Joined arrays in ADIOS2

docs/source/usage/workflow.rst

@@ -3,6 +3,49 @@
 Workflow
 ========
 
+Storing and reading chunks
+--------------------------
+
+1. **Chunks within an n-dimensional dataset**
+
+   Most commonly, chunks within an n-dimensional dataset are identified by their offset and extent.
+   The extent is the size of the chunk in each dimension, NOT the absolute coordinate within the entire dataset.
+
+   In the Python API, this is modeled to conform to the conventional ``__setitem__``/``__getitem__`` protocol.
+
+2. **Joined arrays (write only)**
+
+   (Currently) only supported in ADIOS2 no older than v2.9.0 under the conditions listed in the `ADIOS2 documentation on joined arrays <https://adios2.readthedocs.io/en/latest/components/components.html#shapes>`_.
+
+   In some cases, the concrete chunk within a dataset does not matter and the computation of indexes is a needless computational and mental overhead.
+   This commonly occurs for particle data which the openPMD-standard models as a list of particles.
+   The order of particles does not matter greatly, and making different parallel processes agree on indexing is error-prone boilerplate.
+
+   In such a case, at most one *joined dimension* can be specified in the Dataset, e.g. ``{Dataset::JOINED_DIMENSION, 128, 128}`` (3D for the sake of explanation, particle data would normally be 1D).
+   The chunk is then stored by specifying an empty offset vector ``{}``.
+   The chunk extent vector must be equivalent to the global extent in all non-joined dimensions (i.e. joined arrays allow no further sub-chunking other than concatenation along the joined dimension).
+   The joined dimension of the extent vector specifies the extent that this piece should have along the joined dimension.
+   In the Python API, the slice-based setter syntax can be used as an abbreviation since the necessary information is determined from the passed array, e.g. ``record_component[()] = local_data``.
+   The global extent of the dataset along the joined dimension will then be the sum of all local chunk extents along the joined dimension.
+
+   Since openPMD follows a struct-of-array layout of data, it is important not to lose correlation of data between components. E.g., joining an array must take care that ``particles/e/position/x`` and ``particles/e/position/y`` are joined in uniform way.
+
+   The openPMD-api makes the **following guarantee**:
+
+   Consider a Series written from ``N`` parallel processes between two (collective) flush points. For each parallel process ``n`` and dataset ``D``, let:
+
+    * ``chunk(D, n, i)`` be the ``i``'th chunk written to dataset ``D`` on process ``n``
+    * ``num_chunks(D, n)`` be the count of chunks written by ``n`` to ``D``
+    * ``joined_index(D, c)`` be the index of chunk ``c`` in the joining order of ``D``
+
+  Then for any two datasets ``x`` and ``y``:
+
+    * If for any parallel process ``n`` the condition holds that ``num_chunks(x, n) = num_chunks(y, n)`` (between the two flush points!)...
+    * ...then for any parallel process ``n`` and chunk index ``i`` less than ``num_chunks(x, n)``: ``joined_index(x, chunk(x, n, i)) = joined_index(y, chunk(y, n, i))``.
+
+  **TLDR:** Writing chunks to two joined arrays in synchronous way (**1.** same order of store operations and **2.** between the same flush operations) will result in the same joining order in both arrays.
+
+
 Access modes
 ------------
 

examples/5_write_parallel.py

@@ -14,13 +14,21 @@
 import numpy as np
 import openpmd_api as io
 
+try:
+    import adios2
+    from packaging import version
+    USE_JOINED_DIMENSION = \
+        version.parse(adios2.__version__) >= version.parse('2.9.0')
+except ImportError:
+    USE_JOINED_DIMENSION = False
+
 if __name__ == "__main__":
     # also works with any other MPI communicator
     comm = MPI.COMM_WORLD
 
     # global data set to write: [MPI_Size * 10, 300]
     # each rank writes a 10x300 slice with its MPI rank as values
-    local_value = comm.size
+    local_value = comm.rank
     local_data = np.ones(10 * 300,
                          dtype=np.double).reshape(10, 300) * local_value
     if 0 == comm.rank:
@@ -29,7 +37,9 @@
 
     # open file for writing
     series = io.Series(
-        "../samples/5_parallel_write_py.h5",
+        "../samples/5_parallel_write_py.bp"
+        if USE_JOINED_DIMENSION
+        else "../samples/5_parallel_write_py.h5",
         io.Access.create,
         comm
     )
@@ -51,7 +61,9 @@
         meshes["mymesh"]
 
     # example 1D domain decomposition in first index
-    global_extent = [comm.size * 10, 300]
+    global_extent = [io.Dataset.JOINED_DIMENSION, 300] \
+        if USE_JOINED_DIMENSION else [comm.size * 10, 300]
+
     dataset = io.Dataset(local_data.dtype, global_extent)
 
     if 0 == comm.rank:
@@ -64,7 +76,15 @@
               "mymesh in iteration 1")
 
     # example shows a 1D domain decomposition in first index
-    mymesh[comm.rank*10:(comm.rank+1)*10, :] = local_data
+
+    if USE_JOINED_DIMENSION:
+        # explicit API
+        # mymesh.store_chunk(local_data, [], [10, 300])
+        mymesh[:, :] = local_data
+        # or short:
+        # mymesh[()] = local_data
+    else:
+        mymesh[comm.rank*10:(comm.rank+1)*10, :] = local_data
     if 0 == comm.rank:
         print("Registered a single chunk per MPI rank containing its "
               "contribution, ready to write content to disk")

include/openPMD/Dataset.hpp

@@ -22,7 +22,9 @@
 
 #include "openPMD/Datatype.hpp"
 
+#include <limits>
 #include <memory>
+#include <optional>
 #include <string>
 #include <type_traits>
 #include <vector>
@@ -37,6 +39,11 @@ class Dataset
     friend class RecordComponent;
 
 public:
+    enum : std::uint64_t
+    {
+        JOINED_DIMENSION = std::numeric_limits<std::uint64_t>::max()
+    };
+
     Dataset(Datatype, Extent, std::string options = "{}");
 
     /**
@@ -53,5 +60,9 @@ class Dataset
     Datatype dtype;
     uint8_t rank;
     std::string options = "{}"; //!< backend-dependent JSON configuration
+
+    bool empty() const;
+
+    std::optional<size_t> joinedDimension() const;
 };
 } // namespace openPMD

include/openPMD/IO/ADIOS/ADIOS2IOHandler.hpp

@@ -60,6 +60,8 @@ namespace openPMD
 {
 #if openPMD_HAVE_ADIOS2
 
+std::optional<size_t> joinedDimension(adios2::Dims const &dims);
+
 class ADIOS2IOHandler;
 
 namespace detail
@@ -443,12 +445,35 @@ class ADIOS2IOHandlerImpl
                         std::to_string(actualDim) + ")");
             }
         }
-        for (unsigned int i = 0; i < actualDim; i++)
+        auto joinedDim = joinedDimension(shape);
+        if (joinedDim.has_value())
         {
-            if (offset[i] + extent[i] > shape[i])
+            if (!offset.empty())
             {
                 throw std::runtime_error(
-                    "[ADIOS2] Dataset access out of bounds.");
+                    "[ADIOS2] Offset must be an empty vector in case of joined "
+                    "array.");
+            }
+            for (unsigned int i = 0; i < actualDim; i++)
+            {
+                if (*joinedDim != i && extent[i] != shape[i])
+                {
+                    throw std::runtime_error(
+                        "[ADIOS2] store_chunk extent of non-joined dimensions "
+                        "must be equivalent to the total extent.");
+                }
+            }
+        }
+        else
+        {
+            for (unsigned int i = 0; i < actualDim; i++)
+            {
+                if (!(joinedDim.has_value() && *joinedDim == i) &&
+                    offset[i] + extent[i] > shape[i])
+                {
+                    throw std::runtime_error(
+                        "[ADIOS2] Dataset access out of bounds.");
+                }
             }
         }
 

include/openPMD/IO/IOTask.hpp

@@ -326,6 +326,7 @@ struct OPENPMDAPI_EXPORT Parameter<Operation::CREATE_DATASET>
     Extent extent = {};
     Datatype dtype = Datatype::UNDEFINED;
     std::string options = "{}";
+    std::optional<size_t> joinedDimension;
 
     /** Warn about unused JSON paramters
      *

include/openPMD/RecordComponent.hpp

@@ -537,6 +537,11 @@ OPENPMD_protected
     }
 
     void readBase(bool require_unit_si);
+
+    template <typename T>
+    void verifyChunk(Offset const &, Extent const &) const;
+
+    void verifyChunk(Datatype, Offset const &, Extent const &) const;
 }; // RecordComponent
 
 } // namespace openPMD

include/openPMD/RecordComponent.tpp

@@ -259,8 +259,17 @@ RecordComponent::storeChunk(T_ContiguousContainer &data, Offset o, Extent e)
     // default arguments
     //   offset = {0u}: expand to right dim {0u, 0u, ...}
     Offset offset = o;
-    if (o.size() == 1u && o.at(0) == 0u && dim > 1u)
-        offset = Offset(dim, 0u);
+    if (o.size() == 1u && o.at(0) == 0u)
+    {
+        if (joinedDimension().has_value())
+        {
+            offset.clear();
+        }
+        else if (dim > 1u)
+        {
+            offset = Offset(dim, 0u);
+        }
+    }
 
     //   extent = {-1u}: take full size
     Extent extent(dim, 1u);
@@ -278,38 +287,7 @@ template <typename T, typename F>
 inline DynamicMemoryView<T>
 RecordComponent::storeChunk(Offset o, Extent e, F &&createBuffer)
 {
-    if (constant())
-        throw std::runtime_error(
-            "Chunks cannot be written for a constant RecordComponent.");
-    if (empty())
-        throw std::runtime_error(
-            "Chunks cannot be written for an empty RecordComponent.");
-    Datatype dtype = determineDatatype<T>();
-    if (dtype != getDatatype())
-    {
-        std::ostringstream oss;
-        oss << "Datatypes of chunk data (" << dtype
-            << ") and record component (" << getDatatype() << ") do not match.";
-        throw std::runtime_error(oss.str());
-    }
-    uint8_t dim = getDimensionality();
-    if (e.size() != dim || o.size() != dim)
-    {
-        std::ostringstream oss;
-        oss << "Dimensionality of chunk ("
-            << "offset=" << o.size() << "D, "
-            << "extent=" << e.size() << "D) "
-            << "and record component (" << int(dim) << "D) "
-            << "do not match.";
-        throw std::runtime_error(oss.str());
-    }
-    Extent dse = getExtent();
-    for (uint8_t i = 0; i < dim; ++i)
-        if (dse[i] < o[i] + e[i])
-            throw std::runtime_error(
-                "Chunk does not reside inside dataset (Dimension on index " +
-                std::to_string(i) + ". DS: " + std::to_string(dse[i]) +
-                " - Chunk: " + std::to_string(o[i] + e[i]) + ")");
+    verifyChunk<T>(o, e);
 
     /*
      * The openPMD backend might not yet know about this dataset.
@@ -334,6 +312,7 @@ RecordComponent::storeChunk(Offset o, Extent e, F &&createBuffer)
         dCreate.name = rc.m_name;
         dCreate.extent = getExtent();
         dCreate.dtype = getDatatype();
+        dCreate.joinedDimension = joinedDimension();
         if (!rc.m_dataset.has_value())
         {
             throw error::WrongAPIUsage(
@@ -407,4 +386,9 @@ auto RecordComponent::visit(Args &&...args)
         getDatatype(), *this, std::forward<Args>(args)...);
 }
 
+template <typename T>
+void RecordComponent::verifyChunk(Offset const &o, Extent const &e) const
+{
+    verifyChunk(determineDatatype<T>(), o, e);
+}
 } // namespace openPMD

include/openPMD/backend/BaseRecordComponent.hpp

@@ -143,6 +143,8 @@ class BaseRecordComponent : virtual public Attributable
      */
     bool constant() const;
 
+    std::optional<size_t> joinedDimension() const;
+
     /**
      * Get data chunks that are available to be loaded from the backend.
      * Note that this is backend-dependent information and the returned

include/openPMD/backend/PatchRecordComponent.hpp

@@ -20,6 +20,7 @@
  */
 #pragma once
 
+#include "openPMD/Error.hpp"
 #include "openPMD/RecordComponent.hpp"
 #include "openPMD/auxiliary/ShareRawInternal.hpp"
 #include "openPMD/backend/BaseRecordComponent.hpp"
@@ -85,6 +86,9 @@ class PatchRecordComponent : public RecordComponent
     template <typename T>
     void store(uint64_t idx, T);
 
+    template <typename T>
+    void store(T);
+
     // clang-format off
 OPENPMD_private
     // clang-format on
@@ -180,4 +184,33 @@ inline void PatchRecordComponent::store(uint64_t idx, T data)
     auto &rc = get();
     rc.m_chunks.push(IOTask(this, std::move(dWrite)));
 }
+
+template <typename T>
+inline void PatchRecordComponent::store(T data)
+{
+    Datatype dtype = determineDatatype<T>();
+    if (dtype != getDatatype())
+    {
+        std::ostringstream oss;
+        oss << "Datatypes of patch data (" << dtype << ") and dataset ("
+            << getDatatype() << ") do not match.";
+        throw std::runtime_error(oss.str());
+    }
+
+    if (!joinedDimension().has_value())
+    {
+        throw error::WrongAPIUsage(
+            "[PatchRecordComponent::store] API call without explicit "
+            "specification of index only allowed when a joined dimension is "
+            "specified.");
+    }
+
+    Parameter<Operation::WRITE_DATASET> dWrite;
+    dWrite.offset = {};
+    dWrite.extent = {1};
+    dWrite.dtype = dtype;
+    dWrite.data = std::make_shared<T>(data);
+    auto &rc = get();
+    rc.m_chunks.push(IOTask(this, std::move(dWrite)));
+}
 } // namespace openPMD