  • at the map transformation.

  • So the MapDataset op, you can see that one of its inputs

  • is actually a variant, which is the downstream data set that

  • produces the elements that the map transformation transforms.

  • Other inputs are-- they're called other arguments.

  • And these are actually the captured inputs

  • for the function.

  • In this particular case, that input

  • would be empty, because the function doesn't

  • have any captured inputs, at least not as outlined

  • in the example.

  • And the round boxes are not inputs.

  • They are attributes.

  • The difference between inputs and attributes

  • is that the attribute values do not

  • change with different executions of the op.

  • They are constant.

  • And the attributes here are function,

  • which identifies the function parse, which

  • is stored separately in TensorFlow runtime--

  • but it allows the op to look it up when it executes--

  • as well as the type of the arguments

  • that the function inputs.

  • And again, like the TFRecordDataset,

  • it produces an output variant.

  • So a little more about the use of support

  • for user-defined functions in tf.data.

  • A number of tf.data transformations

  • are operations that actually allow users

  • to specify their own functions.

  • Examples of those are filter, flat_map, interleave, map,

  • or reduce.

  • And irrespective of the mode of the execution,

  • tf.data will convert the user-defined function

  • into a graph.

  • And as illustrated on the previous slide,

  • the function graph is--

  • a handle to the function graph is passed to the respective op

  • through an attr.

  • A little more detail on the tracing implementation-- it

  • was originally based on framework.function.Defun

  • and recently switched to the same tracing implementation

  • that's used for TF functions in 2.0.

  • This provided a number of benefits,

  • including control flow version 2,

  • support for resource variables, TensorArrayV2, and also

  • the ability for users to specify user-defined functions that

  • are not necessarily graph-compatible as

  • long as they're supported by AutoGraph.

  • And it's marked as work in progress

  • here because this functionality is actually

  • temporarily disabled.

  • And we're working on enabling it back on very soon.

  • So to tie it together, if we look at the input pipeline

  • definition, the four lines, this definition of an input pipeline

  • will roughly correspond to the following ops, and inputs,

  • and attributes.

  • Now, up to this point, we've only

  • talked about how to define the input pipeline.

  • But naturally, the thing that you

  • would want to do with the input pipeline

  • is that you would like to enumerate the elements

  • inside of it.

  • And that's where the iterator ops come in play.

  • Because iterator, it can be thought

  • of as an instance of a data set that has a state

  • and allows you to enumerate the elements in a sequential order.

  • So what are the iterator lifecycle ops?

  • The op on the left top corner called

  • Iterator that takes no input and produces a single output called

  • handle is an op that creates an empty iterator resource, which

  • is a way to pass the state, iterator state,

  • between different operations, while the MakeIterator op takes

  • two different inputs.

  • It takes iterator resource, which is something

  • that we've created by the iterator op,

  • and a data set variant.

  • And what this MakeIterator op does,

  • it instantiates the data set-- sorry,

  • the iterator resource with that particular data set.

  • So at that point, you have an iterator resource

  • that has been initialized to start producing

  • elements for that particular data set as defined by the data

  • set variant.

  • Now, the actual iteration happens

  • by the means of the IteratorGetNext op,

  • which takes an iterator resource handle

  • and produces the actual elements, which

  • can be a tensor, or a nest of tensors, or possibly

  • also non-tensor types.

  • And later in the presentation, I'll

  • talk about what exactly is supported

  • in tf.data in terms of types.

  • And finally, there's also a DeleteIterator op

  • that takes the iterator resource and makes

  • sure that the iterator state is properly

  • disposed of when the iterator is no longer needed.

  • This final op, as you can imagine,

  • is very important to make sure that iterator resources are not

  • being left behind.

  • Because it is not uncommon for the iterator resource

  • state to amass hundreds of megabytes or gigabytes

  • of memory.

  • And leaving these around can result in your computation

  • running out of memory.

  • As a side note, when you're looking at the performance

  • or profiling performance of your program or input pipeline,

  • you will see IteratorGetNext op in something

  • like a timeline, or an [INAUDIBLE],, or CPU profile

  • trace.

  • And this is the op that indicates the output

  • latency of your input pipeline.

  • And so if that op is very small in its runtime--

  • I would say on the order of microseconds--

  • it means that your input pipeline is not a bottleneck.

  • And if it's larger than that, chances

  • are you are bottlenecked by input, at least to some extent.

  • So now that we've talked about the different ops, let's

  • actually see how the execution of the Python program

  • corresponds or maps to the execution-- or creation

  • and execution-- of the different ops.

  • And what I'm going to do is I'm going to contrast the TF 2.0

  • eager mode style of execution with the TF 1.x graph mode

  • style of execution to help folks understand what are

  • the differences between TF--

  • between the two modes as far as tf.data is concerned.

  • So let's start with the 2.0 eager mode.

  • In eager mode, ops are created and executed

  • as the program runs.

  • So when the Python line that creates the TFRecodrDataset()

  • runs, we end up creating-- both creating and executing--

  • the RFRecordDataset() op.

  • And similarly, with the next line,

  • we create and execute the shuffle() data set op,

  • feeding the output of the previous op inside of it

  • as part of the input variant.

  • That way, we tie-- we're starting to build the input

  • pipeline, trying it to--

  • connecting the two stages together.

  • When the .map transformation is executed,

  • the user-defined function is traced and stored

  • in the TensorFlow runtime.

  • And a handle to it is passed, as an attribute,

  • to the map data set op along with the input variant

  • representing the input pipeline up to that point.

  • And finally, the batch() op is created and executed,

  • creating the final stage of the input pipeline.

  • Now, when the idiomatic way of iterating through tf.data is

  • used-- that is, the for loop for element in data set--

  • what happens under the hood is that an entire method is

  • called on the data set object.

  • And that actually triggers the creation and execution

  • of two ops.

  • We first create the iterator resource through an op.

  • It's called Anonymous Iterator.

  • And I'm going to point out the difference between that

  • and the iterator as I talk about the graph more execution.

  • And then we associate the iterator resource

  • with the input pipeline that we've created

  • via the MakeIterator op.

  • And as the Python for loop iterates,

  • we end up invoking next() on the Python iterator object.

  • And this translates to the IteratorGetNext op

  • being created and subsequently executed.

  • It's only created once, and it's executed as many times

  • as there's elements in the data set.

  • And finally, when the Python iterator object goes out

  • of scope, the DeleteIterator op is invoked,

  • which makes sure that the iterator state, iterator

  • resource state, is properly disposed of.

  • So let's contrast that with how this

  • would work in 1.x graph mode.

  • So in graph mode, the execution happens lazily,

  • which means we create the ops as the Python lines are invoked.

  • But they're not executed--

  • the execution is postponed until the particular ops are

  • run using decision mechanism.

  • So just stepping through the program,

  • we see that we are building a graph but not executing it.

  • There is a particular mechanism for creating the iterator

  • resource op and the MakeIterator op,

  • as well as creating the op that is later used for iteration.

  • And it's only within the run part of your program

  • that ops are executed.

  • When the iterator initializer op is executed,

  • we actually end up executing the entire graph of the input

  • pipeline, including the iterator op.

  • Now, the difference between the iterator op

  • and the anonymous iterator op that was used in the eager mode

  • is that anonymous iterator op creates a new resource

  • every time it's executed, while iterator op creates

  • a resource only the first time it's executed.

  • And any subsequent execution returns a handle

  • to that resource.

  • And the reason for that is when we run the get_next op,

  • that get_next op will actually execute the iterator op as well

  • by the nature of the difference between the graph

  • mode and eager mode executions.

  • And so that's kind of an artifact of graph mode

  • execution.

  • And thus we need different types of resource creation

  • ops for eager mode and graph mode inside of tf.data.

  • And there is no explicit delete iterator op.

  • And that's because the iterator resource

  • lifetime is tied to the lifetime of the surrounding session.

  • And so when the session is destroyed,

  • so is the iterator resource.

  • So far, so good?

  • OK.

  • So let's now, after--

  • now that we've kind of peeled the Python layer,

  • and we talked about an op-level view of tf.data, let's dive

  • a level deeper.

  • And let's talk about what actually

  • happens inside of the kernels that implement these ops.

  • And like with most other TensorFlow op kernels,

  • these are implemented in C++.

  • So before we retrace our steps or take a look at the example

  • program from a C++ point of view,

  • let's talk about what are the important tf.data C++

  • abstractions.

  • So the top-level one is a data set op kernel which

  • implements the op kernel API.

  • And this provides a mechanism for implementing different

  • types of data set ops through a single interface where

  • the different implementations of the data set op kernel

  • interface just need to override or implement the MakeDataset()

  • method.

  • What the MakeDataset() does, it returns a DatasetBase object.

  • Now, the purpose of the data set op kernel is to provide

  • a translation between a graph representation of the op

  • and a C++ representation of the op.

  • Now the data set object--

  • DatasetBase object-- in turn has a method for creating

  • an iterator for that particular data set as well as a method

  • called AsGraphDef(), which provides the reverse of what I

  • just talked about, which allows you to basically go from a C++

  • representation of a data set back to a graph representation

  • of a data set, which will come in handy when I talk about

  • static optimizations of tf.data.

  • The MakeIterator() method of the DatasetBase returns

  • an IteratorBase, which is an interface representing

  • the different types of iterators we have.

  • And the single most important method in that interface is

  • GetNext(), which is the actual C++ method used for iterating

  • through the state of the input pipeline.

  • And coupled with that is the IteratorResource,

  • which holds the state.

  • And so as we will see, the IteratorResource is actually

  • the entry point into the connected structure

  • of different C++ iterators through which ops like

  • IteratorGetNext get receive data.

  • And the SetIteratorFromDataset() method corresponds

  • to the MakeIterator op, as we'll shortly see.

  • Last but not least, tf.data has two C++ abstractions

  • for representing functions, its CapturedFunction

  • and InstantiatedCapturedFunction.

  • The CapturedFunction provides a mechanism

  • for bundling a function with its captured inputs

  • and later instantiating it.

  • And the InstantiateCapturedFunction

  • provides tf.data with a mechanism to actually run

  • the user-defined functions.

  • And so you can perhaps see how there

  • is a simple relationship between DatasetBase and IteratorBase

  • and CapturedFunction and InstantiatedCapturedFunction

  • where the letter is an instance of the former in both

  • of those cases.

  • All right, so let's go back to our example.

  • And now we're going to take a look at what happens when we

  • execute the different lines of the input pipeline,

  • but what happens in the C++ world.

  • And unlike in the previous Python view section,

  • in this section, the diagram at the bottom will not be graph

  • objects, but they will be C++ objects.

  • So in this case, the TFRecord Data set down below

  • is actually an instance of the TFRecord Data set

  • that's of type DatasetBase.

  • And for context, we are, again, in TF 2.0 eager mode.

  • So when the Python program executes a tf.data TFRecord

  • Data set with files argument, we end up creating the data set

  • variant through the following C++ code.

  • And just for illustration, I'm showing here

  • how that op kernel fetches the set of file names.

  • And the set of file names can either

  • be a single string or a list of strings.

  • So there is some subsequent string parsing

  • that's elided here.

  • But the important bit here is that we're then

  • storing the TFRecord Data set op data set in the output, where

  • the data set object itself is a variant, which

  • allows us to pass it on as an input to another op.

  • And that another op is the ShuffleDataset, which gets

  • executed immediately after.

  • And so here I'm illustrating how the op receives

  • or kind of extracts the variant tensor input from the op kernel

  • context and then passes it inside of the ShuffleDatasetOp

  • so that the ShuffleDatasetOp now understands

  • what stage is producing elements for it to consume.

  • Next, it's the map transformation.

  • What I want to illustrate here is how the CapturedFunction

  • mechanism works.

  • We use the CapturedFunction Create factory

  • that takes the identifier of the function, which

  • is a list of attributes, as well as

  • any captured inputs if there were any stored in the op

  • kernel context.

  • And similar to the ShuffleDatasetOp,

  • we end up passing the captured function as well as the input

  • to the downstream data set inside of the constructor.

  • And finally, there is not much new to see here

  • for the BatchDatasetOp.

  • So I pretty much elided all the details from this slide.

  • OK, so now for the interesting stuff,

  • because this is where we are going to start

  • iterating through the data set.

  • So the first thing that happens when you call--

  • or when you write "for element in dataset"--

  • under the hood, this gets translated

  • to a Python iter invocation on the data set object.

  • And the first thing that happens is

  • that we create the anonymous Iterator Resource.

  • And here is just an illustration of the actual mechanism that

  • does this as well as the code that then produces

  • the handle to the iterator.

  • And this handle, along with the variant tensor

  • representing the batch data set is then

  • passed to the MakeIteratorOp.

  • So here you can see how we extract both the data set

  • variant as well as the resource handle and use these two

  • to pass them into the SetIteratorFromDataset() method

  • that, as we will shortly see, will, in a cascading fashion,

  • create a sequence of connected iterator objects.

  • So let's take a closer look at what SetIteratorFromDataset()

  • does.

  • It takes a look at the outermost data set,

  • because that's the variant that it received.

  • And it invokes MakeIterator on that particular data set.

  • And this will prompt a creation of a Batch Iterator using

  • the MakeIterator() method on the Batch Data set,

  • but also trigger a recursive MakeIterator() invocation

  • on the input of Batch Data set, which is Map Data set.

  • And so in that fashion, the Map-- in a similar fashion,

  • Map Iterator is created, where the Map Iterator

  • creator will also instantiate the captured function.

  • So now we'll see that we have a parse_fn instance in addition

  • to just parse_fn CapturedFunction object.

  • And similarly, we create a Shuffle Iterator, and finally,

  • the TFRecord Iterator.

  • And because TFRecord Data set has no inputs,

  • this is where the recursive creation of the input pipeline

  • state stops.

  • And the control bubbles up back to IteratorResource, that

  • returns a resource handle--

  • actually, MakeIterator() doesn't return a resource handle.

  • We already have the resource handle.

  • AUDIENCE: Question.

  • YURI: Uh-huh?

  • AUDIENCE: Does the input pipeline have to be a line?

  • Or can it be a DAG?

  • YURI: So at the data set level, the input pipeline

  • can be a DAG.

  • At the iterator level, it will always

  • be a tree, if that makes sense.

  • OK.

  • So next, let's take a look at next().

  • So when the IteratorGetNextOp is invoked because we're starting

  • to iterate through the elements of the input pipeline,

  • we again look up the resource using to LookupResource()

  • method and then call to GetNext() method

  • on the resource.

  • And as I mentioned earlier, iterator resource

  • is thus the entry point to the state of the iterator.

  • And what happens is this recursively calls GetNext()

  • on the Batch Iterator.

  • And the Batch Iterator says, well, I need

  • batch size worth of elements.

  • So let me get them one by one.

  • So it calls to Map Iterator to say, please give me an element.

  • Map Iterator says, well, I need an element

  • to apply a user-defined function on.

  • So it's going to ask Shuffle for one.

  • And Shuffle says, well, I need a buffer size worth of elements

  • to do reasonable shuffling.

  • So it's going to call to TFRecord Iterator.

  • And TFRecord Iterator will say, OK, well, I have these files,

  • and I'm going to open and start reading elements out of them.

  • So at this point, we start returning data back

  • up this round trip between Shuffle and TFRecord Iterators.

  • It might happen multiple times initially.

  • And at some point, Shuffle or has filled its buffer

  • off of elements used for shuffling,

  • and produces a random element back up to the Map Iterator,

  • which then applies the user-defined function on it,

  • and takes its output, and returns it back

  • to the Batch Iterator.

  • And this would be repeated batch size number of times.

  • Then the Batch Iterator would take all those elements

  • and create one higher-level, higher-dimensional element out

  • of the individual slices of the batch

  • and pass it on to Iterator Resource.

  • And that would get created--

  • returned out to the Python program.

  • Now finally, when the Python iterator goes out of scope,

  • the Iterator Resource is deleted.

  • And in a cascading fashion, the other iterators

  • get created because their ref count goes to 0,

  • or, we actually use smart pointers that

  • are kind of connected between the different iterator objects.

  • So far, so good?

  • Any questions?

  • OK.

  • So up to this point, I talked primarily

  • about input pipeline that wasn't trying to be performance-savvy.

  • And if it wasn't obvious from the walkthrough

  • of the stages of the iterator, it

  • seems like there's a lot of steps

  • that would need to happen to produce a single batch.

  • And if all these steps lie on the critical path

  • of your computation, then you're probably not executing

  • at the best possible performance,

  • or at the peak of your performance.

  • So we have a tf.data performance guideline

  • that talks about different mechanisms

  • to make sure that your input pipeline is performant.

  • And the three main ones are software pipelining,

  • processing parallelization, and I/O parallelization.

  • And they all use various performance artifacts,

  • either buffers or parallelism, to allow

  • users to specify how their input pipeline should be executed.

  • And in the context of the parallelism--

  • and actually, pre-fetching as well--

  • they all map down to asynchronous threads

  • being started by the data set of kernels.

  • And they are running in the background,

  • disconnected from the GetNext() calls,

  • generating values into an internal buffer.

  • And when a GetNext() call arrives,

  • it just waits until there is something that buffer

  • and returns it.

  • So in the ideal case, there is data in the buffer,

  • and you don't need to wait at all.

  • But in case your consumer of the data

  • is faster than your producer, then

  • you might wait some of the time, but hopefully not all

  • of the time.

  • So let's just take a look at how this would change the diagram

  • that we talked about.

  • So what I did for the sake of an illustration is I

  • added the num_parallel_calls argument

  • to the map transformation as well as

  • a prefetch transformation at the very end of the input pipeline.

  • AUDIENCE: So you can prefetch anywhere [INAUDIBLE]??

  • YURI: You can.

  • Yes.

  • Rule of thumb is that the one at the very end

  • is usually the one that you get the most mileage out

  • of because it allows you to overlap the computation

  • the entire pipeline with the computation that

  • might be happening in the model, either on [INAUDIBLE]..

  • But yes, in theory--

  • I have a pertinent quiz later on.

  • We'll see that prefetching or decoupling the producer

  • and consumer anywhere throughout you input pipeline

  • might be a good idea.

  • So the changes that reflect what happened, or changes

  • in the pipeline code map to the following changes

  • in the diagram.

  • We now have a Prefetch Iterator and Prefetch Data Set

  • at the end of the pipeline.

  • And we also have a ParallelMap Iterator and ParallelMap

  • data set, instead of just regular Map Iterator, Map Data

  • Set.

  • It turns out with tf.data, we have a different op kernel

  • from the one that uses parallelism.

  • AUDIENCE: Do you support Racket tensors yet?

  • YURI: No.

  • But the CL that introduces that support on--

  • actually a couple of CLs-- but we're very close.

  • AUDIENCE: Excellent.

  • And do you see any performance decreases

  • when using Racket tensors?

  • YURI: So because we are not supporting them yet,

  • I don't have a good answer to that.

  • AUDIENCE: Yeah.

  • YURI: I think, through the course of the review process

  • for bringing that support in, we've

  • been cognizant of making sure that the implementation is

  • efficient so that it works well out of the gate.

  • AUDIENCE: Yeah.

  • But you support sparse tensors?

  • YURI: We do.

  • AUDIENCE: OK.

  • YURI: Yeah.

  • So the hope is that the programs that

  • use sparse tensors that also use Racket tensors will see

  • a performance boost by switching to Racket

  • tensors once that support is rolled up.

  • AUDIENCE: Excellent.

  • Thank you.

  • YURI: Mhm.

  • And so the runner threads that are our

  • illustrated here, these are the background threads that

  • decouple the producer and consumer in the Prefetch

  • Iterator and the ParallelMap Iterator respectively.

  • And they're actually not started until the very first getNext

  • invocation.

  • So before you call getNext for the first time,

  • the iterator is idle.

  • There's no activity happening.

  • But the moment you start fetching data out

  • of the iterator, background threads might be started

  • or thread pools might be created that might start

  • performing a background entity.

  • An interesting, exciting thing, or a consequence of this,

  • is that a traditional way to look

  • at the performance of TensorFlow would be a timeline, which

  • gives you a view into what happens

  • in the context of a single stack.

  • And this particular abstraction doesn't match well

  • with the asynchronous nature of tf.data execution.

  • In addition to that, you will only

  • see the iterator get mixed up, which might not necessarily

  • give you a good view into what's actually

  • happening at the different stages of the tf.data

  • into pipeline.

  • And so at the recent Dev Summit, we

  • announced that there is going to be an open source

  • version of a tool that we've had available internally

  • for some time that provides you with all

  • these details of information so that you can debug

  • the performance of your input pipeline

  • using the same tools that we use internally.

  • OK.

  • So that concludes the section where I talked about C++

  • and what happens in C++.

  • And there's a little bit of a switch

  • now, because we're going to go back to Python level

  • and talk about supporting non-tensor types.

  • So tf.data supports more than just regular tensors.

  • The different inputs or outputs of your data

  • transformations can actually be a number of different things--

  • sparse tensors, tensor arrays, nests

  • of any of these optionals, as well as nested data sets.

  • And here, I just illustrate it.

  • It's not an exhaustive list, but I just

  • illustrate some of the transformations in terms

  • of the types that they support either

  • as an input or an output.

  • AUDIENCE: What about NumPy arrays?

  • YURI: NumPy arrays are supported as well.

  • They kind of fall into the category of tensors by virtue

  • being of trivially convertible to tensors.

  • AUDIENCE: Yeah.

  • YURI: Similar to NumPy, there's something

  • called SparseTensorValue, which is really

  • just a Python namedtuple return type.

  • And that's kind of the NumPy equivalent for sparse tensors.

  • And I think Racket tensors have the same.

  • They have a Racket tensor value.

  • AUDIENCE: With the caveat in [INAUDIBLE] and in eager,

  • you don't actually need the value types

  • because you can have a sparse tensor or a Racket tensor whose

  • values are eager tensors, which are trivially

  • convertible to NumPy arrays.

  • YURI: Yeah.

  • Yeah.

  • So the value type is an artifact of graph mode 1.x.

  • So the mechanism that tf.data uses under the hoods

  • could provide support for these different types

  • is the tf.data structure API, which is this interface

  • and will require any type to be supported in tf.data

  • to implement this interface.

  • I'm not going to talk about each of these,

  • but the list of methods is neither short nor long.

  • So for example, the support for TensorArray

  • was introduced less than a month ago.

  • And it was one day of work.

  • So I don't think that the overhead of introducing

  • the support for the type, as long as it's

  • kind of natural how to implement this method is very large.

  • Having said that, the support for Racket tensor

  • has been in the works for some time.

  • And part of it is because we actually

  • want that implementation to be very performant.

  • And so it prompted creation of new C++ kernels to make sure

  • that the performance is good from the get-go.

  • Instead of talking about the individual

  • methods in the interface, what I want to do here

  • is I want to illustrate how this interface is actually

  • used to provide the polymorphism at the Python level

  • for different types of tf.data transformations.

  • So for instance, if we look at the tf.data data set

  • from tensors transformation, which is a data source that

  • just take a memory array and use it as a data set source, what

  • the implementation at the Python level does is it computes,

  • or it involves the structure from value methods,

  • to compute an instance of the structure object and stores

  • internally in its attribute.

  • And then it passes the output of structure to tensor arrays,

  • to the op kernel--

  • so the tensor data set op kernel.

  • I forgot to mention earlier, at the C++ level,

  • tf.data only deals with flat lists of tensors.

  • And so we need a mechanism to go between that representation

  • and the Python numTensor nested structure

  • of also arbitrary types.

  • And it's the to_tensor_list and from_tensor_list

  • that provide us with this boxing and unboxing, if you will,

  • between the two representations.

  • from_tensor_slices is similar.

  • The difference between from_tensors

  • and from_tensor_slices is that instead

  • of viewing data as a single tensor, we end up slicing it.

  • We assume that the value has a rank of at least 1.

  • And we end up slicing it into however many slices

  • it has in the dimension.

  • And these are the invocations of the structure API

  • that would allow us to do this kind of agnostically

  • to the actual type of the Python value.

  • I also want to illustrate how the structure

  • API is used in the context of user defined functions.

  • In particular, the function that we end up tracing

  • is actually a different function than just the function the user

  • process.

  • We end up wrapping the function that

  • gets passed in the Python program in invocations

  • to from_tensor_list and then to_tensor_list.

  • And this is kind of the glue that I

  • talked about where we make sure that the Python record expects

  • a flat list of tensors.

  • And then we reconstruct the structure and the typing,

  • using the from_tensor_list invocation,

  • because that's what user provided function expects.

  • And then we again deconstruct the structure and box

  • all the known tensor types and tensors,

  • because that's what the upstream transformation of tf.data

  • expects.

  • All right.

  • So next up, let's talk about static optimizations,

  • which illustrates that everything that I talked

  • about up to this point about how tf.data works

  • is only part of the truth.

  • So in tf.data, we have a number of transformations or a number

  • of optimizations implemented.

  • Here is a subset of the ones who we have.

  • And the ones that are in italics are

  • the ones that enabled by default.

  • And the ones that are not italic,

  • then those can be enabled through tf.data options.

  • This is how you would, for example,

  • go about enabling a map vectorization transformation

  • if you wanted to.

  • The tf.data options has other features.

  • It's not just for optimizations, it is also, for example,

  • for specifying threading or statistics collection features.

  • But in the context of static optimizations,

  • I'm just illustrating how it's used for those.

  • So what happens at the Python level when iterator is created

  • is that the data set Python object has an options

  • object associated with it.

  • And we use the information in the options

  • object to possibly chain additional data set

  • transformations on the end of the data set.

  • This is something that the user doesn't see in their code,

  • doesn't write in their code.

  • It's something that we do at the Python level

  • to allow us to insert functionality

  • that we would like to insert.

  • And one of the main uses of this is this optimized data set

  • that's used as an entry point for any static optimizations

  • that are to be applied to the input pipeline.

  • So if we take a look at the C++ level,

  • what's happening inside of the optimized data set kernel is

  • we'll again get the input of the data set,

  • and then invoke a data set optimized method

  • on the optimized data set kernel.

  • And the code is actually quite long,

  • so I just summarized it in high level statements here.

  • This is what happens inside of the optimized method.

  • We first use the AsGraphDef functionality to go from

  • the C++ representation of the input object to GraphDef

  • representation of the input object.

  • We then use Grappler to apply the subset of tf.data

  • optimizations that are either enabled by default,

  • or explicitly enabled by a user, which

  • will give us a transformed GraphDef representing the data

  • set.

  • And we then convert the rewritten GraphDef to a C++

  • representation using GraphRunner.

  • And finally, we update the input with the result.

  • So because map and batch optimization

  • is one of the optimizations enabled by default,

  • the map and batch stages that were in our example

  • would be, in fact, replaced with a single map and batch data

  • set, which is a more performant version of diffused map

  • and batch transformation.

  • And last topic that I want to talk about

  • are dynamic optimizations.

  • So I mentioned before that users have

  • a number of mechanisms to make their input pipelines more

  • performant.

  • They can insert prefetching with buffer_size.

  • They can insert map with num_parallel_calls

  • or interleave with num_parallel_calls

  • to apply various performance optimization tools.

  • But what are good values of these arguments,

  • like buffer_size and num_parallel_calls?

  • Well, so in the context of this section,

  • I'm only going to focus on the parallelism optimization.

  • And to get you interested, I have a quiz for you.

  • [LAUGHTER]

  • So imagine that you have an input

  • pipeline that looks like this.

  • It reads from a set of files, and then it

  • applies two transformations.

  • And on the right-hand side in the comment,

  • I mentioned how much time is required

  • to get a single element, assuming constant processing

  • time through the particular stage of the input pipeline.

  • So with this information, how much time do you

  • think you would need, when you call getNext,

  • how long would it take to get a single element out?

  • AUDIENCE: 200 milliseconds.

  • YURI: 320.

  • AUDIENCE: 320.

  • YURI: 320 is the answer, because everything

  • executes sequentially.

  • So when you call into the outermost map,

  • that goes recursively into the innermost

  • or the inner map, which calls recursively

  • into the tf record data set.

  • We spend 20 milliseconds there, then we

  • spend 100 milliseconds in the inner map executing f, and then

  • 200 milliseconds in the order map executing g.

  • So it's 320.

  • I think you kind of jumped one step ahead here.

  • [LAUGHTER]

  • And this was supposed to be a trick question,

  • but you already got the answer.

  • What happens here if we just add num_parallel_calls

  • to the map transformations?

  • Nothing, except something happens.

  • And the reason for this being 200 milliseconds

  • is that num_parallel_calls uses a different op kernel which

  • has a background thread that will be performing activity

  • independently of the consumer of the data.

  • So the very first element will actually take 320 milliseconds.

  • But then over time, there is going

  • to be the processing done for the three different stages will

  • be actually overlapped because there is now two background

  • threads doing everything in parallel.

  • The parallelism is 1 at each stage,

  • but that still gives you 200 milliseconds,

  • in total, in the stable state, assuming--

  • AUDIENCE: Is that the correct mental model to think

  • that this implies prefetch?

  • YURI: Yes.

  • That a very good mental model.

  • Parallel map is, in fact, the prefetching

  • by the virtue of using a background thread.

  • And so in a way, this is an answer to your question

  • where prefetching inside of the input

  • pipeline, not just at the very end, might provide benefits.

  • AUDIENCE: Question.

  • So does that imply that the map function has to be traced?

  • Like, for example, if the map function is just a Python

  • function, and if you have multi-threading

  • on Python function, then really [INAUDIBLE]??

  • YURI: So I think the answer is that you

  • will get these benefits, irrespective of the cost,

  • the constant cost of the Python function is--

  • of the function passed into the map transformation.

  • If it's implemented as a py_func,

  • that function itself might be--

  • oh, I see what you're saying, that multiple functions would

  • be escaping into Python.

  • That's a good point.

  • Possibly.

  • I would want to convince myself that they actually

  • are all content for the same [INAUDIBLE]..

  • AUDIENCE: Either way.

  • AUDIENCE: If you ever use Python,

  • you need to make sure your Python is thread safe.

  • It's very hard for a TensorFlow runtime

  • to not accidentally run Python code from many threads

  • at the same time.

  • YURI: Yeah.

  • I think the bottom line here is that if you

  • can avoid using py_func, for instance, use autograph.

  • So it perhaps might not be surprising

  • that if you increase the values of num_parallel_calls,

  • because you know how much time you're

  • going to spend in each of those stages,

  • you can get to the optimal output latency of this input

  • pipeline.

  • You cannot run any faster than the slowest part of the input

  • pipeline, which in this case is the sequential TFRecordReader.

  • There might actually be a way to speed this up even further,

  • by using interleave, num_parallel_calls

  • over the different readers.

  • But instead of exploring that avenue, what I want to ask

  • is, what do you think happens here?

  • AUDIENCE: Dismal performance.

  • YURI: Yeah.

  • I think the answer is, it depends.

  • It might actually run well, well enough,

  • because num_parallel_calls doesn't

  • mean that you create that many threads, at least

  • in the case of map transformation anyhow.

  • It means that you allow to schedule as many ops

  • into the interop thread pool at the same time.

  • And because you're allowed to schedule them at the same time,

  • you need a place to store them.

  • So if nothing else, the downside of specifying

  • a very large value of num_parallel_calls

  • is that you're going to use more memory to store

  • these intermediate values than you would actually

  • need for equal performance, which

  • might hurt your temporal locality or thread locality.

  • So yes, the performance might actually become worse,

  • but the reasons for why can be subtle and

  • environment-specific.

  • AUDIENCE: You said earlier that ops create their own threads.

  • Is that actually the case?

  • Or do they use the shared thread pools in the executor?

  • YURI: They create their own thread.

  • Parallel map, prefetch end up creating their own thread.

  • Parallel interleave creates its own thread pool.

  • Under the hoods, they end up using this abstraction

  • of an unbounded thread pool, if you are familiar with that,

  • which was introduced recently to combat memory fragmentation

  • issues in the open source memory allocator,

  • resulting from excessive thread creation.

  • So the unbounded thread pool that the tf.data uses creates

  • this illusion of logical threads that are mapped onto a smaller

  • subset of physical threads.

  • But they're different from the inter_op thread pools

  • or any of the core TensorFlow runtime thread pools.

  • We do rely in tf.data data on the inter_op thread

  • pool for the execution of the user-defined functions,

  • by default. But there is also an option to override that.

  • And we also, by default, take advantage

  • or inherit the setting of the inter_op_parallelism.

  • And there is also a way to override that just

  • for the tf.data ops.

  • And as a matter of fact, our experience

  • has been that disabling inter_op_parallelism for--

  • and this is perhaps not a surprise to you,

  • but disabling inter_op_parallelism

  • altogether for CPU content input pipelines

  • gives you 10%, 20% speed up.

  • Because you don't need to parallelize the individual ops,

  • you get the parallelism by running multiple of them

  • in parallel.

  • AUDIENCE: Do we have any guides internally about how

  • you can do tricks with tf.data to get

  • performance enhancements?

  • Or not so much?

  • YURI: Great question.

  • So instead of having guides, why not

  • just have tf.data do the hard work for you?

  • AUDIENCE: Excellent.

  • YURI: And so this is something close to my heart

  • because I worked both on tf.data performance in the early days

  • in terms of exposing these knobs,

  • and then I worked very hard on making sure

  • that users don't need to use these knobs because there

  • is something that does a good enough job automatically.

  • And you might think, well, this seems

  • like a good fit for reinforcement learning,

  • since we're in RMI.

  • And that's something that I explored as an idea.

  • I didn't get it to work, but that

  • might be because of me, not because of reinforcement

  • learning.

  • [LAUGHTER]

  • The issue with reinforcement learning-- so the idea is this.

  • What if you just pick some values for the different paths

  • and knobs in your input pipeline,

  • observed behavior, and then try some different values

  • and use a smart algorithm that kind of converges,

  • hopefully, to global optimum.

  • Well, it turns out that the convergence was very slow.

  • And if you set abysmal parameters for something

  • that could be heavily parallelized,

  • then you would need a very long time

  • to actually realize that this is slow because

  • of poor parameters, as opposed to this

  • is slow because that's how fast the input pipeline runs.

  • So instead of exploring reinforcement

  • learning that tries to modify the parameters, or tf.data

  • has an analytical model that models

  • the performance of the input pipeline

  • that's currently instantiated.

  • And there's a little bit of math in my presentation

  • because I'm originally a formal verification person.

  • So here's some math for you.

  • You can model the output latency of a node

  • as a function of the output latencies of its inputs.

  • And the challenge was, how do you

  • implement these two functions-- the processing time of a node,

  • as well as the lambda function that captures

  • what the node itself does?

  • So the processing time is modeled through a lightweight

  • instrumentation of the C++ implementation we have.

  • By lightweight, I mean it imposes

  • roughly 100 nanoseconds of overhead on the critical path.

  • And we keep track of node creation, deletion,

  • as well as the computation, the processing time,

  • spent within a node and a user-defined function.

  • And the lambda, which is the transformation-specific

  • functionality which maps how the output latencies of the inputs

  • correspond to the output latency of the op,

  • turns out there is a fairly small number of categories

  • of node for each of which there is a different type of lambda.

  • And the autotuning mechanism then

  • takes advantage of these two implementation artifacts

  • to tie it all together.

  • And we start a background thread that periodically performs

  • the following optimization.

  • We snapshot the state of the analytical model, which

  • captures the processing times of the different iterators

  • that are currently floating around.

  • And then we perform a very simple hill-climbing algorithm

  • that allocates threads or cores to parallelism knobs that

  • provide the greatest benefit.

  • So it's really algorithm, assuming that this optimization

  • is actually monotonic.

  • And if you do that, you can specify

  • AUTOTUNE for num_parallel_calls in this example.

  • And as long as you have 16 or more cores,

  • you get the same output latency that you would get out

  • of the manually tuned one.

  • And the key is that it will actually not over-provision

  • by an order of magnitude.

  • It might over-provision a little bit, but not 1,024.

  • And that's it.

  • [APPLAUSE]

YURI: Hi, everyone.

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TensorFlowの内部: tf.data - TF入力パイプライン (Inside TensorFlow: tf.data - TF Input Pipeline)

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    林宜悉 に公開 2021 年 01 月 14 日
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