Scikit-Learn algorithms are single node implementations. Does this mean, that they are not an appropriate choice for building machine learning models on Databricks cluster for the reason that they cannot take advantage of the cluster computing resources ?
They are not appropriate, in the sense that, as you say, they cannot take advantage of the cluster computing resources, which Databricks is arguably all about. The raison d'être of Databricks is Apache Spark, and specifically for ML tasks, its ML library Spark MLlib.
This does not mean that you cannot use scikit-learn in Databricks (you'll find that a Databricks cluster comes by scikit-learn installed by default), only that it is usable for problems that do not actually require a cluster. If you want to exploit the cluster resource capabilities for ML, you need to revert to Spark MLlib.
I think desertnaut hit the nail on the head here. I believe Scikit Learn algos are designed only for non-parallel processing jobs, and all the MLlib stuff is designed to leverage cluster compute resources and parallel processing resources. Take a look at the link below for sample code for standard regression and classification tasks.
https://spark.apache.org/docs/latest/ml-classification-regression.html
In addition, here are some code samples for different clustering tasks.
https://spark.apache.org/docs/latest/ml-clustering.html
That should probably cover most of the things you will be doing.
I believe that it depends on the task at hand. I see two general scenarios:
Your data is big and does not fit into memory. Go with the Spark MLlib and their distributed algos.
Your data is not that big and you want to utilize sheer computing power. The typical use case is hyperparameter search.
Databricks allow for distributing such workloads from the driver node to the executors with hyperopt and its SparkTrials (random + Bayesian search).
Some docs here>
http://hyperopt.github.io/hyperopt/scaleout/spark/
However, there are much more attempts to make the sklearn on spark work. You can supposedly distribute the workloads through UDF, using joblib, or others. I am investigating the issue myself, and will update the answer later.
Related
I'm preparing for the Azure Machine Learning exam, and here is a question confuses me.
You are designing an Azure Machine Learning workflow. You have a
dataset that contains two million large digital photographs. You plan
to detect the presence of trees in the photographs. You need to ensure
that your model supports the following:
Solution: You create a Machine
Learning experiment that implements the Multiclass Decision Jungle
module. Does this meet the goal?
Solution: You create a Machine Learning experiment that implements the
Multiclass Neural Network module. Does this meet the goal?
The answer for the first question is No while for second is Yes, but I cannot understand why Multiclass Decision Jungle doesn't meet the goal since it is a classifier. Can someone explain to me the reason?
I suppose that this is part of a series of questions that present the same scenario. And there should be definitely some constraints in the scenario.
Moreover if you have a look on the Azure documentation:
However, recent research has shown that deep neural networks (DNN)
with many layers can be very effective in complex tasks such as image
or speech recognition. The successive layers are used to model
increasing levels of semantic depth.
Thus, Azure recommends using Neural Networks for image classification. Remember, that the goal of the exam is to test your capacity to design data science solution using Azure so better to use their official documentation as a reference.
And comparing to the other solutions:
You create an Azure notebook that supports the Microsoft Cognitive
Toolkit.
You create a Machine Learning experiment that implements
the Multiclass Decision Jungle module.
You create an endpoint to the
Computer vision API.
You create a Machine Learning experiment that
implements the Multiclass Neural Network module.
You create an Azure
notebook that supports the Microsoft Cognitive Toolkit.
There are only 2 Azure ML Studio modules, and as the question is about constructing a workflow I guess we can only choose between them. (CNTK is actually the best solution as it allows constructing a deep neural network with ReLU whereas AML Studio doesn't, and API call is not about data science at all).
Finally, I do agree with the other contributors that the question is absurd. Hope this helps.
This question is indeed part of a series of questions that present the same scenario with multiple options. Both of the solutions approach the problem as a multi-class classification problem, which is correct. However, the key element here is dimensionality.
Your inputs (images) are highly dimensional which requires a deep learning approach in order to be effective. A decision jungle won't be able to learn effectively in such a high dimensional feature space, where a NN has higher chances to do so.
I hope it helps.
From various sources on the net, I understand that Storm has some machine learning capabilities (via Samoa for example) but is it always for online learning? Corollary question: am I wrong if I say that Storm is not suited for the more classical batch learning because of the inherent iterative nature of this kind of processing?
Thank you for clarifying.
The reason why it is for online learning is because is a solution to process data one by one, that you can scale up by setting up some paralelization (more working processes and nodes).
You typically do batch learning to build or train models with historical data, but for that processing the data one by one is not your best option. Since you may need to process months of data, you want to batch it to optimize the process. This is where other solutions like spark come in place.
I recently took a courser by Andrew Ng on Coursera. After that I shifted to Python and used Pandas, Numpy, Sklearn to implement ML algorithms. Now while surfing I came across tensorFLow and found it pretty amazing, and implemented this example which takes MNIST data as input.
But I am unsure why use such as library(TensorFlow)?
We are not doing any parallel calculations, since the weights updated in the previous epoch are used in the next one???
I am finding it difficult to find a reason to use such a Library?
There are several forms of parallelism that TensorFlow provides when training a convolutional neural network (and many other machine learning models), including:
Parallelism within individual operations (such as tf.nn.conv2d() and tf.matmul()). These operations have efficient parallel implementations for multi-core CPUs and GPUs, and TensorFlow uses these implementations wherever available.
Parallelism between operations. TensorFlow uses a dataflow graph representation for your model, and where there are two nodes that aren't connected by a directed path in the dataflow graph, these may execute in parallel. For example, the Inception image recognition model has many parallel branches in its dataflow graph (see figure 3 in this paper), and TensorFlow can exploit this to run many operations at the same time. The AlexNet paper also describes how to use "model parallelism" to run operations in parallel on different parts of the model, and TensorFlow supports that using the same mechanism.
Parallelism between model replicas. TensorFlow is also designed for distributed execution. One common scheme for parallel training ("data parallelism") involves sharding your dataset across a set of identical workers, performing the same training computation on each of those workers for different data, and sharing the model parameters between the workers.
In addition, libraries like TensorFlow and Theano can perform various optimizations when they can work with the whole dataflow graph of your model. For example, they can eliminate common subexpressions, avoid recomputing constant values, and generate more efficient fused code.
You might be able to find pre-baked models in sklearn or other libraries, but TensorFlow allows for really fast iteration of custom machine learning models. It also comes with a ton of useful functions that you would have to (and probably shouldn't) write yourself.
To me, it's less about performance (though they certainly care about performance), and more about whipping out neural networks really quickly.
Following the Spark MLlib Guide we can read that Spark has two machine learning libraries:
spark.mllib, built on top of RDDs.
spark.ml, built on top of Dataframes.
According to this and this question on StackOverflow, Dataframes are better (and newer) than RDDs and should be used whenever possible.
The problem is that I want to use common machine learning algorithms (e.g: Frequent Pattern Mining,Naive Bayes, etc.) and spark.ml (for dataframes) don't provide such methods, only spark.mllib(for RDDs) provides this algorithms.
If Dataframes are better than RDDs and the referred guide recommends the use of spark.ml, why aren't common machine learning methods implemented in that lib?
What's the missing point here?
Spark 2.0.0
Currently Spark moves strongly towards DataFrame API with ongoing deprecation of RDD API. While number of native "ML" algorithms is growing the main points highlighted below are still valid and internally many stages are implemented directly using RDDs.
See also: Switch RDD-based MLlib APIs to maintenance mode in Spark 2.0
Spark < 2.0.0
I guess that the main missing point is that spark.ml algorithms in general don't operate on DataFrames. So in practice it is more a matter of having a ml wrapper than anything else. Even native ML implementation (like ml.recommendation.ALS use RDDs internally).
Why not implement everything from scratch on top of DataFrames? Most likely because only a very small subset of machine learning algorithms can actually benefit from the optimizations which are currently implemented in Catalyst not to mention be efficiently and naturally implemented using DataFrame API / SQL.
Majority of the ML algorithms require efficient linear algebra library not a tabular processing. Using cost based optimizer for linear algebra could be an interesting addition (I think that flink already has one) but it looks like for now there is nothing to gain here.
DataFrames API gives you very little control over the data. You cannot use partitioner*, you cannot access multiple records at the time (I mean a whole partition for example), you're limited to a relatively small set of types and operations, you cannot use mutable data structures and so on.
Catalyst applies local optimizations. If you pass a SQL query / DSL expression it can analyze it, reorder, apply early projections. All of that is that great but typical scalable algorithms require iterative processing. So what you really want to optimize is a whole workflow and DataFrames alone are not faster than plain RDDs and depending on an operation can be actually slower.
Iterative processing in Spark, especially with joins, requires a fine graded control over the number of partitions, otherwise weird things happen. DataFrames give you no control over partitioning. Also, DataFrame / Dataset don't provide native checkpoint capabilities (fixed in Spark 2.1) which makes iterative processing almost impossible without ugly hacks
Ignoring low level implementation details some groups of algorithms, like FPM, don't fit very well into a model defined by ML pipelines.
Many optimizations are limited to native types, not UDT extensions like VectorUDT.
There is one more problem with DataFrames, which is not really related to machine learning. When you decide to use a DataFrame in your code you give away almost all benefits of static typing and type inference. It is highly subjective if you consider it to be a problem or not but one thing for sure, it doesn't feel natural in Scala world.
Regarding better, newer and faster I would take a look at Deep Dive into Spark SQL’s Catalyst Optimizer, in particular the part related to quasiquotes:
The following figure shows that quasiquotes let us generate code with performance similar to hand-tuned programs.
* This has been changed in Spark 1.6 but it is still limited to default HashPartitioning
What's the best way to run machine learning algorithms on Google Cloud Dataflow? I can imagine that using Mahout would be one option given it's Java based.
The answer is probably no, but is there a way to invoke R or Python (that have strong support for algorithms) based scripts to offload ML execution?
-Girish
You can already implement many algorithms in terms of Dataflow transforms.
A class of algorithms that may not be as easy to implement are iterative algorithms, where the pipeline's execution graph depends on the data itself. Simplifying implementation of iterative algorithms is something that we are interested in, and you can expect future improvements and simplifications in this area.
Invoking Python (or any other) executable shouldn't be hard from a Dataflow pipeline. A ParDo can, for example, shell out and start an arbitrary process. You can use, for example, --filesToStage pipeline option to add additional files to the Dataflow worker environment.
There is also http://quickml.org/ (haven't used personally) and Weka. I remember the docs mention that it's possible to launch a new process from within the job, but AFAIK it's not recommended.