quinnj opened a new pull request, #357: URL: https://github.com/apache/arrow-julia/pull/357
Fixes https://github.com/apache/arrow-julia/issues/345. Alternative to https://github.com/apache/arrow-julia/pull/350. Whew, a bit of a rabbit hole here. It turns out this issue doesn't have anything to do w/ the _arrow_ alignment when writing, but actually goes back to a core Julia issue, specifically the inconsistency reported in https://github.com/JuliaLang/julia/issues/42326. Essentially, the issue is that different platforms (intel vs. arm64) are requiring different alignments for types like `UInt128` (8 for intel, 16 for arm64). And it turns out this isn't even a Julia issue, but all the way back to LLVM. So why does this crop up in Arrow.jl? Because when you try to serialize/deserialize a `Arrow.Primitive{Int128}` array, we're going to write it out in the proper arrow format, but when you read it back in, we've been using the zero-copy technique of: ```julia unsafe_wrap(Array, Ptr{Int128}(arrow_ptr), len) ``` in order to give the user a Julia `Vector{Int128}` to the underlying arrow data. _BUT_, in Julia when you allocate any `Vector` with an eltype size of less than 4, then only 8 bytes of alignment are specified. So the fact that most users will pass arrow data as a file (which is mmapped as `Vector{UInt8}`), a byte vector directly (`Vector{UInt8}`), or an IO (which we read as `Vector{UInt8}`), means these vectors are only 8-byte aligned. This then throws the fatal error reported in the original #350 issue about the pointer not being 16-byte aligned. So one option to consider is allowing users to pass in a 16-byte aligned arrow "source" and then that would just work, right? Well, except Julia doesn't expose any way of "upcasting" an array's alignment, so it's purely based off the array eltype. Which in turn is a struggle because the current Arrow.jl architecture assumes the source will be exactly a `Vector{UInt8}` everywhere. So essentially it requires some heruclean effort to try and pass your own aligned array; I think the only option I can think of is if you did something like: * Mmap an arrow file into Vector{UInt8} * Allocate your own Vector{UInt32} and copy arrow data into it * Use unsafe_wrap to make a Vector{UInt8} of the Vector{UInt32} data * But you _MUST_ keep a reference to the original Vector{UInt32} array otherwise, your new Vector{UInt8} gets corrupted * Pass the new Vector{UInt8} into Arrow.Table to read Anyway, not easy to say the least. The proposed solution here is as follows: * We check `sizeof(T)` to see if it's > 16 bytes, which we're using as a proxy for the alignment, regardless of platform (core devs are leaning towards the 8-byte alignment on intel actually being a bug for 16-byte primitives and I agree) * We use sizeof since the `jl_datatype_t` alignment field isn't part of the public Julia API and thus subject to change (and in fact I think it did just change location in Julia#master) * It's a good enough proxy for our purposes anyway * If the original arrow pointer _isn't_ 16-byte aligned, then we'll allocate a new `Vector{T}`, which _will_ be aligned, then copy the arrow data directly into it via pointers. Simple, easy, just one extra allocation/copy. If Julia _does_ get the ability in the future to specify a custom larger-than-eltype-required alignemnt for arrays, then we could potentially do that ourselves when reading, but it's a little tricky because we really only need to do that if there are 16-byte primitives we'll be deserializing and we don't know that until we read a schema message. So *shrug*. -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
