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cosmocomp workshop, Tireste September 2012
Massive generation of mock catalogues with Pinocchio
A progress report P. Monaco, E. Sefusatti, M. Calcioni, S. Borgani, R. Sheth(Univ. Trieste & ICTP)
P. Monaco, E. Sefusatti, S. Borgani, R. Sheth, A. Paranjape, T. Theuns
cosmocomp workshop, Tireste September 2012
Need for samples of >1000 of simulated catalogues
● Simulating galaxies in past light cones over volumes » 1Gpc3
● Need » 20003 particles per realization
● Need » 1000 realizations to compute covariance matrices
● Disc space may be an issue more than CPU time!
cosmocomp workshop, Tireste September 2012
Computers
● Moving to the Blue Gene tech cycle: massively parallel computers with limited resources per core and limited i/o.
● An optimal code should have very good scaling (>1000 tasks) and output only the final results
Fermi@CINECA
cosmocomp workshop, Tireste September 2012
PINpointing Orbit Crossing-Collapsed HIerarchical Objects
He's cheating:he's not N-body,he's way too fast!
http://adlibitum.oats.inaf.it/monaco/pinocchio/
P.M., Tom Theuns, Giuliano Taffoni, Fabio Governato, Tom Quinn & Joachim Stadel, 2002, ApJ, 564, 8
P.M.,, Tom Theuns & Giuliano Taffoni, 2002, MNRAS, 331, 587
Giuliano Taffoni, P.M. & Tom Theuns, 2002, MNRAS, 333, 623
cosmocomp workshop, Tireste September 2012
● Uses Lagrangian Perturbation Theory and the excursion sets approach to predict the hierarchical formation of DM halos
● To take full account of correlation of trajectories, it works on realizations of a linear density field in a rectangular grid (like N-body simulations)
The algorithm
cosmocomp workshop, Tireste September 2012
STEP 1:● Generate a linear density field on a grid● Gaussian-smooth it on ~20 smoothing radii to
generate one trajectory per grid point● For each smoothing and for each grid point compute
the collapse time with the “ellipsoidal truncation” of 3LPT (P.M. 1997)
● Compute the expected collapse time of the “particle” as the first upcrossing of the trajectory and store the velocity field at the same smoothing radius
● It requires 9 FFTs per smoothing
The algorithm
cosmocomp workshop, Tireste September 2012
STEP 1.5:● Redistribute grid points from planes (FFT) to
sub-volumes with a “safety” boundary of ~30 Mpc
The algorithm
cosmocomp workshop, Tireste September 2012
The algorithm: step 2
● scroll the particle list in increasing order of collapse time● peaks of the (inverse of the) collapse time are seeds of new
halos● if a collapsing particle “touches” a halo then
– displace particle and halo with Zel'dovich approx. to check whether they get “near enough” in the Eulerian space
● if they do, then accrete the particle on the halo● if they do not, then tag the particle as “filament”
● if a collapsing particle touches more than a halo then
– check whether the particle is accreted onto a halo– displace halos to check whether they get “near enough”
● if they do, merge them
cosmocomp workshop, Tireste September 2012
peak -> new halo
1 neighbour: accretion
2 neighbours: accretion + merging
.
cosmocomp workshop, Tireste September 2012
Latest developments
✔ No swapping of memory on disc to minimize i/o➔ RAM requirement: 110 bytes per grid point
✔ distribution of sub-boxes on tasks that works for big volumes
✔ Embedded N-GenIC to generate ICs● 2LPT and 3LPT displacements● built-in output in the past-light cone
cosmocomp workshop, Tireste September 2012
Scaling test 17203 grid of size 720 Mpc/h run on PLX@cineca
cosmocomp workshop, Tireste September 2012
grid size scaled with the number of processors
Scaling test 2
cosmocomp workshop, Tireste September 2012
An example
● Millennium-like simulation● 500 Mpc/h box with 21603 grid points● run on 360 cores on PLX@CINECA (linux
cluster)● writing of 6 full halo catalogues (merger
histories are build with very high time sampling)● CPU time: 214.5 hours● Elapsed time: 35min 45s
cosmocomp workshop, Tireste September 2012
Preliminary: power spectrum
Las Damas: “Oriana” simulation