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274 lines
12 KiB
HTML
<HTML>
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<CENTER><A HREF = "http://lammps.sandia.gov">LAMMPS WWW Site</A> - <A HREF = "Manual.html">LAMMPS Documentation</A> - <A HREF = "Section_commands.html#comm">LAMMPS Commands</A>
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<H3>fix balance command
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</H3>
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<P><B>Syntax:</B>
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</P>
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<PRE>fix ID group-ID balance Nfreq dimstr Niter thresh keyword value ...
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</PRE>
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<UL><LI>ID, group-ID are documented in <A HREF = "fix.html">fix</A> command
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<LI>balance = style name of this fix command
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<LI>Nfreq = perform dynamic load balancing every this many steps
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<LI>dimstr = sequence of letters containing "x" or "y" or "z", each not more than once
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<LI>Niter = # of times to iterate within each dimension of dimstr sequence
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<LI>thresh = stop balancing when this imbalance threshhold is reached
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<LI>zero or more keyword/arg pairs may be appended
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</UL>
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<LI>keyword = <I>out</I>
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<PRE> <I>out</I> arg = filename
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filename = output file to write each processor's sub-domain to
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</PRE>
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</UL>
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<P><B>Examples:</B>
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</P>
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<PRE>fix 2 all balance 1000 x 10 1.05
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fix 2 all balance 0 xy 20 1.1 out tmp.balance
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</PRE>
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<P><B>Description:</B>
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</P>
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<P>This command adjusts the size of processor sub-domains within the
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simulation box dynamically as a simulation runs, to attempt to balance
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the number of particles and thus the computational cost (load) evenly
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across processors. The load balancing is "dynamic" in the sense that
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rebalancing is performed periodically during the simulation. To
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perform "static" balancing, before of between runs, see the
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<A HREF = "balance.html">balance</A> command.
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</P>
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<P>Load-balancing is only useful if the particles in the simulation box
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have a spatially-varying density distribution. E.g. a model of a
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vapor/liquid interface, or a solid with an irregular-shaped geometry
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containing void regions. In this case, the LAMMPS default of dividing
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the simulation box volume into a regular-spaced grid of processor
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sub-domain, with one equal-volume sub-domain per procesor, may assign
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very different numbers of particles per processor. This can lead to
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poor performance in a scalability sense, when the simulation is run in
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parallel.
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</P>
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<P>Note that the <A HREF = "processors.html">processors</A> command gives you some
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control over how the box volume is split across
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processors. Specifically, for a Px by Py by Pz grid of processors, it
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lets you choose Px, Py, and Pz, subject to the constraint that Px * Py
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* Pz = P, the total number of processors. This can be sufficient to
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achieve good load-balance for some models on some processor
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counts. However, all the processor sub-domains will still be the same
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shape and have the same volume.
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</P>
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<P>This command does not alter the topology of the Px by Py by Pz grid or
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processors. But it shifts the cutting planes between processors (in
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3d, or lines in 2d), which adjusts the volume (area in 2d) assigned to
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each processor, as in the following 2d diagram. The left diagram is
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the default partitioning of the simulation box across processors (one
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sub-box for each of 16 processors); the right diagram is after
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balancing.
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</P>
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<CENTER><IMG SRC = "JPG/balance.jpg">
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</CENTER>
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<P>IMPORTANT NOTE: This command attempts to minimize the imbalance
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factor, as defined above. But because of the topology constraint that
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only the cutting planes (lines) between processors are moved, there
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are many irregular distributions of particles, where this factor
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cannot be shrunk to 1.0, particuarly in 3d. Also, computational cost
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is not strictly proportional to particle count, and changing the
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relative size and shape of processor sub-domains may lead to
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additional computational and communication overheads, e.g. in the PPPM
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solver used via the <A HREF = "kspace_style.html">kspace_style</A> command. Thus
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you should benchmark the run times of your simulation with and without
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balancing.
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</P>
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<HR>
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<P>The <I>group-ID</I> is currently ignored. In the future it may be used to
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determine what particles are considered for balancing. Normally it
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would only makes sense to use the <I>all</I> group. But in some cases it
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may be useful to balance on a subset of the particles, e.g. when
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modeling large nanoparticles in a background of small solvent
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particles.
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</P>
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<P>The <I>Nfreq</I> setting determines how often a rebalance is performed. If
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<I>Nfreq</I> > 0, then rebalancing will occur every <I>Nfreq</I> steps. Each
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time a rebalance occurs, a reneighboring is triggered, so you should
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not make <I>Nfreq</I> too small. If <I>Nfreq</I> = 0, then rebalancing will be
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done every time reneighboring normally occurs, as determined by the
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the <A HREF = "neighbor.html">neighbor</A> and <A HREF = "neigh_modify.html">neigh_modify</A>
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command settings.
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</P>
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<P>On rebalance steps, rebalancing will only be attempted if the current
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imbalance factor, as defined above, exceeds the <I>thresh</I> setting.
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</P>
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<P>The <I>dimstr</I> argument is a string of characters, each of which must be
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an "x" or "y" or "z". Eacn character can appear zero or one time,
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since there is no advantage to balancing on a dimension more than
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once. You should normally only list dimensions where you expect there
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to be a density variation in the particles.
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</P>
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<P>Balancing proceeds by adjusting the cutting planes in each of the
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dimensions listed in <I>dimstr</I>, one dimension at a time. For a single
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dimension, the balancing operation (described below) is iterated on up
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to <I>Niter</I> times. After each dimension finishes, the imbalance factor
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is re-computed, and the balancing operation halts if the <I>thresh</I>
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criterion is met.
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</P>
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<P>A rebalance operation in a single dimension is performed using a
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density-dependent recursive multisectioning algorithm, where the
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position of each cutting plane (line in 2d) in the dimension is
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adjusted independently. This is similar to a recursive bisectioning
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(RCB) for a single value, except that the bounds used for each
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bisectioning take advantage of information from neighboring cuts if
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possible, as well as counts of particles at the bounds on either side
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of each cuts, which themselves were cuts in previous iterations. The
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latter is used to infer a density of pariticles near each of the
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current cuts. At each iteration, the count of particles on either
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side of each plane is tallied. If the counts do not match the target
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value for the plane, the position of the cut is adjusted based on the
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local density. The low and high bounds are adjusted on each
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iteration, using new count information, so that they become closer
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together over time. Thus as the recustion progresses, the count of
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particles on either side of the plane gets closer to the target value.
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</P>
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<P>The density-dependent part of this algorithm is often an advantage
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when you rebalance a system that is already nearly balanced. It
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typically converges more quickly than the geometric bisectioning
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algorithm used by the <A HREF = "balance.html">balance</A> command. However, if can
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be a disadvants if you attempt to rebalance a system that is far from
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balanced, and converge more slowly. In this case you probably want to
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use the <A HREF = "balance.html">balance</A> command before starting a run, so that
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you begin the run with a balanced system.
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</P>
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<P>Once the rebalancing is complete and final processor sub-domains
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assigned, particles migrate to their new owning processor as part of
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the normal reneighboring procedure.
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</P>
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<P>IMPORTANT NOTE: At each rebalance operation, the RCB operation for
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each cutting plane (line in 2d) typcially starts with low and high
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bounds separated by the extent of a processor's sub-domain in one
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dimension. The size of this bracketing region shrinks based on the
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local density, as described above, which should typically be 1/2 or
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more every iteration. Thus if <I>Niter</I> is specified as 10, the cutting
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plane will typically be positioned to better than 1 part in 1000
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accuracy (relative to the perfect target position). For <I>Niter</I> = 20,
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it will be accurate to better than 1 part in a million. Thus there is
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no need to set <I>Niter</I> to a large value. This is especially true if
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you are rebalancing often enough that each time you expect only an
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incremental adjustement in the cutting planes is necessary. LAMMPS
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will check if the threshold accuracy is reached (in a dimension) is
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less iterations than <I>Niter</I> and exit early.
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</P>
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<P>IMPORTANT NOTE: If a portion of your system is a perfect lattice,
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e.g. a frozen substrate, then the balancer may be unable to achieve
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exact balance. I.e. entire lattice planes will be owned or not owned
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by a single processor. So you you should not expect to achieve
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perfect balance in this case. Nor will it be helpful to use a large
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value for <I>Niter</I>, since it will simply cause the balancer to iterate
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until <I>Niter</I> is reached, without improving the imbalance factor.
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</P>
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<HR>
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<P>The <I>out</I> keyword writes a text file to the specified <I>filename</I> with
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the results of each rebalancing operation. The file contains the
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bounds of the sub-domain for each processor after the balancing
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operation completes. The format of the file is compatible with the
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<A HREF = "pizza">Pizza.py</A> <I>mdump</I> tool which has support for manipulating and
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visualizing mesh files. An example is shown here for a balancing by 4
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processors for a 2d problem:
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</P>
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<PRE>ITEM: TIMESTEP
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1000
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ITEM: NUMBER OF SQUARES
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4
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ITEM: SQUARES
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1 1 1 2 7 6
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2 2 2 3 8 7
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3 3 3 4 9 8
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4 4 4 5 10 9
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ITEM: TIMESTEP
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1000
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ITEM: NUMBER OF NODES
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10
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ITEM: BOX BOUNDS
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-153.919 184.703
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0 15.3919
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-0.769595 0.769595
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ITEM: NODES
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1 1 -153.919 0 0
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2 1 7.45545 0 0
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3 1 14.7305 0 0
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4 1 22.667 0 0
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5 1 184.703 0 0
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6 1 -153.919 15.3919 0
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7 1 7.45545 15.3919 0
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8 1 14.7305 15.3919 0
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9 1 22.667 15.3919 0
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10 1 184.703 15.3919 0
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</PRE>
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<P>The "SQUARES" lists the node IDs of the 4 vertices in a rectangle for
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each processor (1 to 4). The first SQUARE 1 (for processor 0) is a
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rectangle of type 1 (equal to SQUARE ID) and contains vertices
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1,2,7,6. The coordinates of all the vertices are listed in the NODES
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section. Note that the 4 sub-domains share vertices, so there are
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only 10 unique vertices in total.
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</P>
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<P>For a 3d problem, the syntax is similar with "SQUARES" replaced by
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"CUBES", and 8 vertices listed for each processor, instead of 4.
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</P>
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<P>Each time rebalancing is performed a new timestamp is written with new
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NODES values. The SQUARES of CUBES sections are not repeated, since
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they do not change.
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</P>
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<HR>
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<P><B>Restart, fix_modify, output, run start/stop, minimize info:</B>
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</P>
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<P>No information about this fix is written to <A HREF = "restart.html">binary restart
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files</A>. None of the <A HREF = "fix_modify.html">fix_modify</A> options
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are relevant to this fix.
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</P>
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<P>This fix computes a global scalar which is the imbalance factor
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after the most recent rebalance and a global vector of length 3 with
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additional information about the most recent rebalancing. The 3
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values in the vector are as follows:
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</P>
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<UL><LI>1 = max # of particles per processor
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<LI>2 = total # iterations performed in last rebalance
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<LI>3 = imbalance factor right before the last rebalance was performed
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</UL>
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<P>As explained above, the imbalance factor is the ratio of the maximum
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number of particles on any processor to the average number of
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particles per processor.
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</P>
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<P>These quantities can be accessed by various <A HREF = "Section_howto.html#howto_15">output
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commands</A>. The scalar and vector values
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calculated by this fix are "intensive".
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</P>
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<P>No parameter of this fix can be used with the <I>start/stop</I> keywords of
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the <A HREF = "run.html">run</A> command. This fix is not invoked during <A HREF = "minimize.html">energy
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minimization</A>.
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</P>
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<HR>
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<P><B>Restrictions:</B> none
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</P>
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<P><B>Related commands:</B>
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</P>
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<P><A HREF = "processors.html">processors</A>, <A HREF = "balance.html">balance</A>
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</P>
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<P><B>Default:</B> none
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</P>
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</HTML>
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