[Nek5000-users] Large simulations

[nek5000-users at lists.mcs.anl.gov]

Hi JP,

That doesn't sound normal at all... 200K elements is not that large.

I'm suspecting that the time is spent in factoring the XXt-based coarse grid operator..., but
that should really take only 20 minutes for the numbers you report (which is still too long).

My guess is that a switch to AMG for the coarse-grid solve would fix your problem --- My first
guess about the long time is that somehow the partitioning is messed up --- XXt depends
crucially on having a good partition and genmap is designed to provide this.

Please let me know if you have any issues switching to AMG (which we typically only do
at around a million elements or so...)

Best,

Paul

________________________________
From: nek5000-users-bounces at lists.mcs.anl.gov [nek5000-users-bounces at lists.mcs.anl.gov] on behalf of nek5000-users at lists.mcs.anl.gov [nek5000-users at lists.mcs.anl.gov]
Sent: Tuesday, July 07, 2015 9:52 AM
To: nek5000-users
Subject: [Nek5000-users] Large simulations

Dear All,

Is it normal for a nek simulation (220 000 elements with lx1=12) to have long initial setup times (basically gs_setup) such as 5 hours when running on 32768 cores, even when restarting?

As expected, more elements and/or more cores used results in longer setup times. My problem is that I wish to carry out simulations with a higher number of elements but I am limited to 24 hour runs and must carry out restarts.

Is there a way to carry out faster restarts?

Thank you,
jp
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