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Euro MPI 2011 Call for Pariticpation

July 22, 2011 at 5:00 am PST

WHAT: EuroMPI 2011 Conference
WHERE: Santorini, Greece
WHEN: September 18-21, 2011


EuroMPI is the primary meeting where the users and developers of MPI and other message-passing programming environments can interact. The 18th European MPI Users’ Group Meeting will be a forum for the users and developers of MPI, but also welcome hybrid programing models that combine message passing with programming of modern architectures such as multi-core, or accelerators.

Through the presentation of contributed papers, poster presentations and invited talks, attendees will have the opportunity to share ideas and experiences to contribute to the improvement and furthering of message-passing and related parallel programming paradigms.

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Registered Memory (RMA / RDMA) and MPI implementations

July 20, 2011 at 5:00 am PST

In a prior blog post, I talked about RMA (and RDMA) networks, and what they mean to MPI implementations.  In this post, I’ll talk about one of the consequences of RMA networks: registered memory.

Registered memory is something that most HPC administrators and users have at least heard of, but may not fully understand.

Let me clarify it for you: registered memory is both a curse and a blessing.

It’s more of the former than the latter, if you ask me, but MPI implementations need to use (and track) registered memory to get high performance on today’s high-performance networking API stacks.

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“RDMA” — what does it mean to MPI applications?

July 16, 2011 at 8:13 am PST

RDMA standard for Remote Direct Memory Access.  The acronym is typically associated with OpenFabrics networks such as iWARP, IBoIP (a.k.a. RoCE), and InfiniBand.  But “RDMA” is typically just today’s popular flavor du jour of a more general concept: RMA (remote memory access), or directly reading and writing to a peer’s memory space.

RMA implementations (including RDMA-based networks, such as OpenFabrics) typically include one or more of the following technologies:

  1. Operating system bypass: userspace applications directly communicate with network hardware.
  2. Hardware offload: network activity is driven by the NIC, not the main CPU
  3. Hardware or software notification: when messages finish sending or are received

How are these technologies typically used in MPI implementations?

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Unexpected messages = evil

June 11, 2011 at 4:25 am PST

Another term that is not-infrequently used when discussing message passing application is “unexpected messages.”

What are they, and why are they (usually) bad?

The quick definition is that an unexpected message is one that arrives before a corresponding MPI receive has been posted.  In more concrete terms: an MPI process has sent a message to a process that hadn’t yet called some flavor of MPI_RECV to receive the message.

Why is this a Bad Thing?

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“Eager Limits”, part 2

May 31, 2011 at 7:30 am PST

Open MPI actually has multiple different protocols for sending messages — not just eager / rendezvous.

Our protocols were originally founded on the ideas described in this paper.  Many things have changed since that 2004 paper, but some of the core ideas are still the same.

The picture to the right shows how Open MPI divides an MPI message up into segments and sends them in three phases.  Open MPI’s specific definition of the “eager limit” is the max payload size that is sent with MPI match information to the receiver as the first part of the transfer.  If the entire message fits in the eager limit, no further transfers / no CTS is needed.

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