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Probing space oddities with Cosmos code

Speed read
  • Cosmos code testbed helps develop new techniques for computational astrophysics
  • Simulation employs discontinuous Gelarkin methods for improved accuracy
  • XSEDE allocations on TACC supercomputers provided millions of core hours

Black holes make for a great space mystery. They're so massive that nothing, not even light, can escape once it gets close enough. Yet, somehow, powerful jets of electrons and protons shoot out of the top and bottom of some black holes.

A computer code called Cosmos now fuels supercomputer simulations of black hole jets and is starting to reveal the mysteries of black holes and other space oddities.

“Cosmos, the root of the name, came from the fact that the code was originally designed to do cosmology. It's morphed into doing a broad range of astrophysics,” explains Chris Fragile, a professor in the physics and astronomy department of the College of Charleston.<strong>Uncloudy day.</strong> Molecular cloud G2 (orange, left) gets ripped apart as it approaches a black hole (white, right) in this Cosmos code simulation. Courtesy Chris Fragile.

Fragile was part of a team that developed the Cosmos code in 2005 while he worked as a post-doctoral researcher at the Lawrence Livermore National Laboratory (LLNL).

The current iteration of the code is CosmosDG, which utilizes discontinuous Gelarkin methods. “You take the physical domain that you want to simulate,” says Fragile, “and you break it up into a bunch of little, tiny computational cells, or zones. You're basically solving the equations of fluid dynamics in each of those zones.”

CosmosDG has allowed much higher order of accuracy than ever before, according to results published in Astrophysical Journal.

XSEDE ECSS helps Cosmos develop

Since 2008, the Texas Advanced Computing Center (TACC) has provided computational resources for the development of the Cosmos code — about 6.5 million supercomputer core hours on the Ranger system and 3.6 million core hours on the Stampede system.

“I can't praise enough how meaningful the eXtreme Science and Engineering Discovery Environment (XSEDE) resources are,” Fragile says. “The science that I do wouldn't be possible without resources like that. That's a scale of resources that certainly a small institution like mine could never support.”

<strong>Handle with care. </strong> Thanks to advanced computation, 'We're living in a golden age of astronomy,' says Chris Fragile, professor in the <a href= 'http://physics.cofc.edu'> physics and astronomy department</a>, College of Charleston.

And the fact is that busy scientists can sometimes use a hand with their code. In addition to access, XSEDE also provides a pool of experts through the Extended Collaborative Support Services (ECSS) effort to help researchers take full advantage of some of the world's most powerful supercomputers.

Fragile has recently enlisted the help of ECSS to optimize the CosmosDG code for Stampede2, the 18 petaFLOPS flagship supercomputer at TACC.

Stampede2 features 4,200 Knights Landing (KNL) nodes and 1,736 Intel Xeon Skylake nodes.

Taking Advantage of Knights Landing and Stampede2

The manycore architecture of KNL presents new challenges for researchers trying to get the best compute performance, according to Damon McDougall, a research associate at TACC and also at the Institute for Computational Engineering and Sciences, at The University of Texas at Austin

Each Stampede2 KNL node has 68 cores, with four hardware threads per core. That's a lot of moving pieces to coordinate.

“This is a computer chip that has lots of cores compared to some of the other chips one might have interacted with on other systems,” McDougall explains. “More attention needs to be paid to the design of software to run effectively on those types of chips.”

We're living in a golden age of astronomy. What we do in modern-day astronomy couldn't be done without computers. ~Chris Fragile

Through ECSS, McDougall has helped Fragile optimize CosmosDG for Stampede2. “We promote a certain type of parallelism, called hybrid parallelism, where you might mix Message Passing Interface (MPI) protocols, which is a way of passing messages between compute nodes, and OpenMP, which is a way of communicating on a single compute node,” McDougall says.

“Mixing those two parallel paradigms is something that we encourage for these types of architectures. That's the type of advice we can help give and help scientists to implement on Stampede2 though the ECSS program.”

Black Hole wobble

Some of the science Fragile and colleagues have already done with the help of the Cosmos code has helped study accretion, the fall of molecular gases and space debris into a black hole. Black hole accretion powers its jets. “One of the things I guess I'm most famous for is studying accretion disks where the disk is tilted,” explains Fragile.

<strong>Seeing double. </strong> Two different views of black hole accretion simulated by Cosmos code. Molecular gas (orange) falls turbulently into a black hole, noted here at '0' mark on far left. Courtesy Chris Fragile.

As it turns out, black holes spin, and so do the disk of gasses and debris that surround and fall into it. However, they spin on different axes of rotation, and Einstein's general theory of relativity shows that rotating bodies can exert a torque on other rotating bodies that aren't aligned with it.

Fragile's team was the first to study cases where the axis of rotation of the disk is not aligned with the axis of rotation of the black hole. His simulations show the black hole wobbles, a movement called precession, from the torque of the spinning accretion disk.

“The really interesting thing is that over the last five years or so, observers — the people who actually use telescopes to study black hole systems — have seen evidence that the disks might actually be doing this precession that we first showed in our simulations,” Fragile says.

Read the original article on the TACC website.

Be ure to talk with the TACC folks at booth #1343 at SC17 in Denver.

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