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Why MPI is Good for You (part 2)

A while ago, I posted “Why MPI is Good For You,” describing a one-byte change in Open MPI’s code base that fixed an incredibly subtle IPv6-based bug.

The point of that blog entry was that MPI represents an excellent layered design: it lets application developers focus on their applications while shielding them from all the complex wilderbeasts that roam under the covers in the implementation.

MPI implementors like me don’t know — and don’t really want to know — anything about complex numerical analysis, protein folding, seismic wave propagation, or any one of a hundred other HPC application areas.  And I’m assuming that MPI application developers don’t know — and don’t want to know — about the tricky underpinnings of how modern MPI implementations work.

Today, I present another motivating example for this thesis.

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The MPI C++ bindings are gone: what does it mean to you?

Jeff Hammond at Argonne tells me that there’s some confusion in the user community about MPI and C++.  I explained how/why we got here in my first post; let Jeff (Hammond) and I now explain what this means to you.

The short version is: DON’T PANIC.

MPI implementations that provided the C++ bindings will likely continue to do so for quite a while.  I know that we have no intention of removing them from Open MPI any time soon, for example.  The MPICH guys have told me the same.

I’ll discuss below what this means to both applications that are written in C++, and applications that use the MPI C++ bindings. Read More »

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The MPI C++ bindings: what happened, and why?

Jeff Hammond at Argonne tells me that there’s some confusion in the user community about MPI and C++.

Let me see if I can clear up some of the issues.

In this blog entry, I’ll describe what has happened to the C++ bindings over time (up to and including their removal in MPI-3), and why.  In a second blog entry, I’ll describe what this means to real-world C++ MPI applications.

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Negative MPI tags: Just. Say. No.

A user on the Open MPI mailing list recently asked about using negative tags for point-to-point MPI sending/receiving operations.  He was using bit-mapped tags, and needed just one more bit.

Although this may seem like an innocent, easy-to-fulfill feature request, there are several reasons why we can’t allow the use of negative tags.

First and foremost, the MPI standard disallows negative tags.  It is explicitly mentioned in MPI-3 section 3.2.3 that tag values must be non-negative integers.

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Algo Boost Series: Part 1 Nexus 3548 Latency Innovations

The Nexus 3548 with Algo Boost was announced last week and received a lot of positive buzz around this game changing innovation. To follow up on Berna Devrim’s Introduction Blog, I am introducing a multipart series that goes into more specifics by Cisco experts. As part 1 of the series, I recently had the opportunity to have a chat with Will Ochandarena about the latency enhancements. Will is a Senior Product Manager in the Server Access and Virtualization Business Unit.  In this role, he is responsible for the Nexus 3548 switch, and Cisco’s low latency switching strategy.

GD: The Cisco Nexus 3548 switch with Algo Boost was announced on September 19th and received a lot of positive attention. Can you elaborate a little more on the latency that this switch can achieve? How does this benefit our financial customers?

<WO>: The custom switching ASIC in the Nexus 3548, codenamed Monticello, sets a new bar for switching latency.  Our engineers worked tirelessly to eliminate unnecessary nanoseconds from the forwarding path, tweaking it down to as low as 190 nanoseconds (ns).  Best of all,  this latency is achieved even when we are doing full layer-2 and layer-3 switching, with features such as Network Address Translation (NAT) enabled.   We actually went as far as to offer a few different switching modes, each with different latency and forwarding characteristics, in order to give our customers the most flexibility in their deployments.

In terms of the impact on our end customers, we consistently hear from companies in the financial community that switch latency has a direct impact on the profitability of their business. Trading firms – as well as the exchanges and other participants – gain significant business advantage if the supporting infrastructure enables them to acquire data and execute trades nanoseconds faster than the competition.

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