
.. _extending_theano_gpu:

==============================
Extending Theano with a GPU Op
==============================

.. note::

    This covers the :ref:`gpuarray <gpuarray>` back-end for the GPU.

This tutorial covers how to extend Theano with an op that offers a GPU
implementation.  It assumes you are familiar with how to write new
Theano ops.  If that is not the case you should probably follow the
:ref:`extending_theano` and :ref:`extending_theano_c` sections before
continuing on.

Writing a new GPU op can be done in Python for some simple tasks, but
will usually done in C to access the complete API and avoid paying the
overhead of a Python function call.

Dealing With the Context
========================

One of the major differences with GPU ops is that they require a
context (a.k.a. device) to execute.  Most of the time you can infer
the context to run on from your inputs.  There is a way for the user
to transfer things between contexts and to tag certain variables for
transfer.  It might also be the case that your inputs are not all from
the same context and you would have to choose which one to run on.

In order to support all of those options and have a consistent
interface, :func:`theano.gpuarray.basic_ops.infer_context_name` was
written.  An example usage is below::

    def make_node(self, a, b, c):
        ctx = infer_context_name(a, b, c)
        a = as_gpuarray_variable(a, ctx)
        b = as_gpuarray_variable(b, ctx)
        c = as_gpuarray_variable(c, ctx)
        return Apply(self, [a, b, c], [a.type()])

In this example the Op takes three inputs, all on the GPU.  In case
one or more of your inputs is not supposed to be on the GPU, you
should not pass it to :func:`infer_context_name` or call
:func:`as_gpuarray_variable` on it.

Also note that :func:`theano.gpuarray.basic_ops.as_gpuarray_variable`
takes ``context_name`` as a mandatory parameter.  This is because it's
not enough to know you want the value to be on the GPU, you also want
to know which GPU to put it on.  In almost all cases, you can pass in
the return value of :func:`infer_context_name` there.

If you also need the context during runtime (for example to allocate
the output), you can use the context of one of your inputs to know
which one to use.  Here is another example::

    def perform(self, node, inputs, output_storage):
        A, B = inputs
        C, = output_storage
        C[0] = pygpu.empty([A.shape[0], B.shape[1]], dtype=A.dtype, A.context)
        pygpu.blas.gemm(1, A, B, 0, C, overwrite_c=True)

Finally if you require the context before perform, such as during
make_thunk() to initialize kernels and such, you can access the
context of your inputs through the type of the variables::

    def make_thunk(self, node, storage_map, compute_map, no_recycling):
        ctx = node.inputs[0].type.context

Note that ``GpuArrayType`` objects also have a ``context_name``
attribute which is the symbolic equivalent of ``context``.  It can't
be used for calls to pygpu or libgpuarray, but it should be used for
theano operations and variables.

The last place where you might need the context is in the C
initialization code.  For that you will have to use the :ref:`params
<extending_op_params>`.  The params type should be
:data:`theano.gpuarray.type.gpu_context_type` and the params object
should be a context object from one of your input variables::

    def get_params(self, node):
        return node.inputs[0].type.context

If you don't have any input variables on the GPU you can follow the
the example of :class:`GpuFromHost
<theano.gpuarray.basic_ops.GpuFromHost>` or :class:`GpuEye
<theano.gpuarray.basic_ops.GpuEye>`.  This is not a case that you
should encounter often, so it will not be covered further.

Defining New Kernels
====================

If your op needs to do some transformation on the data, chances are
that you will need to write a new kernel.  The best way to do this is
to leverage :class:`GpuKernelBase
<theano.gpuarray.basic_ops.GpuKernelBase>` (or :class:`CGpuKernelBase
<theano.gpuarray.basic_ops.CGpuKernelBase>` if you want to use the
:class:`ExternalCOp <theano.gof.op.ExternalCOp>` functionality).

For plain :class:`GpuKernelBase
<theano.gpuarray.basic_ops.GpuKernelBase>`, you have to define a
method called ``gpu_kernels`` which returns a list of :class:`Kernel
<theano.gpuarray.basic_ops.Kernel>` objects.  You can define as many
kernels as you want for a single op.  An example would look like
this::

    def gpu_kernels(self, node, name):
        code = """
    KERNEL void k(GLOBAL_MEM ga_double *a, ga_size n, ga_size m) {
        ga_size nb = n < m ? n : m;
        for (ga_size i = LID_0; i < nb; i += LDIM_0) {
            a[i*m + i] = 1;
        }
    }"""
        return [Kernel(
                code=code, name="k",
                params=[gpuarray.GpuArray, gpuarray.SIZE, gpuarray.SIZE],
                flags=Kernel.get_flags('float64'))]

If you want to use ``ExternalCOp``, then you should use ``CGpuKernelBase``
instead.  It adds a new section to the parsed files whose tag is
``kernels``.  Inside that section you can define some kernels with
``#kernel name:params:flags``.

Here ``name`` is the name of the kernel function in the following
code, ``params`` is a comma-separeted list of numpy typecode names.
There are three exceptions for ``size_t`` which should be noted as
``size``, ``ssize_t`` which should be noted as ``ssize`` and a pointer
which should be noted as ``*``.

``flags`` is a ``|``-separated list of C kernel flag values (can be
empty).  The same kernel definition as above would look like this with
``CGpuKernelBase``::

    #section kernels

    #kernel k : *, size, size : GA_USE_DOUBLE

    KERNEL void k(GLOBAL_MEM ga_double *a, ga_size n, ga_size m) {
        ga_size nb = n < m ? n : m;
        for (ga_size i = LID_0; i < nb; i += LDIM_0) {
        a[i*m + i] = 1;
        }
    }

The second method is to handle the kernel compilation and cache on
your own.  This is not recommended because there are lots of details
to pay attention to that can cripple your performance if not done
right, which GpuKernelBase handles for you.  But if you really want to
go this way, then you can look up the C API for kernels in
libgpuarray.

In any case you will need to call your compiled kernel with some data,
in most cases in your :meth:`c_code` method.  This is done by using
the provided wrapper function. An example calling the above kernel
would be::

    size_t dims[2];
    size_t n = 256;

    // ...

    err = k_scall(1, &n, 0, input->ga.data, dims[0], dims[1]);

    // ...

If you want explicit control over the scheduling, you can use the
`_call` wrapper instead which works like this::

    size_t ls, gs;

    // ...

    gs = 1;
    ls = 256;
    err = k_call(1, &gs, &ls, 0, input->ga.data, dims[0], dims[1]);


The name of the wrapper function depends on the name you passed to
``Kernel()`` when you declared it (or the name in your `#kernel`
statement).  It defaults to `name + '_call' or '_scall'`.

For other operations in the C code you should refer to the
`libgpuarray documentation
<http://deeplearning.net/software/libgpuarray/>`_.

Dealing with float16
====================

To support limited-precision storage in a kernel you have to be
careful to load values properly, declare working memory in float32 and
write results properly.  To help with that some functions have been
declared in :mod:`theano.gpuarray.fp16_help`.

To load the inputs you should wrap the read with the function returned
by :func:`load_w() <theano.gpuarray.fp16_help.load_w>`. Similarly writes
should be wrapped in the function returned by :func:`write_w()
<theano.gpuarray.fp16_help.write_w>`.  Finally working data should
have the type returned by :func:`work_dtype()
<theano.gpuarray.fp16_help.work_dtype>`.

Here is a +1 kernel that is not ready to deal with float16 input::

    type_x = dtype_to_ctype(x.dtype)
    type_y = dtype_to_ctype(y.dtype)
    """
    KERNEL void k(const ga_size n, %(type_x)s *x, %(type_y)s *y) {
        ga_size i = GID_0 * LDIM_0 + LID_0;
        %(type_x) z = x[i];
        z += 1;
        y[i] = z;
    }
    """ % dict(dtype_x=dtype_x, dtype_y=dtype_y)

Here is the same kernel, but now ready to handle float16::

    type_x = dtype_to_ctype(x.dtype)
    type_y = dtype_to_ctype(y.dtype)
    work_x = dtype_to_ctype(work_dtype(x.dtype))
    load_x = load_w(x.dtype)
    write_y = write_w(y.dtype)
    """
    KERNEL void k(const ga_size n, %(type_x)s *x, %(type_y)s *y) {
        ga_size i = GID_0 * LDIM_0 + LID_0;
        %(work_x) z = %(load_x)(x[i]);
        z += 1;
        y[i] = %(write_y)(z);
    }
    """ % dict(dtype_x=dtype_x, dtype_y=dtype_y, work_x=work_x, load_x=load_x,
               write_y=write_y)

Once you have converted your kernels for float16 support you need to
tag your op with ``_f16_ok = True`` so that the linker will accept to
generate C code on float16 types.  This is done by inserting it as a
class property like this::

    class SomeOp(COp):
        _f16_ok = True

If this attribute is not present or is False, the linker will print a
message saying that it's refusing to use C code for float16 for the
op.

A Complete Example
==================

This is a complete example using both approches for a implementation
of the Eye operation.

GpuKernelBase
-------------

Python File
~~~~~~~~~~~

.. literalinclude:: ../../theano/gpuarray/basic_ops.py
    :language: python
    :pyobject: GpuEye

CGpuKernelBase
--------------

Python File
~~~~~~~~~~~

.. literalinclude:: ../../theano/gpuarray/tests/test_cgpukernelbase.py
    :language: python
    :pyobject: GpuEye

``tstgpueye.c``
~~~~~~~~~~~~~~~

.. literalinclude:: ../../theano/gpuarray/tests/c_code/tstgpueye.c
    :language: C

Wrapping Exisiting Libraries
============================

PyCUDA
------

For things in PyCUDA (or things wrapped with PyCUDA), we usually need
to create a PyCUDA context.  This can be done with the following
code::

    with gpuarray_cuda_context:
        pycuda_context = pycuda.driver.Context.attach()

If you don't need to create a context, because the library doesn't
require it, you can also just use the pygpu context and a `with`
statement like above for all your code which will make the context the
current context on the cuda stack.

GpuArray objects are compatible with PyCUDA and will expose the
necessary interface so that they can be used in most things.  One
notable exception is PyCUDA kernels which require native objects.  If
you need to convert a pygpu GpuArray to a PyCUDA GPUArray, this code
should do the trick::

    assert pygpu_array.flags['IS_C_CONTIGUOUS']
    pycuda_array = pycuda.gpuarray.GPUArray(pygpu_array.shape,
                                            pygpu_array.dtype,
                                            base=pygpu_array,
                                            gpudata=(pygpu_array.gpudata +
                                                     pygpu_array.offset))

As long as the computations happen on the NULL stream there are no
special considerations to watch for with regards to synchronization.
Otherwise, you will have to make sure that you synchronize the pygpu
objects by calling the `.sync()` method before scheduling any work and
synchronize with the work that happens in the library after all the
work is scheduled.
