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DTSTAMP:20210916T132452Z
LOCATION:Michel Mayor
DTSTART;TZID=Europe/Stockholm:20210708T113000
DTEND;TZID=Europe/Stockholm:20210708T120000
UID:submissions.pasc-conference.org_PASC21_sess157_msa341@linklings.com
SUMMARY:Status of NEMO Scalability and Inputs from POP Assessment
DESCRIPTION:Minisymposium\n\nStatus of NEMO Scalability and Inputs from PO
 P Assessment\n\nMasson\n\nThis presentation will give an overview of the s
 calability issues of the ocean model NEMO. We will show that because NEMO 
 is a memory bandwidth limited application, its « good
  » scalability curve is in fact hiding severe scalability probl
 ems. The benchmark configuration tested during the POP assessment was chos
 en to enhance NEMO defaults and facilitate the analyses. It was shown that
  the main bottleneck in term of MPI communications is coming from a unique
  routine, which does about 50 point-to-point communications per time step 
 at very high frequency. When reaching the end of the scalability curve wit
 h very small MPI subdomains, the frequency call is so high that the comput
 ation weight becomes almost negligible! A load unbalance between the MPI p
 rocesses due to the north pole treatment is making the performance even wo
 rse. Several solutions were proposed by POP experts and have been tes
 ted. The simplest (MPI_WAITALL, MPI_ISEND) ones have already been implemen
 ted. We are currently testing MPI persistent calls and considering differe
 nt solutions to reduce the frequency of MPI calls, improve the computation
 /communication ratio and possibly recover a part of the communications by 
 computations.\n\nDomain: Chemistry and Materials, Climate and Weather, Phy
 sics, Solid Earth Dynamics, Life Sciences, Engineering
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