Re: warn-maybe-out-of-memory

[Dmitry Antipov <dmantipov@yandex.ru>]

On 07/11/2014 10:50 AM, Eli Zaretskii wrote:

> But if this warning is to be useful, it should catch the majority of
> the cases, otherwise we are just wasting cycles.  With the current
> test, many cases of visiting files that cannot be accommodated will go
> undetected, thus rendering all this feature useless.

With this feature, I'm just trying to redirect user to `find-file-literally'
if file looks so large that `find-file-noselect' is likely to run out of
memory.  What other cases do you mean?

> Since it's only a warning that doesn't prevent visiting a file, it is
> better to err on the other side of the truth.  E.g., apply some
> heuristics as to the average expansion factor, perhaps dependent on
> the locale.

IIUC there are no reliable heuristics here.  For example, produce_charset
and compose_text may allocate substantial amount of memory to represent
`composition' and `charset' extra properties, and we need to read and
decode everything to find all places where such an extra properties
should be attached.

> And don't forget that even for plain-ASCII files we
> allocate the gap in addition to the text itself, so the mere size of
> the file is simply _never_ the correct value, it is always an
> underestimation.  IOW, this test is always biased towards lower
> values.

Unless someone still uses a machine with 64K RAM, initial gap size
is very small in comparison with file sizes we're talking about.

> If we don't do something like that, then I wonder what exactly is the
> use case that benefits from this warning?

We don't need to do "something like that", i.e. we don't need a precise
estimation.  OSes are tricky about memory management; overall VM picture
may drastically change when file reading is in progress, etc.  With a lot
of constraints you can't predict, estimate and control, precise estimation
just makes no sense.

> It is IMO inappropriate for a minor feature like this one to abort
> Emacs.  Who knows what the Linux kernel developers will introduce
> tomorrow that could possibly fail the function?  It is none of this
> feature's business to be the place where C stack smashing is detected.
> So my vote is for returning nil and perhaps displaying a warning.

OK.

Dmitry