arrow-dev mailing list archives

Site index · List index
Message view « Date » · « Thread »
Top « Date » · « Thread »
From Wes McKinney <wesmck...@gmail.com>
Subject Re: [DISCUSS] C-level in-process array protocol
Date Tue, 01 Oct 2019 20:28:32 GMT
On Tue, Oct 1, 2019 at 3:22 PM Jed Brown <jed@jedbrown.org> wrote:
>
> I'd just like to chime in with the use case of in-situ data analysis for
> simulations.  This domain tends to be cautious with dependencies and
> there is a lot of C and Fortran, but the in-situ analysis tools will
> preferably reside in separate processes while sharing memory via shared
> memory (/dev/shm or MPI_Win_allocate_shared).  An in-memory protocol
> that holds raw pointers would be problematic because they are typically
> in different virtual address spaces when shared between processes.  I
> think this is a potential application for a C interface with lean
> dependencies, but it wouldn't be useful if it can't be shared
> out-of-process.
>

hi Jed -- I will respond to Jacques's e-mail when I have some time to
compose, but we're looking at a pretty different use case which is C
libraries (or libraries with C FFI) exposing in-memory data structures
to each other in the same process using the same virtual address
space.

- Wes

> Jacques Nadeau <jacques@apache.org> writes:
>
> > I disagree with this statement:
> >
> > - the IPC format is meant for serialization while the C data protocol is
> > meants for in-memory communication, so different concerns apply
> >
> > If that is how the a particular implementation presents it, that is a
> > weaknesses of the implementation, not the format. The primary use case I
> > was focused on when working on the initial format was communication within
> > the same process. It seems like this is being used as a basis for the
> > introduction of new things when the premise is inconsistent with the
> > intention of the creation. The specific reason we used flatbuffers in the
> > project was to collapse the separation of in-process and out-of-process
> > communication. It means the same thing it does with the Arrow data itself:
> > that a consumer doesn't have to use a particular library to interact with
> > and use the data.
> >
> > It seems like there are two ideas here:
> >
> > 1) How do we make it easier for people to use Arrow?
> > 2) Should we implement a new in memory representation of Arrow that is
> > language specific.
> >
> > I'm entirely in support of number one. If for a particular type of domain,
> > people want an easier way to interact with Arrow, let's make a new library
> > that helps with that. In easy of our current libraries, we do many things
> > to make it easier to work with Arrow. None of those require a change to the
> > core format or are formalized as a new in-memory standard. The in-memory
> > representation of rust or javascript or java objects are implementation
> > details.
> >
> > I'm against number two as it creates a fragmentation problem. Arrow is
> > about having a single canonical format for memory for both metadata and
> > data. Having multiple in-memory formats (especially when some are not
> > language independent) is counter to the goals of the project.
> >
> > Two other, separate comments:
> > 1) I don't understand the idea that we need to change the way Arrow
> > fundamentally works so that people can avoid using a dependency. If the
> > dependency is small, open source and easy to build, people can fork it and
> > include directly if they want to. Let's not violate project principles
> > because DuckDB has a religious perspective on dependencies. If the problem
> > is people have to swallow too large of a pill to do basic things with Arrow
> > in C, let's focus on fixing that (to our definition of ease, not someone
> > else's). If FlatCC solves some those things, great. If we need to build a
> > baby integration library that is more C centric, great. Neither of those
> > things require implementing something at the format level.
> >
> > 2) It seems like we should discuss the data structure problem separately
> > from the reference management concern.
> >
> >
> > On Tue, Oct 1, 2019 at 5:42 AM Wes McKinney <wesmckinn@gmail.com> wrote:
> >
> >> hi Antoine,
> >>
> >> On Tue, Oct 1, 2019 at 4:29 AM Antoine Pitrou <antoine@python.org> wrote:
> >> >
> >> >
> >> > Le 01/10/2019 à 00:39, Wes McKinney a écrit :
> >> > > A couple things:
> >> > >
> >> > > * I think a C protocol / FFI for Arrow array/vectors would be better
> >> > > to have the same "shape" as an assembled array. Note that the C
> >> > > structs here have very nearly the same "shape" as the data structure
> >> > > representing a C++ Array object [1]. The disassembly and reassembly
> >> > > here is substantially simpler than the IPC protocol. A recursive
> >> > > structure in Flatbuffers would make RecordBatch messages much larger,
> >> > > so the flattened / disassembled representation we use for serialized
> >> > > record batches is the correct one
> >> >
> >> > I'm not sure I agree:
> >> >
> >> > - indeed, it's not a coincidence that the ArrowArray struct looks quite
> >> > closely like the C++ ArrayData object :-)  We have good experience with
> >> > that abstraction and it has proven to work quite well
> >> >
> >> > - the IPC format is meant for serialization while the C data protocol is
> >> > meants for in-memory communication, so different concerns apply
> >> >
> >> > - the fact that this makes the layout slightly larger doesn't seem
> >> > important at all; we're not talking about transferring data over the wire
> >> >
> >> > There's also another argument for having a recursive struct: it
> >> > simplifies how the data type is represented, since we can encode each
> >> > child type individually instead of encoding it in the parent's format
> >> > string (same applies for metadata and individual flags).
> >> >
> >>
> >> I was saying something different here. I was making an argument about
> >> why we use the flattened array-of-structs in the IPC protocol. One
> >> reason is that it's a more compact representation. That is not very
> >> important here because this protocol is only for *in-process* (for
> >> languages that have a C FFI facility) rather than *inter-process*
> >> communication.
> >>
> >> I agree also that the type encoding is simple, here, too, since we
> >> aren't having to split the schema and record batch between different
> >> serialized messages. There is some potential waste with having to
> >> populate the type fields multiple times when communicating a sequence
> >> of "chunks" from the same logical dataset.
> >>
> >> > > * The "formal" C protocol having the "assembled" shape means that
many
> >> > > minimal Arrow users won't have to implement any separate data
> >> > > structures. They can just use the C struct directly or a slightly
> >> > > wrapped version thereof with some convenience functions.
> >> >
> >> > Yes, but the same applies to the current proposal.
> >> >
> >> > > * I think that requiring building a Flatbuffer for minimal use cases
> >> > > (e.g. communicating simple record batches with primitive types) passes
> >> > > on implementation burden to minimal users.
> >> >
> >> > It certainly does.
> >> >
> >> > > I think the mantra of the C protocol should be the following:
> >> > >
> >> > > * Users of the protocol have to write little to no code to use it.
For
> >> > > example, populating an INT32 array should require only a few lines
of
> >> > > code
> >> >
> >> > Agreed.  As a sidenote, the spec should have an example of doing this in
> >> > raw C.
> >> >
> >> > Regards
> >> >
> >> > Antoine.
> >>

Mime
View raw message