Tornado: Don't use BytesIO to make interface of read-only data file-like

[kmike]

Hi,

When doing https://github.com/tornadoweb/tornado/pull/1109 and checking https://github.com/scrapy/scrapy/pull/803 I've run a couple of benchmarks:

• http://nbviewer.ipython.org/gist/kmike/b23a3b12fac6654b1c90 (Python 3.4)
• http://nbviewer.ipython.org/gist/kmike/4e79964c1d589faef174 (Python 2.7)

The problem is that BytesIO copies the data, so using it just to make interface file-like for readonly data could be quite wasteful.

I believe this is a problem in curl_httpclient, simple_httpclient and wsgi tornado modules, and by using another wrapper (or at least by falling back to cStringIO in Python 2.x) they can be made faster and more memory efficient.

[bdarnell]

It's unfortunate that BytesIO is so slow. We have designed parts of the API around the untested assumption that StringIO or BytesIO can be faster because they don't have to allocate a contiguous block of memory until getvalue(). If that's not true and it is in fact slower, we should probably make HTTPResponse.body a regular variable and make HTTPResponse.buffer the lazy property.

[kmike]

BytesIO uses PyMem_Realloc / memset / memcpy to copy constructor argument contents to the internal buffer; it doesn't store a reference to original data. In pure-Python version the code is

def __init__(self, initial_bytes=None):
    buf = bytearray()
    if initial_bytes is not None:
        buf += initial_bytes
    self._buffer = buf
    self._pos = 0

[kmike]

See also: http://bugs.python.org/issue22003

[greedo]

It looks like this issue has been resolved
http://hg.python.org/cpython/rev/79a5fbe2c78f

io.BytesIO() now defers making a copy until it is mutated

[kmike]

Yes, kudos to David Wilson!

It is likely the fix will be in Python 3.5 (~ November 2015?), but until it is released the issue affects all Python 3.x versions and all Python 2.x versions. It seems the fix won't be in 3.4.x.

[bdarnell]

Support for python 3.4 and older versions has been dropped, so this issue is moot.