[Python-Dev] Prefetching on buffered IO files

[Antoine Pitrou]

Hello,

While trying to solve #3873 (poor performance of pickle on file
objects, due to the overhead of calling read() with very small values),
it occurred to me that the prefetching facilities offered by
BufferedIOBase are not flexible and efficient enough.

Indeed, if you use seek() and read(), 1) you limit yourself to seekable
files 2) performance can be hampered by very bad seek() performance
(this is true on GzipFile).

If instead you use peek() and read(), the situation is better, but you
end up doing multiple copies of data; also, you must call read() to
advance the file pointer even though you don't care about the results.

So I would propose adding the following method to BufferedIOBase:

prefetch(self, buffer, skip, minread)

Skip `skip` bytes from the stream.  Then, try to read at
least `minread` bytes and write them into `buffer`. The file
pointer is advanced by at most `skip + minread`, or less if
the end of file was reached. The total number of bytes written
in `buffer` is returned, which can be more than `minread`
if additional bytes could be prefetched (but, of course,
cannot be more than `len(buffer)`).

Arguments:
- `buffer`: a writable buffer (e.g. bytearray)
- `skip`: number of bytes to skip (must be >= 0)
- `minread`: number of bytes to read (must be >= 0 and <= len(buffer))


Also, the BufferedIOBase ABC can then provide default implementations of
read(), readinto() and peek(), simply by calling prefetch().
(how read1() can fit into the picture is not obvious)

What do you think?

Regards

Antoine.