cpu/__init__.py

-from pystencils.cpu.kernelcreation import createKernel, addOpenMP
-from pystencils.cpu.cpujit import makePythonFunction
-from pystencils.backends.cbackend import generateC

cpu/cpujit.py

-import os
-import subprocess
-from ctypes import cdll, c_double, c_float, sizeof
-from tempfile import TemporaryDirectory
-from pystencils.backends.cbackend import generateC
-import numpy as np
-
-
-CONFIG_GCC = {
-    'compiler': 'g++',
-    'flags': '-Ofast -DNDEBUG -fPIC -shared -march=native -fopenmp',
-}
-CONFIG_INTEL = {
-    'compiler': '/software/intel/2017/bin/icpc',
-    'flags': '-Ofast -DNDEBUG -fPIC -shared -march=native -fopenmp -Wl,-rpath=/software/intel/2017/lib/intel64',
-    'env': {
-        'INTEL_LICENSE_FILE': '1713@license4.rrze.uni-erlangen.de',
-        'LM_PROJECT': 'iwia',
-    }
-}
-CONFIG_CLANG = {
-    'compiler': 'clang++',
-    'flags': '-Ofast -DNDEBUG -fPIC -shared -march=native -fopenmp',
-}
-CONFIG = CONFIG_GCC
-
-
-def ctypeFromString(typename, includePointers=True):
-    import ctypes as ct
-
-    typename = typename.replace("*", " * ")
-    typeComponents = typename.split()
-
-    basicTypeMap = {
-        'double': ct.c_double,
-        'float': ct.c_float,
-        'int': ct.c_int,
-        'long': ct.c_long,
-    }
-
-    resultType = None
-    for typeComponent in typeComponents:
-        typeComponent = typeComponent.strip()
-        if typeComponent == "const" or typeComponent == "restrict" or typeComponent == "volatile":
-            continue
-        if typeComponent in basicTypeMap:
-            resultType = basicTypeMap[typeComponent]
-        elif typeComponent == "*" and includePointers:
-            assert resultType is not None
-            resultType = ct.POINTER(resultType)
-
-    return resultType
-
-
-def ctypeFromNumpyType(numpyType):
-    typeMap = {
-        np.dtype('float64'): c_double,
-        np.dtype('float32'): c_float,
-    }
-    return typeMap[numpyType]
-
-
-def compileAndLoad(kernelFunctionNode):
-    with TemporaryDirectory() as tmpDir:
-        srcFile = os.path.join(tmpDir, 'source.cpp')
-        with open(srcFile, 'w') as sourceFile:
-            print('#include <iostream>', file=sourceFile)
-            print("#include <cmath>", file=sourceFile)
-            print('extern "C" { ', file=sourceFile)
-            print(generateC(kernelFunctionNode), file=sourceFile)
-            print('}', file=sourceFile)
-
-        compilerCmd = [CONFIG['compiler']] + CONFIG['flags'].split()
-        libFile = os.path.join(tmpDir, "jit.so")
-        compilerCmd += [srcFile, '-o', libFile]
-        configEnv = CONFIG['env'] if 'env' in CONFIG else {}
-        env = os.environ.copy()
-        env.update(configEnv)
-        subprocess.call(compilerCmd, env=env)
-
-        showAssembly = False
-        if showAssembly:
-            assemblyFile = os.path.join(tmpDir, "assembly.s")
-            compilerCmd = [CONFIG['compiler'], '-S', '-o', assemblyFile, srcFile] + CONFIG['flags'].split()
-            subprocess.call(compilerCmd, env=env)
-            assembly = open(assemblyFile, 'r').read()
-            kernelFunctionNode.assembly = assembly
-        loadedJitLib = cdll.LoadLibrary(libFile)
-
-    return loadedJitLib
-
-
-def buildCTypeArgumentList(kernelFunctionNode, argumentDict):
-    ctArguments = []
-    for arg in kernelFunctionNode.parameters:
-        if arg.isFieldArgument:
-            field = argumentDict[arg.fieldName]
-            if arg.isFieldPtrArgument:
-                ctArguments.append(field.ctypes.data_as(ctypeFromString(arg.dtype)))
-            elif arg.isFieldShapeArgument:
-                dataType = ctypeFromString(arg.dtype, includePointers=False)
-                ctArguments.append(field.ctypes.shape_as(dataType))
-            elif arg.isFieldStrideArgument:
-                dataType = ctypeFromString(arg.dtype, includePointers=False)
-                baseFieldType = ctypeFromNumpyType(field.dtype)
-                strides = field.ctypes.strides_as(dataType)
-                for i in range(len(field.shape)):
-                    assert strides[i] % sizeof(baseFieldType) == 0
-                    strides[i] //= sizeof(baseFieldType)
-                ctArguments.append(strides)
-            else:
-                assert False
-        else:
-            param = argumentDict[arg.name]
-            expectedType = ctypeFromString(arg.dtype)
-            ctArguments.append(expectedType(param))
-    return ctArguments
-
-
-def makePythonFunctionIncompleteParams(kernelFunctionNode, argumentDict):
-    func = compileAndLoad(kernelFunctionNode)[kernelFunctionNode.functionName]
-    func.restype = None
-
-    def wrapper(**kwargs):
-        from copy import copy
-        fullArguments = copy(argumentDict)
-        fullArguments.update(kwargs)
-        args = buildCTypeArgumentList(kernelFunctionNode, fullArguments)
-        func(*args)
-    return wrapper
-
-
-def makePythonFunction(kernelFunctionNode, argumentDict={}):
-    """
-    Creates C code from the abstract syntax tree, compiles it and makes it accessible as Python function
-
-    The parameters of the kernel are:
-        - numpy arrays for each field used in the kernel. The keyword argument name is the name of the field
-        - all symbols which are not defined in the kernel itself are expected as parameters
-
-    :param kernelFunctionNode: the abstract syntax tree
-    :param argumentDict: parameters passed here are already fixed. Remaining parameters have to be passed to the
-                        returned kernel functor.
-    :return: kernel functor
-    """
-    # build up list of CType arguments
-    try:
-        args = buildCTypeArgumentList(kernelFunctionNode, argumentDict)
-    except KeyError:
-        # not all parameters specified yet
-        return makePythonFunctionIncompleteParams(kernelFunctionNode, argumentDict)
-    func = compileAndLoad(kernelFunctionNode)[kernelFunctionNode.functionName]
-    func.restype = None
-    return lambda: func(*args)

cpu/kernelcreation.py

-import sympy as sp
-from pystencils.transformations import resolveFieldAccesses, makeLoopOverDomain, typingFromSympyInspection, \
-    typeAllEquations, getOptimalLoopOrdering, parseBasePointerInfo, moveConstantsBeforeLoop, splitInnerLoop
-from pystencils.typedsymbol import TypedSymbol
-from pystencils.field import Field
-import pystencils.ast as ast
-
-
-def createKernel(listOfEquations, functionName="kernel", typeForSymbol=None, splitGroups=(),
-                 iterationSlice=None, ghostLayers=None):
-    """
-    Creates an abstract syntax tree for a kernel function, by taking a list of update rules.
-
-    Loops are created according to the field accesses in the equations.
-
-    :param listOfEquations: list of sympy equations, containing accesses to :class:`pystencils.field.Field`.
-           Defining the update rules of the kernel
-    :param functionName: name of the generated function - only important if generated code is written out
-    :param typeForSymbol: a map from symbol name to a C type specifier. If not specified all symbols are assumed to
-           be of type 'double' except symbols which occur on the left hand side of equations where the
-           right hand side is a sympy Boolean which are assumed to be 'bool' .
-    :param splitGroups: Specification on how to split up inner loop into multiple loops. For details see
-           transformation :func:`pystencils.transformation.splitInnerLoop`
-    :param iterationSlice: if not None, iteration is done only over this slice of the field
-    :param ghostLayers: a sequence of pairs for each coordinate with lower and upper nr of ghost layers
-                        if None, the number of ghost layers is determined automatically and assumed to be equal for a
-                        all dimensions
-
-    :return: :class:`pystencils.ast.KernelFunction` node
-    """
-    if not typeForSymbol:
-        typeForSymbol = typingFromSympyInspection(listOfEquations, "double")
-
-    def typeSymbol(term):
-        if isinstance(term, Field.Access) or isinstance(term, TypedSymbol):
-            return term
-        elif isinstance(term, sp.Symbol):
-            return TypedSymbol(term.name, typeForSymbol[term.name])
-        else:
-            raise ValueError("Term has to be field access or symbol")
-
-    fieldsRead, fieldsWritten, assignments = typeAllEquations(listOfEquations, typeForSymbol)
-    allFields = fieldsRead.union(fieldsWritten)
-
-    readOnlyFields = set([f.name for f in fieldsRead - fieldsWritten])
-
-    body = ast.Block(assignments)
-    code = makeLoopOverDomain(body, functionName, iterationSlice=iterationSlice, ghostLayers=ghostLayers)
-
-    if splitGroups:
-        typedSplitGroups = [[typeSymbol(s) for s in splitGroup] for splitGroup in splitGroups]
-        splitInnerLoop(code, typedSplitGroups)
-
-    loopOrder = getOptimalLoopOrdering(allFields)
-
-    basePointerInfo = [['spatialInner0'], ['spatialInner1']]
-    basePointerInfos = {field.name: parseBasePointerInfo(basePointerInfo, loopOrder, field) for field in allFields}
-
-    resolveFieldAccesses(code, readOnlyFields, fieldToBasePointerInfo=basePointerInfos)
-    moveConstantsBeforeLoop(code)
-
-    return code
-
-
-def addOpenMP(astNode, schedule="static"):
-    """
-    Parallelizes the outer loop with OpenMP
-
-    :param astNode: abstract syntax tree created e.g. by :func:`createKernel`
-    :param schedule: OpenMP scheduling policy e.g. 'static' or 'dynamic'
-    """
-    assert type(astNode) is ast.KernelFunction
-    body = astNode.body
-    wrapperBlock = ast.PragmaBlock('#pragma omp parallel', body.takeChildNodes())
-    body.append(wrapperBlock)
-
-    outerLoops = [l for l in body.atoms(ast.LoopOverCoordinate) if l.isOutermostLoop]
-    assert outerLoops, "No outer loop found"
-    assert len(outerLoops) <= 1, "More than one outer loop found. Which one should be parallelized?"
-    outerLoops[0].prefixLines.append("#pragma omp for schedule(%s)" % (schedule,))

doc/conf.py

+#!/usr/bin/env python3
+# -*- coding: utf-8 -*-
+#
+import datetime
+import sphinx_rtd_theme
+import os
+import re
+import sys
+
+sys.path.insert(0, os.path.abspath('.'))
+import pystencils
+
+extensions = [
+    'sphinx.ext.autodoc',
+    'sphinx.ext.doctest',
+    'sphinx.ext.intersphinx',
+    'sphinx.ext.mathjax',
+    'sphinx.ext.napoleon',
+    'nbsphinx',
+    'sphinxcontrib.bibtex',
+    'sphinx_autodoc_typehints',
+]
+
+add_module_names = False
+templates_path = ['_templates']
+source_suffix = '.rst'
+master_doc = 'index'
+
+copyright = f'{datetime.datetime.now().year}, Martin Bauer, Markus Holzer, Frederik Hennig'
+author = 'Martin Bauer, Markus Holzer, Frederik Hennig'
+# The short X.Y version (including .devXXXX, rcX, b1 suffixes if present)
+version = re.sub(r'(\d+\.\d+)\.\d+(.*)', r'\1\2', pystencils.__version__)
+version = re.sub(r'(\.dev\d+).*?$', r'\1', version)
+# The full version, including alpha/beta/rc tags.
+release = pystencils.__version__
+language = 'en'
+exclude_patterns = ['_build', 'Thumbs.db', '.DS_Store', '**.ipynb_checkpoints']
+default_role = 'any'
+pygments_style = 'sphinx'
+todo_include_todos = False
+
+# Options for HTML output
+
+html_theme_path = [sphinx_rtd_theme.get_html_theme_path()]
+html_theme = 'sphinx_rtd_theme'
+htmlhelp_basename = 'pystencilsdoc'
+html_sidebars = {'**': ['globaltoc.html', 'relations.html', 'sourcelink.html', 'searchbox.html']}
+
+# NbSphinx configuration
+nbsphinx_execute = 'never'
+nbsphinx_codecell_lexer = 'python3'
+
+# Example configuration for intersphinx: refer to the Python standard library.
+intersphinx_mapping = {'python': ('https://docs.python.org/3.8', None),
+                       'numpy': ('https://docs.scipy.org/doc/numpy/', None),
+                       'matplotlib': ('https://matplotlib.org/', None),
+                       'sympy': ('https://docs.sympy.org/latest/', None),
+                       }
+
+autodoc_member_order = 'bysource'
+bibtex_bibfiles = ['sphinx/pystencils.bib']
+
+project = 'pystencils'
+html_logo = 'img/logo.png'

doc/img/logo.svg

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doc/index.rst

+pystencils
+==========
+
+Welcome to the documentation of the pystencils code generation tool for stencil codes.
+pystencils can help you to generate blazingly fast code for image processing, numerical simulations or any other task involving numpy arrays.
+
+
+.. toctree::
+   :maxdepth: 2
+
+   sphinx/tutorials.rst
+   sphinx/api.rst
+
+
+.. image:: /img/pystencils_arch_block_diagram.svg
+    :height: 450px
+    :align: center
+
+

doc/notebooks/02_tutorial_basic_kernels.ipynb

+%% Cell type:code id: tags:
+
+``` python
+from pystencils.session import *
+```
+
+%% Cell type:markdown id: tags:
+
+# Tutorial 02: Basic Kernel generation with *pystencils*
+
+Now that you have an [overview of pystencils](01_tutorial_getting_started.ipynb),
+this tutorial shows in more detail how to formulate, optimize and run stencil kernels.
+
+## 1) Kernel Definition
+
+### a) Defining kernels with assignment lists and the  `kernel` decorator
+
+*pystencils* gets a symbolic formulation of the kernel. This can be either an `Assignment` or a sequence of `Assignment`s that follow a set of restrictions.
+
+Lets first create a kernel that consists of multiple assignments:
+
+%% Cell type:code id: tags:
+
+``` python
+src_arr = np.zeros([20, 30])
+dst_arr = np.zeros_like(src_arr)
+
+dst, src = ps.fields(dst=dst_arr, src=src_arr)
+```
+
+%% Cell type:code id: tags:
+
+``` python
+grad_x, grad_y = sp.symbols("grad_x, grad_y")
+
+symbolic_description = [
+    ps.Assignment(grad_x, (src[1, 0] - src[-1, 0]) / 2),
+    ps.Assignment(grad_y, (src[0, 1] - src[0, -1]) / 2),
+    ps.Assignment(dst[0, 0], grad_x + grad_y),
+]
+kernel = ps.create_kernel(symbolic_description)
+symbolic_description
+```
+
+%% Output
+
+
+    $\displaystyle \left[ grad_{x} \leftarrow_{} \frac{{src}_{(1,0)}}{2} - \frac{{src}_{(-1,0)}}{2}, \  grad_{y} \leftarrow_{} \frac{{src}_{(0,1)}}{2} - \frac{{src}_{(0,-1)}}{2}, \  {dst}_{(0,0)} \leftarrow_{} grad_{x} + grad_{y}\right]$
+    ⎡         src_E   src_W            src_N   src_S                         ⎤
+    ⎢gradₓ := ───── - ─────, grad_y := ───── - ─────, dst_C := gradₓ + grad_y⎥
+    ⎣           2       2                2       2                           ⎦
+
+%% Cell type:markdown id: tags:
+
+We created subexpressions, using standard sympy symbols on the left hand side, to split the kernel into multiple assignments. Defining a kernel using a list of `Assignment`s is quite tedious and hard to read.
+To simplify the formulation of a kernel, *pystencils* offers the `kernel` decorator, that transforms a normal Python function with `@=` assignments into an assignment list that can be passed to `create_kernel`.
+
+%% Cell type:code id: tags:
+
+``` python
+@ps.kernel
+def symbolic_description_using_function():
+    grad_x @= (src[1, 0] - src[-1, 0]) / 2
+    grad_y @= (src[0, 1] - src[0, -1]) / 2
+    dst[0, 0] @= grad_x + grad_y
+symbolic_description_using_function
+```
+
+%% Output
+
+
+    $\displaystyle \left[ grad_{x} \leftarrow_{} \frac{{src}_{(1,0)}}{2} - \frac{{src}_{(-1,0)}}{2}, \  grad_{y} \leftarrow_{} \frac{{src}_{(0,1)}}{2} - \frac{{src}_{(0,-1)}}{2}, \  {dst}_{(0,0)} \leftarrow_{} grad_{x} + grad_{y}\right]$
+    ⎡         src_E   src_W            src_N   src_S                         ⎤
+    ⎢gradₓ := ───── - ─────, grad_y := ───── - ─────, dst_C := gradₓ + grad_y⎥
+    ⎣           2       2                2       2                           ⎦
+
+%% Cell type:markdown id: tags:
+
+The decorated function can contain any Python code, only the `@=` operator, and the ternary inline `if-else` operator have different meaning.
+
+### b) Ternary 'if' with `Piecewise`
+
+The ternary operator maps to `sympy.Piecewise` functions, that can be used to introduce branching into the kernel. Piecewise defined functions must give a value for every input, i.e. there must be a 'otherwise' clause in the end that is indicated by the condition `True`. Piecewise objects are standard sympy terms that can be integrated into bigger expressions:
+
+%% Cell type:code id: tags:
+
+``` python
+sp.Piecewise((1.0, src[0,1] > 0), (0.0, True)) + src[1, 0]
+```
+
+%% Output
+
+
+    $\displaystyle {src}_{(1,0)} + \begin{cases} 1.0 & \text{for}\: {src}_{(0,1)} > 0 \\0.0 & \text{otherwise} \end{cases}$
+            ⎛⎧1.0  for src_N > 0⎞
+    src_E + ⎜⎨                  ⎟
+            ⎝⎩0.0    otherwise  ⎠
+
+%% Cell type:markdown id: tags:
+
+Piecewise objects are created by the `kernel` decorator for ternary if-else statements.
+
+%% Cell type:code id: tags:
+
+``` python
+@ps.kernel
+def kernel_with_piecewise():
+    grad_x @= (src[1, 0] - src[-1, 0]) / 2 if src[-1, 0] > 0 else 0.0
+kernel_with_piecewise
+```
+
+%% Output
+
+
+    $\displaystyle \left[ grad_{x} \leftarrow_{} \begin{cases} \frac{{src}_{(1,0)}}{2} - \frac{{src}_{(-1,0)}}{2} & \text{for}\: {src}_{(-1,0)} > 0 \\0.0 & \text{otherwise} \end{cases}\right]$
+    ⎡         ⎧src_E   src_W               ⎤
+    ⎢         ⎪───── - ─────  for src_W > 0⎥
+    ⎢gradₓ := ⎨  2       2                 ⎥
+    ⎢         ⎪                            ⎥
+    ⎣         ⎩     0.0         otherwise  ⎦
+
+%% Cell type:markdown id: tags:
+
+### c) Assignment level optimizations using `AssignmentCollection`
+
+When the kernels get larger and more complex, it is helpful to organize the list of assignment into a more structured way. The `AssignmentCollection` offers optimizating transformation on a list of assignments. It holds two assignment lists, one for subexpressions and one for the main assignments. Main assignments are typically those that write to an array.
+
+%% Cell type:code id: tags:
+
+``` python
+@ps.kernel
+def somewhat_longer_dummy_kernel(s):
+    s.a @= src[0, 1] + src[-1, 0]
+    s.b @= 2 * src[1, 0] + src[0, -1]
+    s.c @= src[0, 1] + 2 * src[1, 0] + src[-1, 0] + src[0, -1] - src[0,0]
+    dst[0, 0] @= s.a + s.b + s.c
+
+ac = ps.AssignmentCollection(main_assignments=somewhat_longer_dummy_kernel[-1:],
+                             subexpressions=somewhat_longer_dummy_kernel[:-1])
+ac
+```
+
+%% Output
+
+    AssignmentCollection: dst_C, <- f(src_C, src_W, src_S, src_N, src_E)
+
+%% Cell type:code id: tags:
+
+``` python
+ac.operation_count
+```
+
+%% Output
+
+    {'adds': 8,
+     'muls': 2,
+     'divs': 0,
+     'sqrts': 0,
+     'fast_sqrts': 0,
+     'fast_inv_sqrts': 0,
+     'fast_div': 0}
+
+%% Cell type:markdown id: tags:
+
+The `pystencils.simp` submodule offers several functions to optimize a collection of assignments.
+It also offers functionality to group optimization into strategies and evaluate them.
+In this example we reduce the number of operations by reusing existing subexpressions to get rid of two unnecessary floating point additions. For more information about assignment collections and simplifications see the [demo notebook](demo_assignment_collection.ipynb).
+
+%% Cell type:code id: tags:
+
+``` python
+opt_ac = ps.simp.subexpression_substitution_in_existing_subexpressions(ac)
+opt_ac
+```
+
+%% Output
+
+    AssignmentCollection: dst_C, <- f(src_C, src_W, src_S, src_N, src_E)
+
+%% Cell type:code id: tags:
+
+``` python
+opt_ac.operation_count
+```
+
+%% Output
+
+    {'adds': 6,
+     'muls': 1,
+     'divs': 0,
+     'sqrts': 0,
+     'fast_sqrts': 0,
+     'fast_inv_sqrts': 0,
+     'fast_div': 0}
+
+%% Cell type:markdown id: tags:
+
+### d) Ghost layers and iteration region
+
+When creating a kernel with neighbor accesses, *pystencils* automatically restricts the iteration region, such that all accesses are safe.
+
+%% Cell type:code id: tags:
+
+``` python
+kernel = ps.create_kernel(ps.Assignment(dst[0,0], src[2, 0] + src[-1, 0]))
+ps.show_code(kernel)
+```
+
+%% Output
+
+
+
+%% Cell type:markdown id: tags:
+
+When no additional ghost layer information is given, *pystencils* looks at all neighboring field accesses and introduces the required number of ghost layers **for all directions**. In the example above the largest neighbor accesses was ``src[2, 0]``, so theoretically we would need 2 ghost layers only the the end of the x coordinate.
+By default *pystencils* introduces 2 ghost layers at all borders of the domain. The next cell shows how to change this behavior. Be careful with manual ghost layer specification, wrong values may lead to SEGFAULTs.
+
+%% Cell type:code id: tags:
+
+``` python
+gl_spec = [(0, 2),   # 0 ghost layers at the left, 2 at the right border
+           (1, 0)]   # 1 ghost layer at the lower y, one at the upper y coordinate
+kernel = ps.create_kernel(ps.Assignment(dst[0,0], src[2, 0] + src[-1, 0]), ghost_layers=gl_spec)
+ps.show_code(kernel)
+```
+
+%% Output
+
+
+
+%% Cell type:markdown id: tags:
+
+## 2 ) Restrictions
+
+
+### a) Independence Restriction
+
+*pystencils* only works for kernels where each array element can be updated independently from all other elements. This restriction ensures that the kernels can be easily parallelized and also be run on the GPU. Trying to define kernels where the results depends on the iteration order, leads to a ValueError.
+
+%% Cell type:code id: tags:
+
+``` python
+invalid_description = [
+    ps.Assignment(dst[1, 0], src[1, 0] + src[-1, 0]),
+    ps.Assignment(dst[0, 0], src[1, 0] - src[-1, 0]),
+]
+try:
+    invalid_kernel = ps.create_kernel(invalid_description)
+    assert False, "Should never be executed"
+except ValueError as e:
+    print(e)
+```
+
+%% Output
+
+    Field dst is written at two different locations
+
+%% Cell type:markdown id: tags:
+
+The independence restriction makes sure that the kernel can be safely parallelized by checking the following conditions: If a field is modified inside the kernel, it may only be modified at a single spatial position. In that case the field may also only be read at this position. Fields that are not modified may be read at multiple neighboring positions.
+
+Specifically, this rule allows for in-place updates that don't access neighbors.
+
+%% Cell type:code id: tags:
+
+``` python
+valid_kernel = ps.create_kernel(ps.Assignment(src[0,0], 2*src[0,0] + 42))
+```
+
+%% Cell type:markdown id: tags:
+
+If a field stores multiple values per cell, as in the next example, this restriction only applies for accesses with the same index.
+
+%% Cell type:code id: tags:
+
+``` python
+v = ps.fields("v(2): double[2D]")
+valid_kernel = ps.create_kernel([ps.Assignment(v[0,0](1), 2*v[0,0](1) + 42),
+                                 ps.Assignment(v[0,1](0), 2*v[0,1](0) + 42)])
+```
+
+%% Cell type:markdown id: tags:
+
+### b) Static Single Assignment Form
+
+All assignments that don't write to a field must be in SSA form
+1. Each sympy symbol may only occur once as a left-hand-side (fields can be written multiple times)
+2. A symbol has to be defined before it is used. If it is never defined it is introduced as function parameter
+
+The next cell demonstrates the first SSA restriction:
+
+%% Cell type:code id: tags:
+
+``` python
+@ps.kernel
+def not_allowed():
+    a, b = sp.symbols("a b")
+    a @= src[0, 0]
+    b @= a + 3
+    a @= src[-1, 0]
+    dst[0, 0] @= a + b
+try:
+    ps.create_kernel(not_allowed)
+    assert False
+except ValueError as e:
+    print(e)
+```
+
+%% Output
+
+    Assignments not in SSA form, multiple assignments to a
+
+%% Cell type:markdown id: tags:
+
+Also it is not allowed to write a field at the same location
+
+%% Cell type:code id: tags:
+
+``` python
+@ps.kernel
+def not_allowed():
+    dst[0, 0] @= src[0, 1] + src[1, 0]
+    dst[0, 0] @= 2 * dst[0, 0]
+
+try:
+    ps.create_kernel(not_allowed)
+    assert False
+except ValueError as e:
+    print(e)
+```
+
+%% Output
+
+    Field dst is written twice at the same location
+
+%% Cell type:markdown id: tags:
+
+This situation should be resolved by introducing temporary variables
+
+%% Cell type:code id: tags:
+
+``` python
+tmp_var = sp.Symbol("a")
+
+@ps.kernel
+def allowed():
+    tmp_var @= src[0, 1] + src[1, 0]
+    dst[0, 0] @= 2 * tmp_var
+
+
+ast = ps.create_kernel(allowed)
+ps.show_code(ast)
+```
+
+%% Output
+
+

doc/notebooks/03_tutorial_datahandling.ipynb

+%% Cell type:code id: tags:
+
+``` python
+import psutil
+from mpl_toolkits.mplot3d import Axes3D
+from matplotlib import pyplot, cm
+from pystencils.session import *
+from pystencils.boundaries import add_neumann_boundary, Neumann, Dirichlet, BoundaryHandling
+from pystencils.slicing import slice_from_direction
+import math
+import time
+%matplotlib inline
+```
+
+%% Cell type:markdown id: tags:
+
+Test to see if cupy is installed which is needed to run calculations on the GPU
+
+%% Cell type:code id: tags:
+
+``` python
+try:
+    import cupy
+    gpu = True
+except ImportError:
+    gpu = False
+    cupy = None
+    print('No cupy installed')
+```
+
+%% Cell type:markdown id: tags:
+
+# Tutorial 03: Datahandling
+
+%% Cell type:markdown id: tags:
+
+This is a tutorial about the `DataHandling` class of pystencils. This class is an abstraction layer to
+- link numpy arrays to pystencils fields
+- handle CPU-GPU array transfer, such that one can write code that works on CPU and GPU
+- makes it possible to write MPI parallel simulations to run on distributed-memory clusters using the waLBerla library
+
+We will look at a small and easy example to demonstrate the usage of `DataHandling` objects. We will define an averaging kernel to every cell of an array, that writes the average of the neighbor cell values to the center.
+
+%% Cell type:markdown id: tags:
+
+## 1. Manual
+
+### 1.1. CPU kernels
+
+In this first part, we set up a scenario manually without a `DataHandling`. In the next sections we then repeat the same setup with the help of the data handling.
+
+One concept of *pystencils* that may be confusing at first, is the differences between pystencils fields and numpy arrays. Fields are used to describe the computation *symbolically* with sympy, while numpy arrays hold the actual values where the computation is executed on.
+
+One option to create and execute a *pystencils* kernel is listed below. For reasons that become clear later we call this the **variable-field-size workflow**:
+
+1. define pystencils fields
+2. use sympy and the pystencils fields to define an update rule, that describes what should be done on *every cell*
+3. compile the update rule to a real function, that can be called from Python. For each field that was referenced in the symbolic description the function expects a numpy array, passed as named parameter
+4. create some numpy arrays with actual data
+5. call the kernel - usually many times
+
+Now, lets see how this actually looks in Python code:
+
+%% Cell type:code id: tags:
+
+``` python
+# 1. field definitions
+src_field, dst_field = ps.fields("src, dst:[2D]")
+
+# 2. define update rule
+update_rule = [ps.Assignment(lhs=dst_field[0, 0],
+                             rhs=(src_field[1, 0] + src_field[-1, 0] +
+                                  src_field[0, 1] + src_field[0, -1]) / 4)]
+
+# 3. compile update rule to function
+kernel_function = ps.create_kernel(update_rule).compile()
+
+# 4. create numpy arrays and call kernel
+src_arr, dst_arr = np.random.rand(30, 30), np.zeros([30, 30])
+
+# 5. call kernel
+kernel_function(src=src_arr, dst=dst_arr)  # names of arguments have to match names passed to ps.fields()
+```
+
+%% Cell type:markdown id: tags:
+
+This workflow separates the symbolic and the numeric stages very cleanly. The separation also makes it possible to stop after step 3, write the C-code to a file and call the kernel from a C program. Speaking of the C-Code - lets have a look at the generated sources:
+
+%% Cell type:code id: tags:
+
+``` python
+ps.show_code(kernel_function.ast)
+```
+
+%% Output
+
+
+
+%% Cell type:markdown id: tags:
+
+Even if it looks very ugly and low-level :) lets look at this code in a bit more detail. The code is generated in a way that it works for different array sizes. The size of the array is passed in the `_size_dst_` variables that specifiy the shape of the array for each dimension. Also, the memory layout (linearization) of the array can be different. That means the array could be stored in row-major or column-major order - if we pass in the array strides correctly the kernel does the right thing. If you're not familiar with the concept of strides check out [this stackoverflow post](https://stackoverflow.com/questions/53097952/how-to-understand-numpy-strides-for-layman) or search in the numpy documentation for strides - C vs Fortran order.
+
+The goal of *pystencils* is to produce the fastest possible code. One technique to do this is to use all available information already on compile time and generate code that is highly adapted to the specific problem. In our case we already know the shape and strides of the arrays we want to apply the kernel to, so we can make use of this information. This idea leads to the **fixed-field-size workflow**. The main difference there is that we define the arrays first and therefore let *pystencils* know about the array shapes and strides, so that it can generate more specific code:
+
+1. create numpy arrays that hold your data
+2. define pystencils fields, this time telling pystencils already which arrays they correspond to, so that it knows about the size and strides
+
+in the other steps nothing has changed
+
+3. define the update rule
+4. compile update rule to kernel
+5. run the kernel
+
+%% Cell type:code id: tags:
+
+``` python
+# 1. create arrays first
+src_arr, dst_arr = np.random.rand(30, 30), np.zeros([30, 30])
+
+# 2. define symbolic fields - note the additional parameter that link an array to each field
+src_field, dst_field = ps.fields("src, dst:[2D]", src=src_arr, dst=dst_arr)
+
+# 3. define update rule
+update_rule = [ps.Assignment(lhs=dst_field[0, 0],
+                             rhs=(src_field[1, 0] + src_field[-1, 0] +
+                                  src_field[0, 1] + src_field[0, -1]) / 4)]
+
+# 4. compile it
+kernel_function = ps.create_kernel(update_rule).compile()
+
+# 5. call kernel
+kernel_function(src=src_arr, dst=dst_arr)  # names of arguments have to match names passed to ps.fields()
+```
+
+%% Cell type:markdown id: tags:
+
+Functionally, both variants are equivalent. We see the difference only when we look at the generated code
+
+%% Cell type:code id: tags:
+
+``` python
+ps.show_code(kernel_function.ast)
+```
+
+%% Output
+
+
+
+%% Cell type:markdown id: tags:
+
+Compare this to the code above! It looks much simpler. The reason is that all index computations are already simplified since the exact field sizes and strides are known. This kernel now only works on arrays of the previously specified size.
+
+Lets try what happens if we use a different array:
+
+%% Cell type:code id: tags:
+
+``` python
+src_arr2, dst_arr2 = np.random.rand(40, 40), np.zeros([40, 40])
+try:
+    kernel_function(src=src_arr2, dst=dst_arr2)
+except ValueError as e:
+    print(e)
+```
+
+%% Output
+
+    Wrong shape of array dst. Expected (30, 30)
+
+%% Cell type:markdown id: tags:
+
+### 1.2. GPU simulations
+
+Let's now jump to a seemingly unrelated topic: running kernels on the GPU.
+When creating the kernel, an additional parameter `target=ps.Target.GPU` has to be passed. Also, the compiled kernel cannot be called with numpy arrays directly, but has to be called with `cupy.array`s instead. That means, we have to transfer our numpy array to GPU first. From this step we obtain a gpuarray, then we can run the kernel, hopefully multiple times so that the data transfer was worth the time. Finally we transfer the finished result back to CPU:
+
+%% Cell type:code id: tags:
+
+``` python
+if cupy:
+    config = ps.CreateKernelConfig(target=ps.Target.GPU)
+    kernel_function_gpu = ps.create_kernel(update_rule, config=config).compile()
+    # transfer to GPU
+    src_arr_gpu = cupy.asarray(src_arr)
+    dst_arr_gpu = cupy.asarray(dst_arr)
+
+    # run kernel on GPU, this is done many times in real setups
+    kernel_function_gpu(src=src_arr_gpu, dst=dst_arr_gpu)
+
+    # transfer result back to CPU
+    dst_arr = dst_arr_gpu.get()
+```
+
+%% Cell type:markdown id: tags:
+
+###  1.3. Summary: manual way
+
+- Don't confuse *pystencils* fields and *numpy* arrays
+    - fields are symbolic
+    - arrays are numeric
+- Use the fixed-field-size workflow whenever possible, since code might be faster. Create arrays first, then create fields from arrays
+- if we run GPU kernels, arrays have to transferred to the GPU first
+
+As demonstrated in the examples above we have to define 2 or 3 corresponding objects for each grid:
+
+- symbolic pystencils field
+- numpy array on CPU
+- for GPU use also cupy to mirror the data on the GPU
+
+Managing these three objects manually is tedious and error-prone. We'll see in the next section how the data handling object takes care of this problem.
+
+%% Cell type:markdown id: tags:
+
+## 2. Introducing the data handling - serial version
+
+### 2.1. Example for CPU simulations
+
+The data handling internally keeps a mapping between symbolic fields and numpy arrays. When we create a field, automatically a corresponding array is allocated as well. Optionally we can also allocate memory on the GPU for the array as well. Lets dive right in and see how our example looks like, when implemented with a data handling.
+
+%% Cell type:code id: tags:
+
+``` python
+dh = ps.create_data_handling(domain_size=(30, 30))
+
+# fields are now created using the data handling
+src_field = dh.add_array('src', values_per_cell=1)
+dst_field = dh.add_array('dst', values_per_cell=1)
+
+# kernel is created just like before
+update_rule = [ps.Assignment(lhs=dst_field[0, 0],
+                             rhs=(src_field[1, 0] + src_field[-1, 0] + src_field[0, 1] + src_field[0, -1]) / 4)]
+kernel_function = ps.create_kernel(update_rule).compile()
+
+# have a look at the generated code - it uses
+# the fast version where array sizes are compiled-in
+#  ps.show_code(kernel_function.ast)
+```
+
+%% Cell type:markdown id: tags:
+
+The data handling has methods to create fields - but where are the corresponding arrays?
+In the serial case you can access them as a member of the data handling, for example to initialize our 'src' array we can write
+
+%% Cell type:code id: tags:
+
+``` python
+src_arr = dh.cpu_arrays['src']
+src_arr.fill(0.0)
+```
+
+%% Cell type:markdown id: tags:
+
+This method is nice and easy, but you should not use it if you want your simulation to run on distributed-memory clusters. We'll see why in the last section. So it is good habit to not access the arrays directly but use the data handling to do so. We can, for example, initialize the array also with the following code:
+
+%% Cell type:code id: tags:
+
+``` python
+dh.fill('src', 0.0)
+```
+
+%% Cell type:markdown id: tags:
+
+To run the kernels with the same code as before, we would also need the arrays. We could do that accessing the `cpu_arrays`:
+
+%% Cell type:code id: tags:
+
+``` python
+kernel_function(src=dh.cpu_arrays['src'],
+                dst=dh.cpu_arrays['dst'])
+```
+
+%% Cell type:markdown id: tags:
+
+but to be prepared for MPI parallel simulations, again a method of the data handling should be used for this.
+Besides, this method is also simpler to use - since it automatically detects which arrays a kernel uses and passes them in.
+
+%% Cell type:code id: tags:
+
+``` python
+dh.run_kernel(kernel_function)
+```
+
+%% Cell type:markdown id: tags:
+
+To access the data read-only instead of using `cpu_arrays` the gather function should be used.
+This function gives you a read-only copy of the domain or part of the domain.
+We will discuss this function later in more detail when we look at MPI parallel simulations.
+For serial simulations keep in mind that modifying the resulting array does not change your original data!
+
+%% Cell type:code id: tags:
+
+``` python
+read_only_copy = dh.gather_array('src', ps.make_slice[:, :], ghost_layers=False)
+```
+
+%% Cell type:markdown id: tags:
+
+### 2.2. Example for GPU simulations
+
+In this section we have a look at GPU simulations using the data handling. Only minimal changes are required.
+When creating the data handling we can pass a 'default_target'. This means for every added field an array on the CPU and the GPU is allocated. This is a useful default, for more fine-grained control the `add_array` method also takes additional parameters controlling where the array should be allocated.
+
+Additionally we also need to compile a GPU version of the kernel.
+
+%% Cell type:code id: tags:
+
+``` python
+if gpu is False:
+    dh = ps.create_data_handling(domain_size=(30, 30), default_target=ps.Target.CPU)
+else:
+    dh = ps.create_data_handling(domain_size=(30, 30), default_target=ps.Target.GPU)
+
+# fields are now created using the data handling
+src_field = dh.add_array('src', values_per_cell=1)
+dst_field = dh.add_array('dst', values_per_cell=1)
+
+# kernel is created just like before
+update_rule = [ps.Assignment(lhs=dst_field[0, 0],
+                             rhs=(src_field[1, 0] + src_field[-1, 0] + src_field[0, 1] + src_field[0, -1]) / 4)]
+config = ps.CreateKernelConfig(target=dh.default_target)
+kernel_function = ps.create_kernel(update_rule, config=config).compile()
+
+dh.fill('src', 0.0)
+```
+
+%% Cell type:markdown id: tags:
+
+The data handling provides function to transfer data between CPU and GPU
+
+%% Cell type:code id: tags:
+
+``` python
+if gpu:
+    dh.to_gpu('src')
+
+dh.run_kernel(kernel_function)
+
+if gpu:
+    dh.to_cpu('dst')
+```
+
+%% Cell type:markdown id: tags:
+
+usually one wants to transfer all fields that have been allocated on CPU and GPU at the same time:
+
+%% Cell type:code id: tags:
+
+``` python
+dh.all_to_gpu()
+dh.run_kernel(kernel_function)
+dh.all_to_cpu()
+```
+
+%% Cell type:markdown id: tags:
+
+We can always include the `all_to_*` functions in our code, since they do nothing if there are no arrays allocated on the GPU. Thus there is only a single point in the code where we can switch between CPU and GPU version: the `default_target` of the data handling.
+
+%% Cell type:markdown id: tags:
+
+### 2.2 Ghost Layers and periodicity
+
+The data handling can also provide periodic boundary conditions. Therefor the domain is extended by one layer of cells, the so-called ghost layer or halo layer.
+
+%% Cell type:code id: tags:
+
+``` python
+print("Shape of domain         ", dh.shape)
+print("Direct access to arrays ", dh.cpu_arrays['src'].shape)
+print("Gather                  ", dh.gather_array('src', ghost_layers=True).shape)
+```
+
+%% Output
+
+    Shape of domain          (30, 30)
+    Direct access to arrays  (32, 32)
+    Gather                   (32, 32)
+
+%% Cell type:markdown id: tags:
+
+ So the actual arrays are 2 cells larger than what you asked for. This additional layer is used to copy over the data from the other end of the array, such that for the stencil algorithm effectively the domain is periodic. This copying operation has to be started manually though:
+
+%% Cell type:code id: tags:
+
+``` python
+dh = ps.create_data_handling((4, 4), periodicity=(True, False))
+dh.add_array("src")
+
+# get copy function
+copy_fct = dh.synchronization_function(['src'])
+
+dh.fill('src', 0.0, ghost_layers=True)
+dh.fill('src', 3.0, ghost_layers=False)
+before_sync = dh.gather_array('src', ghost_layers=True).copy()
+
+copy_fct() # copy over to get periodicity in x direction
+
+after_sync = dh.gather_array('src', ghost_layers=True).copy()
+plt.subplot(1,2,1)
+plt.scalar_field(before_sync);
+plt.title("Before")
+plt.subplot(1,2,2)
+plt.scalar_field(after_sync);
+plt.title("After");
+```
+
+%% Output
+
+
+
+%% Cell type:markdown id: tags:
+
+## 3. Going (MPI) parallel - the parallel data handling
+
+### 3.1. Conceptual overview
+To run MPI parallel simulations the waLBerla framework is used. waLBerla has to be compiled against your local MPI library and thus is a bit hard to install. We suggest to use the version shipped with conda for testing. For production, when you want to run on a cluster the best option is to build waLBerla yourself against the MPI library that is installed on your cluster.
+
+Now lets have a look on how to write code that runs MPI parallel.
+Since the data is distributed, we don't have access to the full array any more but only to the part that is stored locally. The domain is split into so called blocks, where one process might get one (or sometimes multiple) blocks. To do anything with the local part of the data we iterate over the **local** blocks to get the contents as numpy arrays. The blocks returned in the loop differ from process to process.
+
+Copy the following snippet to a python file and run with multiple processes e.g.:
+``mpirun -np 4 myscript.py`` you will see that there are as many blocks as processes.
+
+%% Cell type:code id: tags:
+
+``` python
+dh = ps.create_data_handling(domain_size=(30, 30), parallel=True)
+field = dh.add_array('field')
+for block in dh.iterate():
+    # offset is in global coordinates, where first inner cell has coordiante (0,0)
+    # and ghost layers have negative coordinates
+    print(block.offset, block['field'].shape)
+
+    # use offset to create a local array 'my_data' for the part of the domain
+    #np.copyto(block[field.name], my_data)
+```
+
+%% Output
+
+    (-1, -1) (32, 32)
+    [0][PROGRESS]------(0.000 sec) Initializing SetupBlockForest:
+    [0]                             - AABB: [ <0,0,0>, <30,30,1> ]
+    [0]                             - forest size (root blocks / blocks on the initial grid): 1 x 1 x 1
+    [0]                             - periodicity: false x false x false
+    [0][PROGRESS]------(0.000 sec) Initializing SetupBlockForest: Allocating root blocks ...
+    [0][PROGRESS]------(0.000 sec) Initializing SetupBlockForest: Setting up neighborhood information for each block (with respect to periodicity) ...
+    [0][PROGRESS]------(0.000 sec) Initializing SetupBlockForest: Assigning workload, memory requirements, and SUIDs to blocks ...
+    [0][PROGRESS]------(0.000 sec) Initializing SetupBlockForest: finished!
+    [0]                            The following block structure has been created:
+    [0]                            - AABB: [ <0,0,0>, <30,30,1> ]
+    [0]                            - initial decomposition: 1 x 1 x 1 (= forest size)
+    [0]                            - periodicity: false x false x false
+    [0]                            - number of blocks discarded from the initial grid: 0 (= 0 %)
+    [0]                            - number of levels: 1
+    [0]                            - tree ID digits: 1 (-> block ID bytes = 1)
+    [0]                            - total number of blocks: 1
+    [0]                            - blocks have not yet been distributed to processes
+    [0][PROGRESS]------(0.000 sec) Balancing SetupBlockForest: Creating a process distribution for 1 process(es) ...
+    [0][PROGRESS]------(0.000 sec) Balancing SetupBlockForest: process distribution to 1 process(es) finished!
+    [0]                            - number of worker processes:       1
+    [0]                            - number of empty buffer processes: 0
+    [0]                            - buffer processes are inserted into the process network: no
+    [0]                            The resulting block structure looks like as follows:
+    [0]                            - AABB: [ <0,0,0>, <30,30,1> ]
+    [0]                            - initial decomposition: 1 x 1 x 1 (= forest size)
+    [0]                            - periodicity: false x false x false
+    [0]                            - number of blocks discarded from the initial grid: 0 (= 0 %)
+    [0]                            - number of levels: 1
+    [0]                            - tree ID digits: 1 (-> block ID bytes = 1)
+    [0]                            - total number of blocks: 1
+    [0]                            - number of processes: 1 (1 worker process(es) / 0 empty buffer process(es))
+    [0]                            - buffer processes are inserted into the process network: no
+    [0]                            - process ID bytes: 0
+    [0]                            - blocks/memory/workload per process:
+    [0]                               + blocks:
+    [0]                                  - min       = 1
+    [0]                                  - max       = 1
+    [0]                                  - avg       = 1
+    [0]                                  - stdDev    = 0
+    [0]                                  - relStdDev = 0
+    [0]                               + memory:
+    [0]                                  - min       = 1
+    [0]                                  - max       = 1
+    [0]                                  - avg       = 1
+    [0]                                  - stdDev    = 0
+    [0]                                  - relStdDev = 0
+    [0]                               + workload:
+    [0]                                  - min       = 1
+    [0]                                  - max       = 1
+    [0]                                  - avg       = 1
+    [0]                                  - stdDev    = 0
+    [0]                                  - relStdDev = 0
+    [0][PROGRESS]------(0.000 sec) Adding block data ("cell bounding box")
+    [0][PROGRESS]------(0.000 sec) Adding block data ("field")
+
+%% Cell type:markdown id: tags:
+
+To get some more interesting results here in the notebook we put multiple blocks onto our single notebook process. This makes not much sense for real simulations, but for testing and demonstration purposes this is useful.
+
+%% Cell type:code id: tags:
+
+``` python
+from waLBerla import createUniformBlockGrid
+from pystencils.datahandling import ParallelDataHandling
+
+blocks = createUniformBlockGrid(blocks=(2,1,2), cellsPerBlock=(20, 10, 20),
+                                oneBlockPerProcess=False, periodic=(1, 0, 0))
+dh = ParallelDataHandling(blocks)
+field = dh.add_array('field')
+for block in dh.iterate():
+    print(block.offset, block['field'].shape)
+```
+
+%% Output
+
+    (-1, -1, -1)[0][PROGRESS]------(0.025 sec) Initializing SetupBlockForest:
+    [0]                             - AABB: [ <0,0,0>, <40,10,40> ]
+    [0]                             - forest size (root blocks / blocks on the initial grid): 2 x 1 x 2
+    [0]                             - periodicity: true x false x false
+     (22, 12, 22)
+    (19, -1, -1) (22, 12, 22)
+    (-1, -1, 19) (22, 12, 22)
+    (19, -1, 19) (22, 12, 22)
+    [0][PROGRESS]------(0.025 sec) Initializing SetupBlockForest: Allocating root blocks ...
+    [0][PROGRESS]------(0.025 sec) Initializing SetupBlockForest: Setting up neighborhood information for each block (with respect to periodicity) ...
+    [0][PROGRESS]------(0.025 sec) Initializing SetupBlockForest: Assigning workload, memory requirements, and SUIDs to blocks ...
+    [0][PROGRESS]------(0.025 sec) Initializing SetupBlockForest: finished!
+    [0]                            The following block structure has been created:
+    [0]                            - AABB: [ <0,0,0>, <40,10,40> ]
+    [0]                            - initial decomposition: 2 x 1 x 2 (= forest size)
+    [0]                            - periodicity: true x false x false
+    [0]                            - number of blocks discarded from the initial grid: 0 (= 0 %)
+    [0]                            - number of levels: 1
+    [0]                            - tree ID digits: 3 (-> block ID bytes = 1)
+    [0]                            - total number of blocks: 4
+    [0]                            - blocks have not yet been distributed to processes
+    [0][PROGRESS]------(0.025 sec) Balancing SetupBlockForest: Creating a process distribution for 1 process(es) ...
+    [0][PROGRESS]------(0.025 sec) Balancing SetupBlockForest: process distribution to 1 process(es) finished!
+    [0]                            - number of worker processes:       1
+    [0]                            - number of empty buffer processes: 0
+    [0]                            - buffer processes are inserted into the process network: no
+    [0]                            The resulting block structure looks like as follows:
+    [0]                            - AABB: [ <0,0,0>, <40,10,40> ]
+    [0]                            - initial decomposition: 2 x 1 x 2 (= forest size)
+    [0]                            - periodicity: true x false x false
+    [0]                            - number of blocks discarded from the initial grid: 0 (= 0 %)
+    [0]                            - number of levels: 1
+    [0]                            - tree ID digits: 3 (-> block ID bytes = 1)
+    [0]                            - total number of blocks: 4
+    [0]                            - number of processes: 1 (1 worker process(es) / 0 empty buffer process(es))
+    [0]                            - buffer processes are inserted into the process network: no
+    [0]                            - process ID bytes: 0
+    [0]                            - blocks/memory/workload per process:
+    [0]                               + blocks:
+    [0]                                  - min       = 4
+    [0]                                  - max       = 4
+    [0]                                  - avg       = 4
+    [0]                                  - stdDev    = 0
+    [0]                                  - relStdDev = 0
+    [0]                               + memory:
+    [0]                                  - min       = 4
+    [0]                                  - max       = 4
+    [0]                                  - avg       = 4
+    [0]                                  - stdDev    = 0
+    [0]                                  - relStdDev = 0
+    [0]                               + workload:
+    [0]                                  - min       = 4
+    [0]                                  - max       = 4
+    [0]                                  - avg       = 4
+    [0]                                  - stdDev    = 0
+    [0]                                  - relStdDev = 0
+    [0][PROGRESS]------(0.025 sec) Adding block data ("cell bounding box")
+    [0][PROGRESS]------(0.026 sec) Adding block data ("field")
+
+%% Cell type:markdown id: tags:
+
+Now we see that we have four blocks with (20, 10, 20) block each, and the global domain is (40, 10, 40) big.
+All subblock also have a ghost layer around them, which is used to synchronize with their neighboring blocks (over the network). For ghost layer synchronization the same `synchronization_function` is used that we used above for periodic boundaries, because copying between blocks and copying the ghost layer for periodicity uses the same mechanism.
+
+%% Cell type:code id: tags:
+
+``` python
+dh.gather_array('field').shape
+```
+
+%% Output
+
+
+    $\displaystyle \left( 40, \  10, \  40\right)$
+    (40, 10, 40)
+
+%% Cell type:markdown id: tags:
+
+### 3.2. Parallel example
+
+To illustrate this in more detail, lets run a simple kernel on a parallel domain. waLBerla can handle 3D domains only, so we choose a z-size of 1.
+
+%% Cell type:code id: tags:
+
+``` python
+blocks = createUniformBlockGrid(blocks=(2,4,1), cellsPerBlock=(20, 10, 1),
+                                oneBlockPerProcess=False, periodic=(1, 1, 0))
+dh = ParallelDataHandling(blocks, dim=2)
+src_field = dh.add_array('src')
+dst_field = dh.add_array('dst')
+
+update_rule = [ps.Assignment(lhs=dst_field[0, 0 ],
+                             rhs=(src_field[1, 0] + src_field[-1, 0] +
+                                  src_field[0, 1] + src_field[0, -1]) / 4)]
+
+# 3. compile update rule to function
+kernel_function = ps.create_kernel(update_rule).compile()
+```
+
+%% Output
+
+    [0][PROGRESS]------(0.119 sec) Initializing SetupBlockForest:
+    [0]                             - AABB: [ <0,0,0>, <40,40,1> ]
+    [0]                             - forest size (root blocks / blocks on the initial grid): 2 x 4 x 1
+    [0]                             - periodicity: true x true x false
+    [0][PROGRESS]------(0.119 sec) Initializing SetupBlockForest: Allocating root blocks ...
+    [0][PROGRESS]------(0.119 sec) Initializing SetupBlockForest: Setting up neighborhood information for each block (with respect to periodicity) ...
+    [0][PROGRESS]------(0.119 sec) Initializing SetupBlockForest: Assigning workload, memory requirements, and SUIDs to blocks ...
+    [0][PROGRESS]------(0.119 sec) Initializing SetupBlockForest: finished!
+    [0]                            The following block structure has been created:
+    [0]                            - AABB: [ <0,0,0>, <40,40,1> ]
+    [0]                            - initial decomposition: 2 x 4 x 1 (= forest size)
+    [0]                            - periodicity: true x true x false
+    [0]                            - number of blocks discarded from the initial grid: 0 (= 0 %)
+    [0]                            - number of levels: 1
+    [0]                            - tree ID digits: 4 (-> block ID bytes = 1)
+    [0]                            - total number of blocks: 8
+    [0]                            - blocks have not yet been distributed to processes
+    [0][PROGRESS]------(0.119 sec) Balancing SetupBlockForest: Creating a process distribution for 1 process(es) ...
+    [0][PROGRESS]------(0.119 sec) Balancing SetupBlockForest: process distribution to 1 process(es) finished!
+    [0]                            - number of worker processes:       1
+    [0]                            - number of empty buffer processes: 0
+    [0]                            - buffer processes are inserted into the process network: no
+    [0]                            The resulting block structure looks like as follows:
+    [0]                            - AABB: [ <0,0,0>, <40,40,1> ]
+    [0]                            - initial decomposition: 2 x 4 x 1 (= forest size)
+    [0]                            - periodicity: true x true x false
+    [0]                            - number of blocks discarded from the initial grid: 0 (= 0 %)
+    [0]                            - number of levels: 1
+    [0]                            - tree ID digits: 4 (-> block ID bytes = 1)
+    [0]                            - total number of blocks: 8
+    [0]                            - number of processes: 1 (1 worker process(es) / 0 empty buffer process(es))
+    [0]                            - buffer processes are inserted into the process network: no
+    [0]                            - process ID bytes: 0
+    [0]                            - blocks/memory/workload per process:
+    [0]                               + blocks:
+    [0]                                  - min       = 8
+    [0]                                  - max       = 8
+    [0]                                  - avg       = 8
+    [0]                                  - stdDev    = 0
+    [0]                                  - relStdDev = 0
+    [0]                               + memory:
+    [0]                                  - min       = 8
+    [0]                                  - max       = 8
+    [0]                                  - avg       = 8
+    [0]                                  - stdDev    = 0
+    [0]                                  - relStdDev = 0
+    [0]                               + workload:
+    [0]                                  - min       = 8
+    [0]                                  - max       = 8
+    [0]                                  - avg       = 8
+    [0]                                  - stdDev    = 0
+    [0]                                  - relStdDev = 0
+    [0][PROGRESS]------(0.119 sec) Adding block data ("cell bounding box")
+    [0][PROGRESS]------(0.119 sec) Adding block data ("src")
+    [0][PROGRESS]------(0.120 sec) Adding block data ("dst")
+
+%% Cell type:markdown id: tags:
+
+Now lets initialize the arrays. To do this we can get arrays (meshgrids) from the block with the coordinates of the local cells. We use this to initialize a circular shape.
+
+%% Cell type:code id: tags:
+
+``` python
+dh.fill('src', 0.0)
+for block in dh.iterate(ghost_layers=False, inner_ghost_layers=False):
+    x, y = block.midpoint_arrays
+    inside_sphere = (x -20)**2 + (y-25)**2  < 8 ** 2
+    block['src'][inside_sphere] = 1.0
+plt.scalar_field( dh.gather_array('src') );
+```
+
+%% Output
+
+
+
+%% Cell type:markdown id: tags:
+
+Now we can run our compute kernel on this data as usual. We just have to make sure that the field is synchronized after every step, i.e. that the ghost layers are correctly updated.
+
+%% Cell type:code id: tags:
+
+``` python
+sync = dh.synchronization_function(['src'])
+for i in range(40):
+    sync()
+    dh.run_kernel(kernel_function)
+    dh.swap('src', 'dst')
+```
+
+%% Cell type:code id: tags:
+
+``` python
+plt.scalar_field( dh.gather_array('src') );
+```
+
+%% Output
+
+