[Python-ideas] Support parsing stream with `re`

Erik Bray erik.m.bray at gmail.com
Mon Oct 8 08:02:05 EDT 2018

On Mon, Oct 8, 2018 at 12:20 PM Cameron Simpson <cs at cskk.id.au> wrote:
> On 08Oct2018 10:56, Ram Rachum <ram at rachum.com> wrote:
> >That's incredibly interesting. I've never used mmap before.
> >However, there's a problem.
> >I did a few experiments with mmap now, this is the latest:
> >
> >path = pathlib.Path(r'P:\huge_file')
> >
> >with path.open('r') as file:
> >    mmap = mmap.mmap(file.fileno(), 0, access=mmap.ACCESS_READ)
> Just a remark: don't tromp on the "mmap" name. Maybe "mapped"?
> >    for match in re.finditer(b'.', mmap):
> >        pass
> >
> >The file is 338GB in size, and it seems that Python is trying to load it
> >into memory. The process is now taking 4GB RAM and it's growing. I saw the
> >same behavior when searching for a non-existing match.
> >
> >Should I open a Python bug for this?
> Probably not. First figure out what is going on. BTW, how much RAM have you
> got?
> As you access the mapped file the OS will try to keep it in memory in case you
> need that again. In the absense of competition, most stuff will get paged out
> to accomodate it. That's normal. All the data are "clean" (unmodified) so the
> OS can simply release the older pages instantly if something else needs the
> RAM.
> However, another possibility is the the regexp is consuming lots of memory.
> The regexp seems simple enough (b'.'), so I doubt it is leaking memory like
> mad; I'm guessing you're just seeing the OS page in as much of the file as it
> can.

Yup. Windows will aggressively fill up your RAM in cases like this
because after all why not?  There's no use to having memory just
sitting around unused.  For read-only, non-anonymous mappings it's not
much problem for the OS to drop pages that haven't been recently
accessed and use them for something else.  So I wouldn't be too
worried about the process chewing up RAM.

I feel like this is veering more into python-list territory for
further discussion though.

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