Re: very slow archive-mode

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> > I regularly open Java source archive (JAR of several tens of megabytes)
>> > in Emacs.  Recently I recompiled Emacs from CVS and noticed that process
>> > of opening and parsing archive had become times slower (didn't measure
>> > precisely, but I guess about 5--10x slowdown is there).  Can anyone
>> > investigate the problem or just guess what changes caused it?  I don't
>> > remember when I compiled Emacs previously, I guess it was a couple month
>> > old.

>> I reported a similar problem.  I believe set-buffer-multibyte is *a lot*
>> slower now, and may even have a time complexity of O(N^2).

> I suspect so too.  Now set-buffer-multibyte must convert
> more 8-bit bytes to mutlibyte forms and that results in more
> movement and increasing of the gap.  This code:

Also, IIUC the 8-bit bytes that are not represented as a single byte and
not only more numerous, but they also take up more space (they used to
take up just 2 bytes but now they take up what 3? 4? 5 bytes?).

For this reason, we should rewrite the set-buffer-multibyte to do things
in two passes: either a first that computes the final size and allocates
the destination and a second that does no re-allocation, or a first that
converts into too-large a destination and a second that shrinks it back
to a reasonable size.


        Stefan