MPI parallelization

The module mpy implements parallelization by domain decomposition (distributed memory) using the Message Passing Interface (MPI) implemented by OpenMPI. It aims at exploiting large numbers of compute nodes by running independent instances of yade on them. The shared memory and the distributed memory approaches are compatible, i.e. it is possible to run hybrid jobs using both, and it may well be the optimal solution in some cases.

Most (initially all) calls to OpenMPI library are done in Python using mpi4py. However for the sake of efficiency some critical communications are triggered via python wrappers of C++ functions, wherein messages are produced, sent/received, and processed.

This module development was started in 2018. It received contributions during a HPC hackathon. An extension enables parallel coupling with OpenFoam.

Note

Disclaimer: even though the yade.mpy module provides the function mpirun, which may seem as a simple replacement for O.run(), setting up a simulation with mpy might be deceptively triavial. As of now, it is anticipated that, in general, a simple replacement of “run” by “mpirun” in an arbitrary script will not speedup anything and may even fail miserably (it could be improved in the future). To understand why, and to tackle the problems, basic knowledge of how MPI works will certainly help (specifically mpi4py).

Concepts

subdomain: a (sub)set of bodies attached to one MPI process after domain decomposition - with or without spatial coherence. The corresponding class in Yade is Subdomain, a Shape instance with helper functions for MPI communications. In some sense Subdomain is to subscribed bodies what Clump (another Shape) is to clump members.

rank: subdomain index from 0 to N-1 (with N the number of mpi processes) to identify subdomains. The rank of the subdomain a body belongs to can be retrieved as Body.subdomain. Each subdomain corresponds to an instance of yade and a specific scene during parallel execution. The rank of the scene is given by Scene.subdomain.

master: refers to subdomain with rank =0. This subdomain does not behave like others. In general master will handle boundary conditions and it will control transitions and termination of the whole simulation. Unlike standard subdomains it may not contain a large number of raw bodies (i.e. not beyond objects bounding the scene such as walls or boxes). In interactive execution master is the process responding to the python prompt.

splitting and merging: cutting a master Scene into a set of smaller, distributed, scenes is called “splitting”. The split is undone by a ‘merge’, by which all bodies and (optionally) all interactions are sent back to the master thread. Splitting, running, then merging, should leave the scene just as if no MPI had been used at all (i.e. as if the same number of iterations had been executed in single-thread). Therefore normal O.run() after that should work as usual.

intersections: subsets of bodies in a subdomain intersected by the bounding box of other subdomains (see fig-subdomains). intersection(i,j) refers to the bodies owned by current (i) subdomain and intersecting subdomain j (retrieved as O._sceneObj.subD.intersections[j]); mirrorIntersection(i,j) refers to bodies owned by j and intersecting current domain (retrieved as O._sceneObj.subD.mirrorIntersections[j]). The bodies are listed by Body.id. By definition intersection(i,j)=mirrorIntersection(j,i).

The intersections and mirror intersections are updated automatically as part of parallel collision detection. They define which body states need to be communicated. The bodies in intersections need to be sent to other subdomains (in pratice only updated position and velocity are sent at every iteration), the bodies in mirrorIntersections need to be received from other subdomains.

_images/subdomains.png

Two overlapping subdomains and their intersections. In this situation we have SubD1.intersections[SubD2.subdomain]=[id4,id5] and SubD1.mirrorIntersections[SubD2.subdomain]=[id1], with SubD1 and SubD2 instances of Subdomain.

Walkthrough

For demonstrating the main internal steps in the implemented parallel algorithm let us consider the example script examples/mpi/testMPI_2D.py. Executing this script (interactive or passive mode) with three MPI processes generates the scene as shown in fig-scene-mpi. It then executes mpirun, which triggers the steps described hereafter.

_images/mpyschema0.svg

In this scene, we have three MPI processes (three subdomains) and the raw bodies are partitioned among the subdomains/ranks 1 and 2. The master process with subdomain=0 holds the boundary/wall type body. Bodies can be manually assigned or automatically assigned via a domain decomposition algorithm. Details on the dommain decomposition algorithm is presented in the later section of this document.

Scene splitting :

In the function mpy.splitScene, called at the beginning of mpi execution, specific engines are added silently to the scene in order to handle what will happen next. That very intrusive operation can even change settings of some pre-existing engines, in particular InsertionSortCollider, to make them behave with MPI-friendlyness. InsertionSortCollider.verletDist is an important factor controlling the efficiency of the simulations. The reason for this will become evident in the later steps.

Bounds dispatching : In the next step, the Body.bound is dispatched with the Aabb extended as shown in figure fig-regularbounds (in dotted lines). Note that the Subdomain Aabb is obtained from taking the min and max of the owned bodies, see figure fig-subDBounds with solid coloured lines for the subdomain Aabb. At this time, the min and max of other subdomains are unknown.

_images/mpyschema1a.svg
_images/mpyschema1b.svg

Update of Domain bounds : Once the bounds for the regular bodies and the local subdomain has been dispatched, information on the other subdomain bounds are obtained via the function mpy.updateDomainBounds. In this collective communication, each subdomain broadcasts its Aabb.min and Aabb.max to other subdomains. Figure fig-subdomain-bounds shows a schematic in which each subdomain has received the Aabb.min and Aabb.max of the other subdomains.

_images/mpyschema2.svg

Parallel Collision detection :

  • Once the Aabb.min and Aabb.max of the other subdomains are obtained, the collision detection algorithm is used to determine the bodies that have intersections with the remote subdomains. The ids of the identified bodies are then used to build the Subdomain.intersections list.
_images/mpyschema3.svg
  • Next step involves obtaining the ids of the remote bodies intersecting with the current subdomain (Subdomain.mirrorIntersections). Each subdomain sends its list of local body intersections to the respective remote subdomains and also receives the list of intersecting ids from the other subdomains. If the remote bodies do not exist within the current subdomain’s BodyContainer, the subdomain then requests these remote bodies from the respective subdomain. A schematic of this operation is shown in figure fig-mirrorIntersections, in which subdomain=1 receives three bodies from subdomain=2, and 1 body from subdomain=0. subdomain=2 receives three bodies from subdomain=1. subdomain=0 only sends its bodies and does not receive from the worker subdomains. This operation sets the stage for communication of the body states to/from the other subdomains.
_images/mpysendBodies.svg

Update states :

Once the subdomains and the associated intersecting bodies, and remote bodies are identified, State of these bodies are sent and received every timestep, by peer-to-peer communications between the interacting subdomains. In the case of an interaction with the master subdomain (subdomain=0), only the total force and torque exerted on master’s bodies by a given subdomain are sent. Figure fig-sendRecvStates shows a schematic in which the states of the remote bodies between subdomain=1 and subdomain=2 are communicated. Subdomain=0 receives forces and torques from subdomain=1 and subdomain=2.

_images/mpyschema4.svg

MPI initialization and communications

The mpy modules tries to retain from yade design an important feature: interactive access to the objects of scene (or of multiple scenes in this case), as explained below. Interactive execution does not use the mpiexec command of OpenMPI, a pool of workers is spawned by the mpy module after yade startup, instead. In production one may use passive jobs, and in that case mpiexec will preceed the call to yade.

Note

Most examples in this page use 4 mpi processes. It is not a problem, in principle, to run the examples even if the number of available cores is less than 4 (this is called oversubscribing (it may also fail depending on OS and MPI implementation). There is no performance gain to expect from oversubscribing but it is useful for experiments (e.g. for testing the examples in this page on a single-core machine).

Interactive mode

The interactive mode aims primarily at inspecting the simulation after some MPI execution for debugging. Functions shown here (especially sendCommand) may also be usefull in the general case, to achieve advanced tasks such as controlling transitions between phases of a simulation, collecting and processing results.

Explicit initialization from python prompt

A pool of yade instances can be spawned with mpy.initialize() as illustrated hereafter. Mind that the next sequences of commands are supposed to be typed directly in the python prompt after starting yade, it will not give exactly the same result if it is pasted into a script executed by Yade (see the next section on automatic initialization):

@suppress
Yade [1]: from yade.utils import *

@suppress
Yade [1]: O.engines=yade.utils.defaultEngines

Yade [2]: wallId=O.bodies.append(box(center=(0,0,0),extents=(2,0,1),fixed=True))

Yade [3]: for x in range(-1,2):
   ...:    O.bodies.append(sphere((x,0.5,0),0.5))
   ...:

Yade [5]: from yade import mpy as mp

@suppress
Yade [5]: mp.COLOR_OUTPUT=False

@doctest
Yade [6]: mp.initialize(4)
Master: I will spawn  3  workers
->  [6]: (0, 4)

After mp.initialize(np) the parent instance of yade takes the role of master process (rank=0). It is the only one executing the commands typed directly in the prompt. The other instances (rank=1 to rank=np-1) are idle and they wait for commands sent from master. Sending commands to the other instances can be done with mpy.sendCommand(), which by default returns the result or the list of results. We use that command below to verify that the spawned workers point to different (still empty) scenes:

Yade [8]: len(O.bodies)
 ->  [8]: 4

Yade [10]: mp.sendCommand(executors="all",command="len(O.bodies)",wait=True) #check content
->  [10]: [4, 0, 0, 0]

Yade [9]: mp.sendCommand(executors="all",command="str(O)") # check scene pointers
 ->  [9]:
['<yade.wrapper.Omega object at 0x7f9c0a399490>',
'<yade.wrapper.Omega object at 0x7f9231213490>',
'<yade.wrapper.Omega object at 0x7f20086a1490>',
'<yade.wrapper.Omega object at 0x7f622b47f490>']

Sending commands makes it possible to manage all types of message passing using calls to the underlying mpi4py (see mpi4py documentation). Be carefull with sendCommand “blocking” behavior by default. Next example would hang without “wait=False” since both master and worker would be waiting for a message from each other.

Yade [3]: mp.sendCommand(executors=1,command="message=comm.recv(source=0); print('received',message)",wait=False)

Yade [4]: mp.comm.send("hello",dest=1)
received hello

Every picklable python object (namely, nearly all Yade objects) can be transmitted this way. Remark hereafter the use of mpy.mprint (identifies the worker by number and by font colors). Note also that the commands passed via sendCommand are executed in the context of the mpy module, for this reason comm, mprint, rank and all objects of the module are accessed without the mp. prefix.

Yade [3]: mp.sendCommand(executors=1,command="O.bodies.append(comm.recv(source=0))",wait=False) # leaves the worker idle waiting for an argument to append()

Yade [4]: b=Body(shape=Sphere(radius=0.7))  # now create body in the context of master

Yade [5]: mp.comm.send(b,dest=1) # send it to worker 1

Yade [6]: mp.sendCommand(executors="all",command="mprint('received',[b.shape.radius if hasattr(b.shape,'radius') else None for b in O.bodies])")
Master: received [None, 0.5, 0.5, 0.5]
Worker1: received [0.7]
Worker3: received []
Worker2: received []
->  [5]: [None, None, None, None] # printing yields no return value, hence that empty list of returns, "wait=False" argument to sendCommand would suppress it

Explicit initialization from python script

Though usefull for advanced operations, the function sendCommand() is limited. Basic features of the python language are missing, e.g. function definitions and loops are a problem - in fact every code fragment which can’t fit on a single line is. In practice the mpy module provides a mechanism to initialize from a script, where functions and variables will be declared.

Whenever Yade is started with a script as argument the script name will be remembered, and if mpy.initialize() is called (by the script itself or interactively in the prompt) all Yade instances will be initialized with that same script. It makes distributing function definitions and simulation parameters trivial (and even distributing scene constructions as seen below).

This behaviour is what happens usually with MPI: all processes execute the same program. It is also what happens with “mpiexec -np N yade …”.

If the first commands above are pasted into a script used to start Yade, all workers insert the same bodies as master (with interactive execution only master was inserting). Here is the script:

# script 'test1.py'
wallId=O.bodies.append(box(center=(0,0,0),extents=(2,0,1),fixed=True))
for x in range(-1,2):
        O.bodies.append(sphere((x,0.5,0),0.5))
from yade import mpy as mp
mp.initialize(4)
print( mp.sendCommand(executors="all",command="str(O)",wait=True) )
print( mp.sendCommand(executors="all",command="len(O.bodies)",wait=True) )

and the output reads:

yade test1.py
...
Running script test1.py
Master: will spawn  3  workers
None
None
None
None
None
None
['<yade.wrapper.Omega object at 0x7feb979403a0>', '<yade.wrapper.Omega object at 0x7f5b61ae9440>', '<yade.wrapper.Omega object at 0x7fdd466b8440>', '<yade.wrapper.Omega object at 0x7f8dc7b73440>']
[4, 4, 4, 4]

That’s because all instances execute the script in the initialize() phase. “None” is printed 2x3 times because the script contains print( mp.sendCommand(…)) twice, the workers try to execute that too, but for them sendCommand returns by default, hence the None.

Though logical, this result is not what we want if we try to split a simulation into pieces. The solution (typical of all mpi programs) is to use the rank of the process in conditionals. Different parts of the script can then be executed, differently, by each worker, depending on its rank. In order to produce the same result as before for instance, the script can be modified as follows:

# script 'test2.py'
from yade import mpy as mp
mp.initialize(4)
if mp.rank==0: # only master
        wallId=O.bodies.append(box(center=(0,0,0),extents=(2,0,1),fixed=True))
        for x in range(-1,2):
        O.bodies.append(sphere((x,0.5,0),0.5))

        print( mp.sendCommand(executors="all",command="str(O)",wait=True) )
        print( mp.sendCommand(executors="all",command="len(O.bodies)",wait=True) )
        print( mp.sendCommand(executors="all",command="str(O)",wait=True) )

Resulting in:

Running script test2.py
Master: will spawn  3  workers
['<yade.wrapper.Omega object at 0x7f21a8c8d3a0>', '<yade.wrapper.Omega object at 0x7f3142e43440>', '<yade.wrapper.Omega object at 0x7fb699b1a440>', '<yade.wrapper.Omega object at 0x7f1e4231e440>']
[4, 0, 0, 0]

We could also use rank to assign bodies from different regions of space to different workers, as found in example examples/mpi/helloMPI.py, with rank-dependent positions:

# rank is accessed without "mp." prefix as it is interpreted in mpy module's scope
mp.sendCommand(executors=[1,2],command= "ids=O.bodies.append([sphere((xx,1.5+rank,0),0.5) for xx in range(-1,2)])")

Keep in mind that the position of the call mp.initialize(N) relative to the other commands has no consequence for the execution by the workers (for them initialize() just returns), hence program logic should not rely on it. The workers execute the script from begin to end with the same MPI context, already set when the first line is executed. It can lead to counter intuitive behavior, here is a script:

# testInit.py
# script.py
O.bodies.append([Body() for i in range(100)])

from yade import mpy as mp
mp.mprint("before initialize: rank ", mp.rank,"/", mp.numThreads,"; ",len(O.bodies)," bodies")
mp.initialize(2)
mp.mprint("after initialize: rank ", mp.rank,"/", mp.numThreads,"; ",len(O.bodies)," bodies")

and the output:

Running script testInit.py
Master: before initialize: rank  0 / 1 ;  100  bodies
Master: will spawn  1  workers
Master: after initialize: rank  0 / 2 ;  100  bodies
Worker1: before initialize: rank  1 / 2 ;  100  bodies
Worker1: after initialize: rank  1 / 2 ;  100  bodies

mpirun (automatic initialization)

Effectively running a distributed DEM simulation on the basis of just the above commands would be tedious. The mpy modules thus provides the function mpy.mpirun to automatize most of the steps, as described in introduction. Mainly, splitting the scene in subdomains based on rank assigned to bodies and handling collisions between the subdomains as time integration proceeds (includes changing the engine list agressively to make this all happen).

If needed, the first execution of mpirun will call the function initialize(), which can therefore be omitted on user’s side. The subdomains will be merged into a centralized scene on master process at the end of the iterations depending on argument withMerge.

Here is a concrete example where a floor is assigned to master and multiple groups of spheres are assigned to subdomains:

import os
from yade import mpy as mp

NSTEPS=5000 #turn it >0 to see time iterations, else only initilization
numThreads = 4 # number of threads to be spawned, (in interactive mode).

#materials
young = 5e6
compFricDegree = 0.0
O.materials.append(FrictMat(young=young, poisson=0.5, frictionAngle = radians(compFricDegree), density= 2600, label='sphereMat'))
O.materials.append(FrictMat(young=young*100, poisson = 0.5, frictionAngle = compFricDegree, density =2600, label='wallMat'))


#add spheres

mn,mx=Vector3(0,0,0),Vector3(90,180,90)
pred = pack.inAlignedBox(mn,mx)
O.bodies.append(pack.regularHexa(pred,radius=2.80,gap=0, material='sphereMat'))

#walls (floor)

wallIds=aabbWalls([Vector3(-360,-1,-360),Vector3(360,360,360)],thickness=10.0, material='wallMat')
O.bodies.append(wallIds)

#engines
O.engines=[
        ForceResetter(),
        InsertionSortCollider([
                Bo1_Sphere_Aabb(),
                Bo1_Box_Aabb()], label = 'collider'), # always add labels.
        InteractionLoop(
                [Ig2_Sphere_Sphere_ScGeom(),Ig2_Box_Sphere_ScGeom()],
                [Ip2_FrictMat_FrictMat_FrictPhys()],
                [Law2_ScGeom_FrictPhys_CundallStrack()],
                label="interactionLoop"
        ),
        GlobalStiffnessTimeStepper(timestepSafetyCoefficient=0.3,  timeStepUpdateInterval=100, parallelMode=True, label = 'timeStepper'),
        NewtonIntegrator(damping=0.1,gravity = (0, -0.1, 0), label='newton'),
        VTKRecorder(fileName='spheres/3d-vtk-', recorders=['spheres', 'intr', 'boxes'], parallelMode=True,iterPeriod=500), #use .pvtu to open spheres, .pvtp for ints, and .vtu for boxes.
]

#set a custom verletDist for efficiency.
collider.verletDist = 1.5

#########  RUN  ##########
# customize mpy
mp.ERASE_REMOTE_MASTER = True   #keep remote bodies in master?
mp.DOMAIN_DECOMPOSITION= True   #automatic splitting/domain decomposition
#mp.mpirun(NSTEPS)              #passive mode run
mp.MERGE_W_INTERACTIONS = False
mp.mpirun(NSTEPS,numThreads,withMerge=True) # interactive run, numThreads is the number of workers to be initialized, see below for withMerge explanation.
mp.mergeScene()  #merge scene after run.
if mp.rank == 0: O.save('mergedScene.yade')

#demonstrate getting stuff from workers, here we get kinetic energy from worker subdomains, notice that the master (mp.rank = 0), uses the sendCommand to tell workers to compute kineticEnergy.
if mp.rank==0:
        print("kinetic energy from workers: "+str(mp.sendCommand([1,2],"kineticEnergy()",True)))

The script is then executed:

yade script.py

For running further timesteps, the mp.mpirun command has to be executed in yade prompt:

Yade [0]: mp.mpirun(100,4,withMerge=False) #run for 100 steps and no scene merge.

Yade [1]: mp.sendCommand([1,2],"kineticEnergy()",True) # get kineticEnergy from workers 1 and 2.

Yade [2]: mp.mpirun(1,4,withMerge=True) # run for 1 step and merge scene into master. Repeat multiple time to watch evolution in QGL view

Non-interactive execution

Instead of spawning mpi processes after starting yade, it is possible to run yade with the classical “mpiexec” from OpenMPI. Importantly, it may be the only method allowed through HPC job submission systems. When using mpiexec there is no interactive shell, or a broken one (which is ok in general in production). The job needs to run (or “mpirun”) and terminate by itslef.

The functions initialize and mpirun described above handle both interactive and passive executions transparently, and the user scripts should behave the same in both cases. “Should”, since what happens behind the scene is not exactly the same at startup, and it may surface in some occasions (let us know).

Provided that a script calls mpy.mpirun with a number of timesteps, the simulation (see e.g. examples/mpi/vtkRecorderExample.py) is executed with the following command:

mpiexec -np NUMSUBD+1 yade vtkRecorderExample.py

where NUMSUBD corresponds to the required number of subdomains.

Note

Remember that the master process counts one while it does not handle an ordinary subdomain, therefore the number of processes is always NUMSUBD +1.

Splitting

Splitting an initial scene into subdomains and updating the subdomains after particle motion are two critical issues in terms of efficiency. The decomposition can be prescribed on users’s side (first section below), but mpy module also provides algorithms for both.

Note

The mpy module has no requirement in terms of how the subdomains are defined, and using the helper functions described here is not a requirement. Even assigning randomly the bodies from a large cloud to a number of subdomains (such that the subdomains overlap each other and the scene entirely) would work. It would only be suboptimal as the number of interactions between subdomains would increase compared to a proper partition of space.

Split by yourself

In order to impose a decomposition it is enough to assign Body.subdomain a value corresponding to the process rank it should belong to. This can be done either in one centralized scene that is later splitted, or by inserting the correct subsets of bodies independently in each subdomain (see section on scene construction)

In the example script examples/mpi/testMPI_2D.py the spheres are generated as follows (centralized construction in this example, easily turned into distributed one). For each available worker a bloc of spheres is generated with a different position in space. The spheres in each block are assigned a subdomain rank (and a color for visualisation) so that they will be picked up by the right worker after mpirun().:

for sd in range(0,numThreads-1):
        col = next(colorScale)
        ids=[]
        for i in range(N):#(numThreads-1) x N x M spheres, one thread is for master and will keep only the wall, others handle spheres
                for j in range(M):
                        id = O.bodies.append(sphere((sd*N+i+j/30.,j,0),0.500,color=col)) #a small shift in x-positions of the rows to break symmetry
                        ids.append(id)
                for id in ids: O.bodies[id].subdomain = sd+1

Don’t know how to split? Leave it to mpirun

Initial split

mpirun will decide by itself how to distribute the bodies across several subdomains if DOMAIN_DECOMPOSITION =True. In such case the difference between the sequential script and its mpi version is limited to importing mpy and calling mpirun after turning the DOMAIN_DECOMPOSITION flag.

The automatic splitting of bodies to subdomains is based on the Orthogonal Recursive Bisection Algortithm of Berger [Berger1987], and [Fleissner2007]. The partitioning is based on bisecting the space at several levels, with the longest axis in each level chosen as the bisection axis. The number of levels is determined as \(int(log_{2}(N_{w}))\) with \(N_{w}\) being the number of worker subdomains. A schematic of this decomposition is shown in fig-bisectionAlgo, with 4 worker subdomains. At the initial stage (level = 0), we assume that subdomain=1 contains the information of the body positions (and bodies), the longest axis is first determined, this forms the bisectioning axis/plane. The list containing the body positions is sorted along the bisection axis, and the median of this sorted list is determined. The bodies with positions (bisection coordinate) less than the median is coloured with the current subdomain, (SD=1) and the other half is coloured with SD = 2, the subdomain colouring at each level is determined using the following rule:

if (subdomain <  1<<level) : this subdomain gets the bodies with position lower than the median.
if ((subdomain >  1<<level) and (subdomain <  1<<(level+1) ) ) : this subdomain gets the bodies with position greater than median, from subdomain - (1<<level)

This process is continued until the number of levels are reached.

_images/mpyrecursuveBisection.svg

Figure fig-domainDecompose shows the resulting partitioning obtained using the ORB algorithm : (a) for 4 subdomains, (b) for 8 subdomains. Odd number of worker subdomains are also supported with the present implementation.

_images/mpyddcmp.svg

The present implementation can be found in py/bisectionDecomposition.py, and a parallel version can be found here.

Note

importing py/bisectionDecomposition.py triggers the import of mpi4py and ultimately of the MPI library and related environment variables. The mpy module may change some mpi settings on import, therefore it is safer to only import bisectionDecomposition after some mpy.initialize().

Updating the decomposition (load balancing)

As the bodies move each subdomain may experience overall distorsion and diffusion of bodies to/from other subdomains. We want to keep the subdomains as compact as possible to minimize communications, instead. An algorithm does that dynamically if mpy.REALLOCATE_FREQUENCY >0. It exploits InsertionSortCollider to reassign bodies efficiently and in synchronicity with collision detection.

The algorithm is not centralized in order to preserve scalability. It involves only peer-to-peer communications between the workers which share an intersection. The re-assignment depends on a filter for local decisions. The currently available filter is mpy.medianFilter. Custom filters may be used instead.

The criterion for re-allocating bodies in the median filter involves finding the position a median plane between two subdomains such that after discriminating bodies on the “+” and “-” side of that plane the total number in each subdomain is preserved. It results in the type of split shown in the video hereafter. Even though the median planes seem to rotate rather quickly at some point in this video, there are actually five collision detections between each re-allocation, i.e. thousands of time iterations to effectively rotate the split between two different colors. These progressive rotations are for good since the initial split would have resulted in flat discs otherwise.

Note

This is not a load balancing in the sense of achieving an equal amount of work per core. In fact that sort of balancing is achieved by definition already as soon as each worker is assigned the same amount of bodies (and because a subdomain is really ultimately a list of bodies, not a specific region of space). Instead the objective is to decrease the communication times overall.

Centralized versus distributed scene construction

In the centralized method of scene construction, the master process creates all the bodies of a scene and assigns subdomains to them. As part of mpy initialization some engines will be modified or inserted, then the scene is broadcasted to the workers. Each worker receives the entire scene, identifies its assigned bodies via Body.subdomain (if worker’s rank==b.subdomain the bodies are retained) and erase the others. Such a scene construction was used in the previous example and it is by far the simplest. It makes no real difference with building a scene for non-MPI execution besides calling mp.mpirun instead or jusr O.run.

For large number of bodies and processes, though, the centralized scene construction and distribution can take a significant time. It can also be memory bound since the memory usage is quadratic: suppose N bodies per thread on a 32-core node, centralized construction implies that 32 copies of the entire scene exist simultaneously in memory at some point in time (during the split), i.e. \(32^2 N\) bodies on one single node. For massively parallel applications distributed construction should be prefered.

In distributed mode each worker instantiates its own bodies and insert them in the local BodyContainer. Attention need to be paid to properly assign bodies ids since no index should be owned by two different workers initially. Insertion of bodies in BodyContainer with imposed ids is done with BodyContainer.insertAtId. The distributed mode is activated by setting the DISTRIBUTED_INSERT flag ON, the user is in charge of setting up the subdomains and partitioning the bodies, an example showing the use of distributed insertion can be found in examples/mpi/parallelBodyInsert3D.py.

The relevant fragment, where the filtering is done by skipping all steps of a loop but the one with proper rank (keep in mind that all workers will run the same loop but they all have a different rank each), reads:

#add spheres
subdNo=0
import itertools
_id = 0 #will be used to count total number of bodies regardless of subdomain attribute, so that same ids are not reused for different bodies
for x,y,z in itertools.product(range(int(Nx)),range(int(Ny)),range(int(Nz))):
        subdNo+=1
        if mp.rank!=subdNo: continue
        ids=[]
        for i in range(L):#(numThreads-1) x N x M x L spheres, one thread is for master and will keep only the wall, others handle spheres
                for j in range(M):
                        for k in range(N):
                                dxOndy = 1/5.; dzOndy=1/15.  # shifts in x/y-positions to make columns inclines
                                px= x*L+i+j*dxOndy; pz= z*N+k+j*dzOndy; py = (y*M+j)*(1 -dxOndy**2 -dzOndy**2)**0.5 #so they are always nearly touching initialy
                                id = O.bodies.insertAtId(sphere((px,py,pz),0.500),_id+(N*M*L*(subdNo-1)))
                                _id+=1
                                ids.append(id)
        for id in ids: O.bodies[id].subdomain = subdNo

if mp.rank==0: #the wall belongs to master
        WALL_ID=O.bodies.insertAtId(box(center=(Nx*L/2,-0.5,Nz*N/2),extents=(2*Nx*L,0,2*Nz*N),fixed=True),(N*M*L*(numThreads-1)))

The bissection algorithm can be used for defining the initial split, in the distributed case too, since it takes a points dataset as input. Provided that all workers work with the same dataset (e.g. the same sequence of a random number generator) they will all reach the same partitioning, and they can instanciate their bodies on this basis.

Merging

The possibility of a “merge” emerged in previous example, as an optional argument of mpirun or as a standalone function mpy.mergeScene .

If withMerge=True in mpirun then the bodies in master scene are updated to reflect the evolution of their distributed clones. This is done once after finishing the required number of iterations in mpirun. This merge operation can include updating interactions. mpy.mergeScene does the same within current iteration. Merging is an expensive task which requires the communication of large messages and, therefore, it should be done purposely and at a reasonable frequency. It can even be the main bottleneck for massively parallel scenes. Nevertheless it can be usefull for debugging using the 3D view, or for various post-processing tasks. The MERGE_W_INTERACTIONS provides full merge, i.e. the interactions in the worker subdomains and between the subdomains are included, else only the position and states of the bodies are use. Merging with interactions should result in a usual yade scene, ready for further time-stepping in non-mpi mode or (more usefull) for some post-processing. The merge operation is not required for a proper time integration in general.

Hints and problems to expect

MPI support in engines

For MPI cases, the parallelMode flag for GlobalStiffnessTimeStepper and VTKRecorder have to be turned on. They are the only two engines upgraded with MPI support at the moment.

For other things. Read next section and be carefull. If you feel like implementing MPI support for another couple engines that would be great (the two available exemples should help). Let us know!

Reduction (partial sums)

Quantities such as kinetic energy cannot be obtained for the entire scene just by summing the return value of kineticEnergy() from each subdomain. This is because each subdmomain may contain also images of bodies from intersecting subdomains and they may add their velocity, mass, or whatever is summed, to what is returned by each worker. Although some most-used functions of yade may progressively get mpi support to filter out bodies from remote domains, it is not standard yet and therefore partial sums may need to be implemented on a case-by-case basis, with proper filtering in the user script.

This is just an example of why many things may go wrong if run is directly replaced by mpirun in a complex script.

Miscellaneous

  • sendCommand() has a hardcoded latency of 0.001s to not keep all cores 100% busy waiting for a command (with possibly little left to OS). If sendCommand() is used at high frequency in complex algorithms it might be beneficial to decrease that sleep time.

Control variables

  • VERBOSE_OUTPUT : Details on each operation/step (such as mpy.splitScene, mpy.parallelCollide etc) is printed on the console, useful for debugging purposes
  • ACCUMULATE_FORCES : Control force summation on bodies owned by the master.
  • ERASE_REMOTE_MASTER : Erase remote bodies in the master subdomain or keep them as unbounded ? Useful for fast merge.
  • OPTIMIZE_COM, USE_CPP_MPI : Use optimized communication functions and MPI functions from Subdomain class
  • YADE_TIMING : Report timing statistics, prints time spent in communications, collision detection and other operations.
  • DISTRIBUTED_INSERT : Bodies are created and inserted by each subdomain, used for distributed scene construction.
  • DOMAIN_DECOMPOSITION : If true, the bisection decomposition algorithm is used to assign bodies to the workers/subdomains.
  • MINIMAL_INTERSECTIONS : Reduces the size of position/velocity communications (at the end of the colliding phase, we can exclude those bodies with no interactions besides body<->subdomain from intersections).
  • REALLOCATE_FREQUENCY : if > 0, bodies are migrated between subdomains for efficient load balancing. If =1 realloc. happens each time collider is triggered, else every N collision detection
  • REALLOCATE_MINIMAL : Intersections are minimized before reallocations, hence minimizing the number of reallocated bodies
  • USE_CPP_REALLOC : Use optimized C++ functions to perform body reallocations
  • FLUID_COUPLING : Flag for coupling with OpenFOAM.

Benchmark

_images/mpybenchmarkDahu.png

Comments:

  • From 1k particles/core to 8k particles/core there is a clear improvement. Obviously 1k is too small and most of the time is spent in comunications.
  • From 8k/core to 64k/core the throughput per core is more or less the same, and the performance is not too far from linear. The data includes elimination of random noise, and overall it is not clear to me which non-linearity comes from the code and which one comes from the hardware.
  • Conclusion, if you don’t have at least 8k spheres/core (maybe less for more compex shapes) mpi is not your friend. This in line with the estimate of 10k by Dion Weatherley (DEM8+beer)
  • It looks like OpenMP sucks, but be aware that the benchmark script is heavily tuned for MPI. It includes huges verletDist and more time wasted on virtual interactions to minimize global updates.
  • I believe tuning for OpenMP could make -j26 (or maybe 2xMPIx -j13) on par or faster than 26 MPI threads for less than a million particle. Given the additional difficulty, MPI’s niche is for more than a million particles or more than one compute node.
  • the nominal per-core throughput is not impressive. On an efficient script my laptop can approach 1e6Cu while we get 0.3e6Cu per core on Dahu. MPI is not to blame here, my laptop would also outperform Dahu on a single core.