OK. Can you open a JIRA about fixing this? I don't recall the rationale for using MAP_PRIVATE to begin with, and since the behavior is unspecified on Linux it would be better to be consistent across platforms
On Wed, May 22, 2019 at 11:02 PM John Muehlhausen <j...@jgm.org> wrote: > > Well, it works fine on Linux... and the Linux mmap man page seems to > indicate you are right about MAP_PRIVATE: > > "It is unspecified whether changes made to the file after the mmap() call > are visible in the mapped region." > > The Mac man page has no such note. > > Changing it to MAP_SHARED makes it work as expected on MacOS. Still odd > that the changes are only sometimes visible ... but I guess that is > compatible with it being "unspecified." > > -John > > On Wed, May 22, 2019 at 8:56 PM John Muehlhausen <j...@jgm.org> wrote: > > > I'll mess with this on various platforms and report back. Thanks > > > > On Wed, May 22, 2019 at 8:42 PM Wes McKinney <wesmck...@gmail.com> wrote: > > > >> I tried locally and am not seeing this behavior > >> > >> In [10]: source = pa.memory_map('/tmp/test.batch') > >> > >> In [11]: reader=pa.ipc.open_stream(source) > >> > >> In [12]: batch = reader.get_next_batch() > >> /home/wesm/miniconda/envs/arrow-3.7/bin/ipython:1: FutureWarning: > >> Please use read_next_batch instead of get_next_batch > >> #!/home/wesm/miniconda/envs/arrow-3.7/bin/python > >> > >> In [13]: batch.to_pandas() > >> Out[13]: > >> field1 > >> 0 1.0 > >> 1 NaN > >> > >> Now ran dd to overwrite the file contents > >> > >> In [14]: batch.to_pandas() > >> Out[14]: > >> field1 > >> 0 NaN > >> 1 -245785081.0 > >> > >> On Wed, May 22, 2019 at 8:34 PM John Muehlhausen <j...@jgm.org> wrote: > >> > > >> > I don't think that is it. I changed my mmap to MAP_PRIVATE in the first > >> > raw mmap test and the dd changes are still visible. I also changed to > >> > storing the stream format instead of the file format and got the same > >> > result. > >> > > >> > Where is the code that constructs a buffer/array by pointing it into the > >> > mmap space instead of by allocating space? Sorry I'm so confused about > >> > this, I just don't see how it is supposed to work. > >> > > >> > On Wed, May 22, 2019 at 7:58 PM Wes McKinney <wesmck...@gmail.com> > >> wrote: > >> > > >> > > It seems this could be due to our use of MAP_PRIVATE for read-only > >> memory > >> > > maps > >> > > > >> > > > >> https://github.com/apache/arrow/blob/master/cpp/src/arrow/io/file.cc#L393 > >> > > > >> > > Some more investigation would be required > >> > > > >> > > On Wed, May 22, 2019 at 7:43 PM John Muehlhausen <j...@jgm.org> wrote: > >> > > > > >> > > > Is there an example somewhere of referring to the RecordBatch data > >> in a > >> > > memory-mapped IPC File in a zero-copy manner? > >> > > > > >> > > > I tried to do this in Python and must be doing something wrong. (I > >> > > don't really care whether the example is Python or C++) > >> > > > > >> > > > In the attached test, when I get to the first prompt and hit > >> return, I > >> > > get the same content again. Likewise when I hit return on the second > >> > > prompt I get the same content again. > >> > > > > >> > > > However, if before hitting return on the first prompt I issue: > >> > > > > >> > > > dd conv=notrunc if=/dev/urandom of=/tmp/test.batch bs=478 count=1 > >> > > > > >> > > > > >> > > > i.e. overwrite the contents of the file, I get a garbled result. > >> > > (Replace 478 with the size of your file.) > >> > > > > >> > > > However, if I wait until the second prompt to issue the dd command > >> > > before hitting return, I do not get an error. Instead, > >> batch.to_pandas() > >> > > works the same both before and after the data is overwritten. This > >> was not > >> > > expected as I thought that the batch object was looking at the file > >> > > in-place, i.e. zero-copy? > >> > > > > >> > > > Am I tying together the memory-mapping and the batch construction > >> in the > >> > > wrong way? > >> > > > > >> > > > Thanks, > >> > > > John > >> > > > >> > >