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fly is the tree n - body code we design , develop and use to run very big simulations of the large scale structure of the universe using parallel systems mpp and smp . fly uses the leapfrog numerical integration scheme for performance reasons , and incorporates fully periodic boundary conditions using the ewald method . the i / o data format is integrated with the astromd package . + astromd is an analysis and visualization tool specifically designed to deal with the visualization and analysis of astrophysical data . astromd can find structures having a not well defined shape or symmetries , and performs quantitative calculations on a selected region or structure . astromd makes use of virtual reality techniques which are particularly effective for understanding the three dimensional distribution of the fields , their geometry , topology and specific patterns . the display of data gives the illusion of a surrounding medium into which the user is immersed . the result is that the user has the impression of travelling through a computer - based multi - dimensional model which could be directly hand - manipulated . the fly code , written in fortran 90 and c languages , uses the one - side communication paradigm : it has been developed on the cray t3e using the shmem library . fly is based on the following main characteristics . it adopts a simple domain decomposition , a grouping strategy and a data buffering that allows us to minimize data communication . fly does not split the domain with orthogonal planes , but the domain decomposition is done by assigning an equal number of particles to each processor . the input data particles is a sorted file containing the fields of position and velocity , so that particles with a near tag number are also near in the physical space , and the arrays containing the tree properties are distributed using a fine grain data distribution .      during the tree walk procedure , fly builds a single interaction list ( il ) to be applied to all particles inside a grouping cell ( @xmath1 ) . this reduces the number of the tree accesses to build the il . we consider a hypothetical particle we call _ virtual body _ ( vb ) placed in the center of mass of the @xmath1 : the vb interaction list @xmath2 is formed by two parts : @xmath3 where @xmath4 includes the elements more distant than a threshold parameter from vb and @xmath5 includes the elements near vb . using the two lists it is possible to compute the force @xmath6 of each _ p _ particle in @xmath1 as the sum of two components : @xmath7 the component @xmath8 is computed only once for vb , and it is applied to all the _ p _ particles , while the @xmath9 component is computed separately for each particle . the size of the @xmath1 , and the tree - level where it can be considered , is constrained by the maximum allowed value of the overall error of this method . in this sense the performance of fly is a level - based code . the data buffer is managed with a policy of a simulated cache in the local ram . every time the pe has to access a remote element , at first it looks for the local simulated cache and , if the element is not found , the pe executes the get calls to down - load the remote element and stores it in the buffer . in a simulation with 16-million - particles clustered , with 32 pes and 256 mbytes of local memory , without the use of the simulated cache , the pes execute about @xmath10 remote gets . this value , using the data buffering , decreases at @xmath11 remote gets , with an enormous advantage in terms of scalability and performance . 1 shows the code performance on cray t3e system , obtained by running simulations with 32 and 64 pes , and fly scalability , considering the case of 16,777,216 particles , where a speed - up factor of 118 is reached using 128 pes . the highest performance obtained in a clustered configuration is a positive effect of the grouping characteristic . the obtained results show that fly has a very good scalability and a very high performance and it can be used to run very big simulations . astromd is developed using the visualization toolkit ( vtk ) by kitware , a freely available software portable on several platforms which range from the pc to the most powerful visualization systems , with a good scalability . data are visualized with respect to a box which can describe the whole computational mesh or a sub - mesh . astromd can find structures having a not well defined shape or symmetries , and performs quantitative calculations on a selected region or structure . the user can choose the sample of the loaded particle type ( i.e. stars , dark matter and gas particles ) , the size of the visualization box and the starting time from which he wants to show the evolution of the simulation . density _ entries control the visualization of the iso - surfaces . these are calculated on a grid whose resolution can be user - selectable . to allow the user to investigate with more accuracy a subset of the visualized system it is possible to use a cubic sampler . if the sampler is selected ( _ show / hide _ menu ) , visualized in the scene and enabled ( _ sampler _ menu ) , all the computations are performed only inside the region of the sampler ( fig . astromd can also show the evolution in time of the simulated system over all the interval of time for which the data are available , performing interpolations at intermediate frames . during the evolution the updated time of evolution is displayed in the _ time _ entry of the _ cloud _ section . finally snapshots of the images displayed can be created using the button _ take it _ in the _ screenshot _ menu . astromd is developed by the visit ( visual information technology ) laboratory at cineca ( casalecchio di reno - bologna ) in collaboration with the astrophysical observatory of catania . antonuccio - delogu , v. , becciani , u. , pagliaro , a. , van kampen , e. , colafrancesco , s. , german , a. and gambera , m.  2000 , submitted to barnes , j.e . and hut , p.  1986 , nature , 324 , 446schroeder , w. , ken , m. , lorensen , b.  1999 , `` the visualization toolkit '' , prentice hall becciani , u. , antonuccio - delogu , v.  2000 , j. comput . phys . , 163 , 118 earnshaw , r.a . , watson , d.  1993 , `` animation and scientific visualization '' , academic press ltd . qinn , t. , stadel , c. tipsy http://www-hpcc.astro.washington.edu/tools/tipsy salmon , j. and warren , m.s . 1997 , proc . of the eight conf . on parallel processing for scientific computing , siam 1997
we present a new code allowing to evolve three - dimensional self - gravitating collisionless systems with a large number of particles @xmath0 . * 
 fly * ( fast level - based n - body code ) is a fully parallel code based on a tree algorithm . 
 it adopts periodic boundary conditions implemented by means of the ewald summation technique . 
 fly is based on the one - side communication paradigm to share data among the processors that access remote private data avoiding any kind of synchronism . 
 the code was originally developed on a cray t3e system using the _ shmem _ library and it was ported on sgi origin 2000 and on ibm sp ( on the latter making use of the _ lapi _ library ) . 
 version 1.1 is an open source _ freely available code_. + fly data output can be analysed with astromd , an analysis and visualization tool specifically designed to deal with the visualization and analysis of astrophysical data . 
 astromd can manage different physical quantities 
 . it can find out structures without well defined shape or symmetries , and perform quantitative calculations on selected regions . 
 is a _ freely available _ code .