starpu-max/doc/doxygen/chapters/07data_management.doxy

/*
 * This file is part of the StarPU Handbook.
 * Copyright (C) 2009--2011  Universit@'e de Bordeaux
 * Copyright (C) 2010, 2011, 2012, 2013, 2014  Centre National de la Recherche Scientifique
 * Copyright (C) 2011, 2012 Institut National de Recherche en Informatique et Automatique
 * See the file version.doxy for copying conditions.
 */

/*! \page DataManagement Data Management

intro qui parle de coherency entre autres

\section DataManagement Data Management

When the application allocates data, whenever possible it should use
the function starpu_malloc(), which will ask CUDA or OpenCL to make
the allocation itself and pin the corresponding allocated memory. This
is needed to permit asynchronous data transfer, i.e. permit data
transfer to overlap with computations. Otherwise, the trace will show
that the <c>DriverCopyAsync</c> state takes a lot of time, this is
because CUDA or OpenCL then reverts to synchronous transfers.

By default, StarPU leaves replicates of data wherever they were used, in case they
will be re-used by other tasks, thus saving the data transfer time. When some
task modifies some data, all the other replicates are invalidated, and only the
processing unit which ran that task will have a valid replicate of the data. If the application knows
that this data will not be re-used by further tasks, it should advise StarPU to
immediately replicate it to a desired list of memory nodes (given through a
bitmask). This can be understood like the write-through mode of CPU caches.

\code{.c}
starpu_data_set_wt_mask(img_handle, 1<<0);
\endcode

will for instance request to always automatically transfer a replicate into the
main memory (node <c>0</c>), as bit <c>0</c> of the write-through bitmask is being set.

\code{.c}
starpu_data_set_wt_mask(img_handle, ~0U);
\endcode

will request to always automatically broadcast the updated data to all memory
nodes.

Setting the write-through mask to <c>~0U</c> can also be useful to make sure all
memory nodes always have a copy of the data, so that it is never evicted when
memory gets scarse.

Implicit data dependency computation can become expensive if a lot
of tasks access the same piece of data. If no dependency is required
on some piece of data (e.g. because it is only accessed in read-only
mode, or because write accesses are actually commutative), use the
function starpu_data_set_sequential_consistency_flag() to disable
implicit dependencies on that data.

In the same vein, accumulation of results in the same data can become a
bottleneck. The use of the mode ::STARPU_REDUX permits to optimize such
accumulation (see \ref DataReduction). To a lesser extent, the use of
the flag ::STARPU_COMMUTE keeps the bottleneck, but at least permits
the accumulation to happen in any order.

Applications often need a data just for temporary results.  In such a case,
registration can be made without an initial value, for instance this produces a vector data:

\code{.c}
starpu_vector_data_register(&handle, -1, 0, n, sizeof(float));
\endcode

StarPU will then allocate the actual buffer only when it is actually needed,
e.g. directly on the GPU without allocating in main memory.

In the same vein, once the temporary results are not useful any more, the
data should be thrown away. If the handle is not to be reused, it can be
unregistered:

\code{.c}
starpu_data_unregister_submit(handle);
\endcode

actual unregistration will be done after all tasks working on the handle
terminate.

If the handle is to be reused, instead of unregistering it, it can simply be invalidated:

\code{.c}
starpu_data_invalidate_submit(handle);
\endcode

the buffers containing the current value will then be freed, and reallocated
only when another task writes some value to the handle.

\section DataPrefetch Data Prefetch

The scheduling policies <c>heft</c>, <c>dmda</c> and <c>pheft</c>
perform data prefetch (see \ref STARPU_PREFETCH):
as soon as a scheduling decision is taken for a task, requests are issued to
transfer its required data to the target processing unit, if needed, so that
when the processing unit actually starts the task, its data will hopefully be
already available and it will not have to wait for the transfer to finish.

The application may want to perform some manual prefetching, for several reasons
such as excluding initial data transfers from performance measurements, or
setting up an initial statically-computed data distribution on the machine
before submitting tasks, which will thus guide StarPU toward an initial task
distribution (since StarPU will try to avoid further transfers).

This can be achieved by giving the function starpu_data_prefetch_on_node()
the handle and the desired target memory node.

\section PartitioningData Partitioning Data

An existing piece of data can be partitioned in sub parts to be used by different tasks, for instance:

\code{.c}
int vector[NX];
starpu_data_handle_t handle;

/* Declare data to StarPU */
starpu_vector_data_register(&handle, STARPU_MAIN_RAM, (uintptr_t)vector,
                            NX, sizeof(vector[0]));

/* Partition the vector in PARTS sub-vectors */
struct starpu_data_filter f =
{
    .filter_func = starpu_vector_filter_block,
    .nchildren = PARTS
};
starpu_data_partition(handle, &f);
\endcode

The task submission then uses the function starpu_data_get_sub_data()
to retrieve the sub-handles to be passed as tasks parameters.

\code{.c}
/* Submit a task on each sub-vector */
for (i=0; i<starpu_data_get_nb_children(handle); i++) {
    /* Get subdata number i (there is only 1 dimension) */
    starpu_data_handle_t sub_handle = starpu_data_get_sub_data(handle, 1, i);
    struct starpu_task *task = starpu_task_create();

    task->handles[0] = sub_handle;
    task->cl = &cl;
    task->synchronous = 1;
    task->cl_arg = &factor;
    task->cl_arg_size = sizeof(factor);

    starpu_task_submit(task);
}
\endcode

Partitioning can be applied several times, see
<c>examples/basic_examples/mult.c</c> and <c>examples/filters/</c>.

Wherever the whole piece of data is already available, the partitioning will
be done in-place, i.e. without allocating new buffers but just using pointers
inside the existing copy. This is particularly important to be aware of when
using OpenCL, where the kernel parameters are not pointers, but handles. The
kernel thus needs to be also passed the offset within the OpenCL buffer:

\code{.c}
void opencl_func(void *buffers[], void *cl_arg)
{
    cl_mem vector = (cl_mem) STARPU_VECTOR_GET_DEV_HANDLE(buffers[0]);
    unsigned offset = STARPU_BLOCK_GET_OFFSET(buffers[0]);

    ...
    clSetKernelArg(kernel, 0, sizeof(vector), &vector);
    clSetKernelArg(kernel, 1, sizeof(offset), &offset);
    ...
}
\endcode

And the kernel has to shift from the pointer passed by the OpenCL driver:

\code{.c}
__kernel void opencl_kernel(__global int *vector, unsigned offset)
{
    block = (__global void *)block + offset;
    ...
}
\endcode

StarPU provides various interfaces and filters for matrices, vectors, etc.,
but applications can also write their own data interfaces and filters, see
<c>examples/interface</c> and <c>examples/filters/custom_mf</c> for an example.

\section DataReduction Data Reduction

In various cases, some piece of data is used to accumulate intermediate
results. For instances, the dot product of a vector, maximum/minimum finding,
the histogram of a photograph, etc. When these results are produced along the
whole machine, it would not be efficient to accumulate them in only one place,
incurring data transmission each and access concurrency.

StarPU provides a mode ::STARPU_REDUX, which permits to optimize
that case: it will allocate a buffer on each memory node, and accumulate
intermediate results there. When the data is eventually accessed in the normal
mode ::STARPU_R, StarPU will collect the intermediate results in just one
buffer.

For this to work, the user has to use the function
starpu_data_set_reduction_methods() to declare how to initialize these
buffers, and how to assemble partial results.

For instance, <c>cg</c> uses that to optimize its dot product: it first defines
the codelets for initialization and reduction:

\code{.c}
struct starpu_codelet bzero_variable_cl =
{
        .cpu_funcs = { bzero_variable_cpu },
        .cpu_funcs_name = { "bzero_variable_cpu" },
        .cuda_funcs = { bzero_variable_cuda },
        .nbuffers = 1,
}

static void accumulate_variable_cpu(void *descr[], void *cl_arg)
{
        double *v_dst = (double *)STARPU_VARIABLE_GET_PTR(descr[0]);
        double *v_src = (double *)STARPU_VARIABLE_GET_PTR(descr[1]);
        *v_dst = *v_dst + *v_src;
}

static void accumulate_variable_cuda(void *descr[], void *cl_arg)
{
        double *v_dst = (double *)STARPU_VARIABLE_GET_PTR(descr[0]);
        double *v_src = (double *)STARPU_VARIABLE_GET_PTR(descr[1]);
        cublasaxpy(1, (double)1.0, v_src, 1, v_dst, 1);
        cudaStreamSynchronize(starpu_cuda_get_local_stream());
}

struct starpu_codelet accumulate_variable_cl =
{
        .cpu_funcs = { accumulate_variable_cpu },
        .cpu_funcs_name = { "accumulate_variable_cpu" },
        .cuda_funcs = { accumulate_variable_cuda },
        .nbuffers = 1,
}
\endcode

and attaches them as reduction methods for its handle <c>dtq</c>:

\code{.c}
starpu_variable_data_register(&dtq_handle, -1, NULL, sizeof(type));
starpu_data_set_reduction_methods(dtq_handle,
        &accumulate_variable_cl, &bzero_variable_cl);
\endcode

and <c>dtq_handle</c> can now be used in mode ::STARPU_REDUX for the
dot products with partitioned vectors:

\code{.c}
for (b = 0; b < nblocks; b++)
    starpu_task_insert(&dot_kernel_cl,
        STARPU_REDUX, dtq_handle,
        STARPU_R, starpu_data_get_sub_data(v1, 1, b),
        STARPU_R, starpu_data_get_sub_data(v2, 1, b),
        0);
\endcode

During registration, we have here provided <c>NULL</c>, i.e. there is
no initial value to be taken into account during reduction. StarPU
will thus only take into account the contributions from the tasks
<c>dot_kernel_cl</c>. Also, it will not allocate any memory for
<c>dtq_handle</c> before tasks <c>dot_kernel_cl</c> are ready to run.

If another dot product has to be performed, one could unregister
<c>dtq_handle</c>, and re-register it. But one can also call
starpu_data_invalidate_submit() with the parameter <c>dtq_handle</c>,
which will clear all data from the handle, thus resetting it back to
the initial status <c>register(NULL)</c>.

The example <c>cg</c> also uses reduction for the blocked gemv kernel,
leading to yet more relaxed dependencies and more parallelism.

::STARPU_REDUX can also be passed to starpu_mpi_task_insert() in the MPI
case. That will however not produce any MPI communication, but just pass
::STARPU_REDUX to the underlying starpu_task_insert(). It is up to the
application to call starpu_mpi_redux_data(), which posts tasks that will
reduce the partial results among MPI nodes into the MPI node which owns the
data. For instance, some hypothetical application which collects partial results
into data <c>res</c>, then uses it for other computation, before looping again
with a new reduction:

\code{.c}
for (i = 0; i < 100; i++) {
    starpu_mpi_task_insert(MPI_COMM_WORLD, &init_res, STARPU_W, res, 0);
    starpu_mpi_task_insert(MPI_COMM_WORLD, &work, STARPU_RW, A,
               STARPU_R, B, STARPU_REDUX, res, 0);
    starpu_mpi_redux_data(MPI_COMM_WORLD, res);
    starpu_mpi_task_insert(MPI_COMM_WORLD, &work2, STARPU_RW, B, STARPU_R, res, 0);
}
\endcode

\section TemporaryBuffers Temporary Buffers

There are two kinds of temporary buffers: temporary data which just pass results
from a task to another, and scratch data which are needed only internally by
tasks.

\subsection TemporaryData Temporary Data

Data can sometimes be entirely produced by a task, and entirely consumed by
another task, without the need for other parts of the application to access
it. In such case, registration can be done without prior allocation, by using
the special memory node number <c>-1</c>, and passing a zero pointer. StarPU will
actually allocate memory only when the task creating the content gets scheduled,
and destroy it on unregistration.

In addition to that, it can be tedious for the application to have to unregister
the data, since it will not use its content anyway. The unregistration can be
done lazily by using the function starpu_data_unregister_submit(),
which will record that no more tasks accessing the handle will be submitted, so
that it can be freed as soon as the last task accessing it is over.

The following code examplifies both points: it registers the temporary
data, submits three tasks accessing it, and records the data for automatic
unregistration.

\code{.c}
starpu_vector_data_register(&handle, -1, 0, n, sizeof(float));
starpu_task_insert(&produce_data, STARPU_W, handle, 0);
starpu_task_insert(&compute_data, STARPU_RW, handle, 0);
starpu_task_insert(&summarize_data, STARPU_R, handle, STARPU_W, result_handle, 0);
starpu_data_unregister_submit(handle);
\endcode

The application may also want to see the temporary data initialized
on the fly before being used by the task. This can be done by using
starpu_data_set_reduction_methods() to set an initialization codelet (no redux
codelet is needed).

\subsection ScratchData Scratch Data

Some kernels sometimes need temporary data to achieve the computations, i.e. a
workspace. The application could allocate it at the start of the codelet
function, and free it at the end, but that would be costly. It could also
allocate one buffer per worker (similarly to \ref
HowToInitializeAComputationLibraryOnceForEachWorker), but that would
make them systematic and permanent. A more  optimized way is to use
the data access mode ::STARPU_SCRATCH, as examplified below, which
provides per-worker buffers without content consistency.

\code{.c}
starpu_vector_data_register(&workspace, -1, 0, sizeof(float));
for (i = 0; i < N; i++)
    starpu_task_insert(&compute, STARPU_R, input[i],
                       STARPU_SCRATCH, workspace, STARPU_W, output[i], 0);
\endcode

StarPU will make sure that the buffer is allocated before executing the task,
and make this allocation per-worker: for CPU workers, notably, each worker has
its own buffer. This means that each task submitted above will actually have its
own workspace, which will actually be the same for all tasks running one after
the other on the same worker. Also, if for instance GPU memory becomes scarce,
StarPU will notice that it can free such buffers easily, since the content does
not matter.

The example <c>examples/pi</c> uses scratches for some temporary buffer.

\section TheMultiformatInterface The Multiformat Interface

It may be interesting to represent the same piece of data using two different
data structures: one that would only be used on CPUs, and one that would only
be used on GPUs. This can be done by using the multiformat interface. StarPU
will be able to convert data from one data structure to the other when needed.
Note that the scheduler <c>dmda</c> is the only one optimized for this
interface. The user must provide StarPU with conversion codelets:

\snippet multiformat.c To be included. You should update doxygen if you see this text.

Kernels can be written almost as for any other interface. Note that
::STARPU_MULTIFORMAT_GET_CPU_PTR shall only be used for CPU kernels. CUDA kernels
must use ::STARPU_MULTIFORMAT_GET_CUDA_PTR, and OpenCL kernels must use
::STARPU_MULTIFORMAT_GET_OPENCL_PTR. ::STARPU_MULTIFORMAT_GET_NX may
be used in any kind of kernel.

\code{.c}
static void
multiformat_scal_cpu_func(void *buffers[], void *args)
{
    struct point *aos;
    unsigned int n;

    aos = STARPU_MULTIFORMAT_GET_CPU_PTR(buffers[0]);
    n = STARPU_MULTIFORMAT_GET_NX(buffers[0]);
    ...
}

extern "C" void multiformat_scal_cuda_func(void *buffers[], void *_args)
{
    unsigned int n;
    struct struct_of_arrays *soa;

    soa = (struct struct_of_arrays *) STARPU_MULTIFORMAT_GET_CUDA_PTR(buffers[0]);
    n = STARPU_MULTIFORMAT_GET_NX(buffers[0]);

    ...
}
\endcode

A full example may be found in <c>examples/basic_examples/multiformat.c</c>.

\section DefiningANewDataInterface Defining A New Data Interface

Let's define a new data interface to manage complex numbers.

\code{.c}
/* interface for complex numbers */
struct starpu_complex_interface
{
        double *real;
        double *imaginary;
        int nx;
};
\endcode

Registering such a data to StarPU is easily done using the function
starpu_data_register(). The last
parameter of the function, <c>interface_complex_ops</c>, will be
described below.

\code{.c}
void starpu_complex_data_register(starpu_data_handle_t *handle,
     unsigned home_node, double *real, double *imaginary, int nx)
{
        struct starpu_complex_interface complex =
        {
                .real = real,
                .imaginary = imaginary,
                .nx = nx
        };

        if (interface_complex_ops.interfaceid == STARPU_UNKNOWN_INTERFACE_ID)
        {
                interface_complex_ops.interfaceid = starpu_data_interface_get_next_id();
        }

        starpu_data_register(handleptr, home_node, &complex, &interface_complex_ops);
}
\endcode

Different operations need to be defined for a data interface through
the type starpu_data_interface_ops. We only define here the basic
operations needed to run simple applications. The source code for the
different functions can be found in the file
<c>examples/interface/complex_interface.c</c>.

\code{.c}
static struct starpu_data_interface_ops interface_complex_ops =
{
        .register_data_handle = complex_register_data_handle,
        .allocate_data_on_node = complex_allocate_data_on_node,
        .copy_methods = &complex_copy_methods,
        .get_size = complex_get_size,
        .footprint = complex_footprint,
        .interfaceid = STARPU_UNKNOWN_INTERFACE_ID,
        .interface_size = sizeof(struct starpu_complex_interface),
};
\endcode

Functions need to be defined to access the different fields of the
complex interface from a StarPU data handle.

\code{.c}
double *starpu_complex_get_real(starpu_data_handle_t handle)
{
        struct starpu_complex_interface *complex_interface =
          (struct starpu_complex_interface *) starpu_data_get_interface_on_node(handle, STARPU_MAIN_RAM);
        return complex_interface->real;
}

double *starpu_complex_get_imaginary(starpu_data_handle_t handle);
int starpu_complex_get_nx(starpu_data_handle_t handle);
\endcode

Similar functions need to be defined to access the different fields of the
complex interface from a <c>void *</c> pointer to be used within codelet
implemetations.

\snippet complex.c To be included. You should update doxygen if you see this text.

Complex data interfaces can then be registered to StarPU.

\code{.c}
double real = 45.0;
double imaginary = 12.0;starpu_complex_data_register(&handle1, STARPU_MAIN_RAM, &real, &imaginary, 1);
starpu_task_insert(&cl_display, STARPU_R, handle1, 0);
\endcode

and used by codelets.

\code{.c}
void display_complex_codelet(void *descr[], __attribute__ ((unused)) void *_args)
{
        int nx = STARPU_COMPLEX_GET_NX(descr[0]);
        double *real = STARPU_COMPLEX_GET_REAL(descr[0]);
        double *imaginary = STARPU_COMPLEX_GET_IMAGINARY(descr[0]);
        int i;

        for(i=0 ; i<nx ; i++)
        {
                fprintf(stderr, "Complex[%d] = %3.2f + %3.2f i\n", i, real[i], imaginary[i]);
        }
}
\endcode

The whole code for this complex data interface is available in the
directory <c>examples/interface/</c>.


\section SpecifyingATargetNode Specifying a target node for task data

When executing a task on a GPU for instance, StarPU would normally copy all the
needed data for the tasks on the embedded memory of the GPU.  It may however
happen that the task kernel would rather have some of the datas kept in the
main memory instead of copied in the GPU, a pivoting vector for instance.
This can be achieved by setting the starpu_codelet::specific_nodes flag to
1, and then fill the starpu_codelet::nodes array (or starpu_codelet::dyn_nodes when
starpu_codelet::nbuffers is greater than STARPU_NMAXBUFS) with the node numbers
where data should be copied to, or -1 to let StarPU copy it to the memory node
where the task will be executed. For instance, with the following codelet:

\code{.c}
struct starpu_codelet cl =
{
	.cuda_funcs = { kernel },
	.nbuffers = 2,
	.modes = {STARPU_RW, STARPU_RW},
	.specific_nodes = 1,
	.nodes = {STARPU_MAIN_RAM, -1},
};
\endcode

the first data of the task will be kept in the main memory, while the second
data will be copied to the CUDA GPU as usual.

*/