Tag Archives: Parallel computing

On efficient reading and writing for large scale simulations

Last year, my colleagues and I presented a paper on giga model simulations in an SPE conference: Giga-Model Simulations In A Commercial Simulator – Challenges & Solutions. During this talk, we talked about the complexity of I/O for such simulations. We had ordered data as input that we needed to split in chunks to send them on the relevant MPI ranks, and then the same process was required for writing the results, gathering the chunks and then writing them down to the disk.

The central point is that some clusters have parallel file systems, and these works well when you try to access big blobs of aligned data. In fact, as they are the bottleneck of the whole system, you need to limit the number of accesses to what you actually require. For instance in HDF5, you can specify the alignment of datasets, so you can say that all HDF5 datasets will be aligned on the filesystem specifications (so for instance 1MB if your Lustre/GPFS has a chunk size of 1MB) and read or write chunks that are multiple of these values.

Continue reading On efficient reading and writing for large scale simulations

Book review: Machine Learning for Adaptive Many-Core Machines – A Practical Approach

Nice title, surfing on the many core hype, and with a practical approach! What more could one expect from a book on such an interesting subject?

Continue reading Book review: Machine Learning for Adaptive Many-Core Machines – A Practical Approach

On the importance of not reinventing the wheel in distributed applications

Sometimes, it’s so easy to rewrite some existing code because it doesn’t fit exactly your bill.

I just so the example with an All To All communication that was written by hand. The goal was to share how many elements would be sent from one MPI process to another, and these elements were stored on one process in different structure instances, one for each MPI process. So in the end, you had n structures on each of the n MPI processes.

The MPI_Alltoall cannot map directly to this scattered structure, so it sounds fair to assume that using MPI_Isend and MPI_Irecv would be simpler to implement. The issue is that this pattern uses buffers on each process for each other process it will send values to or receive values from. A lot of MPI library allocate their buffer when needed, but will never let go of the memory until the end. So you end up with a memory consumption that doesn’t scale. In my case, when using more than 1000 cores, the MPI library uses more than 1GB per MPI process when it hits these calls, just for these additional hidden buffers. This is just no manageable.

Now, if you use MPI_Alltoall, two things happen:

  • there are no additional buffer allocated, so this scales nicely when you increase the number of cores
  • it is actually faster than your custom implementation

Now with MPI 3 standard having non-blocking collective operations, there is absolutely no reason to try to outsmart the library when you need a collective operation. It has heuristics when it knows that it is doing a collective call, so let them work. You won’t be smarter if you try, but you will if you use them.

In my case, the code to retrieve all values and store them in an intermediate buffer was smaller that the one with the Isend/Irecv.

Book review: Learning scikit-learn – Machine Learning in Python

It seems that Packt Publishing is on a publishing spree on Machine Learning in Python. After Building Machine Learning Systems In Python for which I was technical reviewer, Packt published Learning Scikit-Learn In Python last November.

Continue reading Book review: Learning scikit-learn – Machine Learning in Python

Book review: CUDA By Example

It has been a while since my last post here, but I’m back! I had access to the French version of this book, thanks to the publisher.

CUDA is now in the trend, and there are several books, one of them I’ve also reviewed.
Continue reading Book review: CUDA By Example

Book review: Introduction to High Performance Computing for Scientists and Engineers

We know now that we won’t have the same serial computing increase we had in the last decades. We have to cope with optimizing serial codes, and programming parallel and concurrent ones, and this means that all coders have to cope with this paradigm shift. If computer scientists are aware of the tools to use, it is not the same for the “average” scientist or engineer. And this is the purpose of this book: educate the average coder.
Continue reading Book review: Introduction to High Performance Computing for Scientists and Engineers

Book review: Programming Massively Parallel Processors: A Hands-on Approach

Massively parallel processors are in the mood today. We had small parallel processors with a few cores and the ability to launch serevral threads on one core, we have now many cores on one processor and at the other end of the spectrum, we have GPUs. CPUs vendors are now going in this direction with Larabee and Fusion, and GPUs will still have more cores/threads/… It’s thus mandatory to understand this shift now.
Continue reading Book review: Programming Massively Parallel Processors: A Hands-on Approach

Review of Intel Parallel Studio

I’ve played a little bit with Intel Parallel Studio. Let’s say it has been a pleasant trip out in the wildness of multithreaded applications.

Intel Parallel Studio is a set of tools geared toward multithreaded applications. It consists of three Visual Studio plugins (so you need a fully-fledged Visual Studio, not an Express edition):

  • Parallel Inspector for memory analysis
  • Parallel Amplifier for thread behavior and concurrency
  • Parallel Composer for parallel debugging

This is an update of the review I’ve done for the beta version. Since this first review, I’ve tried the official first version.

Continue reading Review of Intel Parallel Studio

Interactive RayTracer

Some months ago, I’ve decided to dig into raytracing, and more exactly interactive raytracing. So I’ve started writting my own library, based on several publications.
nVidia announced recently its own framework, Intel wants also to do raytracing on Larrabee, it is the current trend.
Continue reading Interactive RayTracer