My mistake, I confused the input type to kernels as Datums, when they are
in fact Scalar and ArrayData.
I agree that SIMD details should not be exposed in the kernel API.
~Eduardo
On Wed, Jul 28, 2021 at 6:38 PM Wes McKinney <wesmck...@gmail.com> wrote:
> On Wed, Jul 28, 2021 at 5:23 PM Eduardo Ponce <edponc...@gmail.com> wrote:
> >
> > Hi all,
> >
> > I agree with supporting finer-grained parallelism in the compute
> operators.
> > I think that incorporating a Datum-like span, would allow expressing
> > parallelism not only
> > on a per-thread basis but can also be used to represent SIMD spans, where
> > span length
> > is directed by vector ISA, "L2" cache line or NUMA-aware, and the
> > materialized data type being processed.
> > For compute functions, data parallelism between threads is equivalent to
> > data parallelism in
> > SIMD (thread == vector lane). My intention is not to derail this
> > conversation to discuss SIMD
> > but rather to acknowledge the suggested approach as a possible solution
> for
> > it. I can definitely
> > put together a PR for how a span interface looks for SIMDified compute
> > functions.
>
> My principal concern is making sure we have an input/output splitting
> solution (building the arguments that have to be passed to the
> ArrayKernelExec function) that is not as heavyweight as the one we
> have now, without trying to pack in additional functionality.
>
> That said, I'm not sure how SIMD spans are related to Datum, because
> this seems to be something that's done in the kernel implementation
> itself? If a Scalar is passed into a SIMD-enabled kernel, then the
> Scalar value would be replicated however many times as necessary into
> a simd_batch<T> and used for the kernel evaluation. I'm not sure why
> we would externalize SIMD details in the kernel API?
>
> > ~Eduardo
> >
> > On Wed, Jul 28, 2021 at 5:36 PM Wes McKinney <wesmck...@gmail.com>
> wrote:
> >
> > > On Wed, Jul 28, 2021 at 5:39 AM Antoine Pitrou <anto...@python.org>
> wrote:
> > > >
> > > >
> > > > Le 28/07/2021 à 03:33, Wes McKinney a écrit :
> > > > >
> > > > > I don't have the solution worked out for this, but the basic gist
> is:
> > > > >
> > > > > * To be 10-100x more efficient ExecBatch slicing cannot call
> > > > > ArrayData::Slice for every field like it does now
> > > > > * Atomics associated with interacting with shared_ptr<ArrayData> /
> > > > > shared_ptr<Buffer> do add meaningful overhead
> > > > > * The way that array kernels are currently implemented would need
> to
> > > > > shift to accommodate the changes needed to make ExecBatch lighter
> > > > > weight.
> > > > >
> > > > > One initial option is to move the "batch offset" to the top level
> of
> > > > > ExecBatch (to remove the need to copy ArrayData), but then quite a
> bit
> > > > > of code would need to be adapted to combine that offset with the
> > > > > ArrayData's offsets to compute memory addresses. If this memory
> > > > > address arithmetic has leaked into kernel implementations, this
> might
> > > > > be a good opportunity to unleak it. That wouldn't fix the
> > > > > shared_ptr/atomics overhead, so I'm open to ideas about how that
> could
> > > > > be addressed also.
> > > >
> > > > We could have a non-owning ArraySpan:
> > > >
> > > > struct ArraySpan {
> > > > ArrayData* data;
> > > > const int64_t offset, length;
> > > >
> > > > int64_t absolute_offset() const {
> > > > return offset + data->offset;
> > > > }
> > > > };
> > > >
> > > > And/or a more general (Datum-like) Span:
> > > >
> > > > class Span {
> > > > util::variant<ArraySpan*, Scalar*> datum_;
> > > >
> > > > public:
> > > > // Datum-like accessors
> > > > };
> > > >
> > > > or
> > > >
> > > > class Span {
> > > > util::variant<ArrayData*, Scalar*> datum_;
> > > > const int64_t offset_, length_;
> > > >
> > > > public:
> > > > // Datum-like accessors
> > > > };
> > > >
> > > >
> > > > Then ExecBatch could be a glorified std::vector<Span>.
> > >
> > > Yes, something like this might work. To make this complete, we would
> > > also want to change the out-argument of ArrayKernelExec to be
> > > something lighter-weight than Datum*
> > >
> > > std::function<Status(KernelContext*, const ExecBatch&, Datum*)>
> > >
> > > One thing to navigate would be kernels that do zero-copy [1] — the
> > > output passed to a kernel would need to be a mutable Span that can
> > > communicate to zero-copy implementations whether buffers can be
> > > replaced outright or whether the span is a slice of some other
> > > ArrayData a memcopy is required.
> > >
> > > A prototype along with some benchmarks would help assess whether a
> > > proposed design addresses the slicing cost to satisfaction. From a
> > > glance through some of the kernel implementations, the porting would
> > > be a labor-intensive but probably fairly mechanical project once the
> > > details are worked out.
> > >
> > > [1]:
> > >
> https://github.com/apache/arrow/blob/apache-arrow-5.0.0/cpp/src/arrow/compute/kernels/scalar_cast_internal.cc#L226
> > >
> > > > (also, Arrow would probably still benefit from a small vector
> > > > implementation... at least for internals, because we can't easily
> expose
> > > > it in public-facing APIs in place of regular std::vector)
> > > >
> > > > Regards
> > > >
> > > > Antoine.
> > >
>
