Correct. The encapsulated IPC message will just be 4 bytes bigger. On Tue, Jul 2, 2019, 9:31 AM Antoine Pitrou <anto...@python.org> wrote:
> > 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 >
