Re: trouble starting a large N-body run

From: Robert Thompson <>
Date: Tue, 18 Mar 2014 18:38:05 +0200

Hi Volker thanks for your quick reply! I should note that the ICs were generated via N-GenIC and I am running the simulation with Gadget3.

> It looks like your initial conditions file contains incorrect entries for the particle count. Note that 2250^3 > 2^32, i.e. your total particle count does not fit into an ordinary 32-bit unsigned int. In gadget2, the higher-order word is stored in a separate field in the file header (npartTotalHighWord[]).
> Check out the calculation of "All.TotNumPart" as well as of that of "All.MaxPart" in read_ic.c. For some reason you are getting All.MaxPart = 0, likely due to an incorrect value of the computed value of All.TotNumPart, which in turn probably originates in a faulty IC file header.
I had a sneaking suspicion of this. It seems neither N-GenIC nor 2LPTic contains npartTotalHighWord, apparently the values are stored in npartTotal[1] & npartTotal[2], which interestingly enough are 0 in my IC header (probably the source of the problem). In N-GenIC I commented out NO64BITID (and enabled LONGIDS in gadget), are there any other tricks to getting it to create such large ICs?

> Note: 128000 cores is pretty over the top for this particle count. I doubt that Gadget2 (which is nearly 10 years old) will work well for such a large number of MPI ranks - never tried it myself.
I felt that was far too many cores myself; I figured even if I did get it to run the MPI overhead would slow it to a crawl.

Received on 2014-03-18 17:38:20

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