I guess I still dont understand how the IPC stream format works :-/
To put it clearly: what happens in Flight? Will a Flight message automatically get the "stream continuation message" in front of it? Le 02/07/2019 à 16:15, Wes McKinney a écrit : > On Tue, Jul 2, 2019 at 4:23 AM Antoine Pitrou <anto...@python.org> wrote: >> >> >> Le 02/07/2019 à 00:20, Wes McKinney a écrit : >>> Thanks for the references. >>> >>> If we decided to make a change around this, we could call the first 4 >>> bytes a stream continuation marker to make it slightly less ugly >>> >>> * 0xFFFFFFFF: continue >>> * 0x00000000: stop >> >> Do you mean it would be a separate IPC message? > > No, I think this is only about how we could change the message prefix > from 4 bytes to 8 bytes > > https://github.com/apache/arrow/blob/master/docs/source/format/IPC.rst#encapsulated-message-format > > Currently a 0x00000000 (0 metadata size) is used as an end-of-stream > marker. So what I was saying is that the first 8 bytes could be > > <4 bytes: stream continuation><int32_t metadata size> > >> >> >>> >>> On Mon, Jul 1, 2019 at 4:35 PM Micah Kornfield <emkornfi...@gmail.com> >>> wrote: >>>> >>>> Hi Wes, >>>> I'm not an expert on this either, my inclination mostly comes from some >>>> research I've done. I think it is important to distinguish two cases: >>>> 1. unaligned access at the processor instruction level >>>> 2. undefined behavior >>>> >>>> From my reading unaligned access is fine on most modern architectures and >>>> it seems the performance penalty has mostly been eliminated. >>>> >>>> Undefined behavior is a compiler/language concept. The problem is the >>>> compiler can choose to do anything in UB scenarios, not just the "obvious" >>>> translation. Specifically, the compiler is under no obligation to >>>> generate the unaligned access instructions, and if it doesn't SEGVs ensue. >>>> Two examples, both of which relate to SIMD optimizations are linked below. >>>> >>>> I tend to be on the conservative side with this type of thing but if we >>>> have experts on the the ML that can offer a more informed opinion, I would >>>> love to hear it. >>>> >>>> [1] http://pzemtsov.github.io/2016/11/06/bug-story-alignment-on-x86.html >>>> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65709 >>>> >>>> On Mon, Jul 1, 2019 at 1:41 PM Wes McKinney <wesmck...@gmail.com> wrote: >>>>> >>>>> The <0xffffffff><int32_t size> solution is downright ugly but I think >>>>> it's one of the only ways that achieves >>>>> >>>>> * backward compatibility (new clients can read old data) >>>>> * opt-in forward compatibility (if we want to go to the labor of doing >>>>> so, sort of dangerous) >>>>> * old clients receiving new data do not blow up (they will see a >>>>> metadata length of -1) >>>>> >>>>> NB 0xFFFFFFFF <length> would look like: >>>>> >>>>> In [13]: np.array([(2 << 32) - 1, 128], dtype=np.uint32) >>>>> Out[13]: array([4294967295, 128], dtype=uint32) >>>>> >>>>> In [14]: np.array([(2 << 32) - 1, 128], >>>>> dtype=np.uint32).view(np.int32) >>>>> Out[14]: array([ -1, 128], dtype=int32) >>>>> >>>>> In [15]: np.array([(2 << 32) - 1, 128], dtype=np.uint32).view(np.uint8) >>>>> Out[15]: array([255, 255, 255, 255, 128, 0, 0, 0], dtype=uint8) >>>>> >>>>> Flatbuffers are 32-bit limited so we don't need all 64 bits. >>>>> >>>>> Do you know in what circumstances unaligned reads from Flatbuffers >>>>> might cause an issue? I do not know enough about UB but my >>>>> understanding is that it causes issues on some specialized platforms >>>>> where for most modern x86-64 processors and compilers it is not really >>>>> an issue (though perhaps a performance issue) >>>>> >>>>> On Sun, Jun 30, 2019 at 6:36 PM Micah Kornfield <emkornfi...@gmail.com> >>>>> wrote: >>>>>> >>>>>> At least on the read-side we can make this detectable by using something >>>>>> like <0xffffffff><int32_t size> instead of int64_t. On the write side >>>>>> we would need some sort of default mode that we could flip on/off if we >>>>>> wanted to maintain compatibility. >>>>>> >>>>>> I should say I think we should fix it. Undefined behavior is unpaid >>>>>> debt that might never be collected or might cause things to fail in >>>>>> difficult to diagnose ways. And pre-1.0.0 is definitely the time. >>>>>> >>>>>> -Micah >>>>>> >>>>>> On Sun, Jun 30, 2019 at 3:17 PM Wes McKinney <wesmck...@gmail.com> wrote: >>>>>>> >>>>>>> On Sun, Jun 30, 2019 at 5:14 PM Wes McKinney <wesmck...@gmail.com> >>>>>>> wrote: >>>>>>>> >>>>>>>> hi Micah, >>>>>>>> >>>>>>>> This is definitely unfortunate, I wish we had realized the potential >>>>>>>> implications of having the Flatbuffer message start on a 4-byte >>>>>>>> (rather than 8-byte) boundary. The cost of making such a change now >>>>>>>> would be pretty high since all readers and writers in all languages >>>>>>>> would have to be changed. That being said, the 0.14.0 -> 1.0.0 version >>>>>>>> bump is the last opportunity we have to make a change like this, so we >>>>>>>> might as well discuss it now. Note that particular implementations >>>>>>>> could implement compatibility functions to handle the 4 to 8 byte >>>>>>>> change so that old clients can still be understood. We'd probably want >>>>>>>> to do this in C++, for example, since users would pretty quickly >>>>>>>> acquire a new pyarrow version in Spark applications while they are >>>>>>>> stuck on an old version of the Java libraries. >>>>>>> >>>>>>> NB such a backwards compatibility fix would not be forward-compatible, >>>>>>> so the PySpark users would need to use a pinned version of pyarrow >>>>>>> until Spark upgraded to Arrow 1.0.0. Maybe that's OK >>>>>>> >>>>>>>> >>>>>>>> - Wes >>>>>>>> >>>>>>>> On Sun, Jun 30, 2019 at 3:01 AM Micah Kornfield >>>>>>>> <emkornfi...@gmail.com> wrote: >>>>>>>>> >>>>>>>>> While working on trying to fix undefined behavior for unaligned memory >>>>>>>>> accesses [1], I ran into an issue with the IPC specification [2] which >>>>>>>>> prevents us from ever achieving zero-copy memory mapping and having >>>>>>>>> aligned >>>>>>>>> accesses (i.e. clean UBSan runs). >>>>>>>>> >>>>>>>>> Flatbuffer metadata needs 8-byte alignment to guarantee aligned >>>>>>>>> accesses. >>>>>>>>> >>>>>>>>> In the IPC format we align each message to 8-byte boundaries. We then >>>>>>>>> write a int32_t integer to to denote the size of flat buffer metadata, >>>>>>>>> followed immediately by the flatbuffer metadata. This means the >>>>>>>>> flatbuffer metadata will never be 8 byte aligned. >>>>>>>>> >>>>>>>>> Do people care? A simple fix would be to use int64_t instead of >>>>>>>>> int32_t >>>>>>>>> for length. However, any fix essentially breaks all previous client >>>>>>>>> library versions or incurs a memory copy. >>>>>>>>> >>>>>>>>> [1] https://github.com/apache/arrow/pull/4757 >>>>>>>>> [2] https://arrow.apache.org/docs/ipc.html
