Physics for martial arts students

Right after the kickboxing class, I saw one of classmate and friend of mine trying to solve a physics exercise. I studied physics in high school, and I had a exam in college; it was long time ago and even at that time I wasn’t very strong in the matter (and I used to soothe myself (or to fool myself?) quoting Linus Torvalds “While in physics you’re supposed to figure out how the world is made up, in computer science you create the world.” But that’s a different story), but I decided that I could help, after all it didn’t sound too difficult.

The problem

The problem required finding the final velocity (rounded at one decimal digit), $v_f$ of a ball thrown down from a 40 meters tall tower with a initial velocity, $v_0$, of 12 m/s. The exercise book was providing also the solution: 30.5 m/s. Read more of this post

First step of my journey for rediscovering the straight path (aka I’m using again GNU Emacs after many years)

“… mi ritrovai per una selva oscura, ché la diritta via era smarrita …” (Divina Commedia – Canto I)

Like Dante in his allegoric journey, I (also) “found myself deep in a darkened forest, for I had lost all trace of the straight path” in the computing world.

The “darkened forest” in which I found myself is the world of proprietary software, in fact I found myself using some of the wrong (as in closed source) operating systems, programming languages and text editors. Sometimes there is no alternative in using proprietary software (i.e. a compiler for a strange architecture, a library that I absolute need to use for my daytime job).

But why I wasn’t using free software for the operating system and (even more) the text editor when there are the free software alternatives, and many of them are also technically better (way better), than the proprietary software I was using? That’s simple I just followed the hype. Read more of this post

A Coprocessor Sharing-Aware Scheduler for Xeon Phi-Based Compute Clusters

In Parallel and Distributed Processing Symposium, 2014 IEEE 28th International

We propose a cluster scheduling technique for compute clusters with Xeon Phi coprocessors. Even though the Xeon Phi runs Linux which allows multiprocessing, cluster schedulers generally do not allow jobs to share coprocessors because sharing can cause oversubscription of coprocessor memory and thread resources. It has been shown that memory or thread oversubscription on a many core like the Phi results in job crashes or drastic performance loss. We first show that such an exclusive device allocation policy causes severe coprocessor underutilization: for typical workloads, on average only 38% of the Xeon Phi cores are busy across the cluster. Then, to improve coprocessor utilization, we propose a scheduling technique that enables safe coprocessor sharing without resource oversubscription. Jobs specify their maximum memory and thread requirements, and our scheduler packs as many jobs as possible on each coprocessor in the cluster, subject to resource limits. We solve this problem using a greedy approach at the cluster level combined with a knapsack-based algorithm for each node. Every coprocessor is modeled as a knapsack and jobs are packed into each knapsack with the goal of maximizing job concurrency, i.e., as many jobs as possible executing on each coprocessor. Given a set of jobs, we show that this strategy of packing for high concurrency is a good proxy for (i) reducing make span, without the need for users to specify job execution times and (ii) reducing coprocessor footprint, or the number of coprocessors required to finish the jobs without increasing make span. We implement the entire system as a seamless add on to Condor, a popular distributed job scheduler, and show make span and footprint reductions of more than 50% across a wide range of workloads.

Property for Sale: Vari, Syros, Cyclades, Greece

36963sft (3434sqm) of residential land in the locality Vari (prefecture of Posidonia), Syros island (Cyclades, Aegean Sea), Greece.

The real estate is street front and it is composed by two adjacent lots (1666sqm + 1768sqm, 17933sft + 19031sft).

Each batch has a pre-approved buildability of 400sqm (4305sft) for residential purpose, but it can be extended for hotel accommodation.

The real estate is located 800m (2625ft) from the shore of Vari, 5km (3mi) from the town of Ermoupoli, 4km (2.5mi) from the airport and 5km from the sea-dock.

Snapify: capturing snapshots of offload applications on Xeon Phi manycore processors

In Proceedings of the 23nd international symposium on High-performance parallel and distributed computing (HPDC ’14). ACM, New York, NY, USA, 1-12.

Intel Xeon Phi coprocessors provide excellent performance acceleration for highly parallel applications and have been deployed in several top-ranking supercomputers. One popular approach of programming the Xeon Phi is the offload model, where parallel code is executed on the Xeon Phi, while the host system executes the sequential code. However, Xeon Phi’s Many Integrated Core Platform Software Stack (MPSS) lacks fault-tolerance support for offload applications. This paper introduces Snapify, a set of extensions to MPSS that provides three novel features for Xeon Phi offload applications: checkpoint and restart, process swapping, and process migration. The core technique of Snapify is to take consistent process snapshots of the communicating offload processes and their host processes. To reduce the PCI latency of storing and retrieving process snapshots, Snapify uses a novel data transfer mechanism based on remote direct memory access (RDMA). Snapify can be used transparently by single-node and MPI applications, or be triggered directly by job schedulers through Snapify’s API. Experimental results on OpenMP and MPI offload applications show that Snapify adds a runtime overhead of at most 5%, and this overhead is low enough for most use cases in practice.

Continue reading the complete paper …

Decaffeinated: experiencing the effects of caffeine by its abstinence

“I don’t believe in caffeine!” I found myself repeating that sentence more than once, while hearing colleagues or friends talking and debating about the effects that caffeine has on them.

It is said that caffeine gives addiction, that, in a moderate dosage, it can help the humor and increase the alertness; people drink coffee or other caffeinated beverages when they feel drowsy or when they need to stay up, many affirm that they cannot work or study if they don’t have coffee.

The Gulf of Naples as seen from Princeton

The Gulf of Naples as seen from Castel dell’Ovo

I’m just back in Princeton, and I decided to share with you this picture taken few days ago during a walk in Naples (Napoli).

I really enjoyed my stay at Home in Italy, having dinner with my family, visiting some relatives, going around in some amazing places, sharing few beers with friends of a lifetime or just met!

But, please, don’t get me wrong: I’m happy to be back to my usual life, skipping dinner alone, “making the science”, playing volleyball with my colleagues,  training MMA, sharing few beers with friends of a lifetime or just met!

Happy birthday to me!