[Numpy-discussion] Views of memmaps and offset
Sat Sep 22 17:20:02 CDT 2012
On Sat, Sep 22, 2012 at 12:19:53PM -0600, Charles R Harris wrote:
> But you loose the pointer to the filename and the offset. In previous
> versions of numpy c.base used to be the np.memmap instance from which
> c is an array view. That allowed to make efficient pickling without
> any memory copy when doing single machine multiprocessing stuff by
> introspecting the base ancestry.
> This is no longer possible with the current base collapsing that is
> happening in numpy master. The only way would be to replace the
> mmap.mmap instance of a numpy.memmap object by a buffer implementation
> that would wrap or derive from mmap.mmap but also preserve the
> original filename and offset.
> Pickling was left as an unresolved problem after to offset updates to
To be clear, the issue here is not really a pickling issue where pickling
is a general purpose persistence model, but rather a specific and
focussed I/O problem. We don't want to swap out to disk and load back an
array that is a view on the disk.
With the current numpy, we can tell that it is indeed a view on the disk,
and we can tell what offset and strides, but we cannot tell from which
file it comes from, because that information is lost when deriving
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