bug#31138: Native json slower than json.el

[Dmitry Gutov]

On 21.04.2019 12:31, Eli Zaretskii wrote:

> > Eli, how did you reach that conclusion?
>
> By adding the percentage of all the functions involved in decoding
> strings.

Was the total sum of the profile 100%?

> > One: looking at the profile, libjansson is somewhere down below, and
> > most of the runtime is taken by Emacs.
>
> I only accounted for time used by Emacs code, i.e. 100% doesn't
> include the library.

I still don't understand. I my profiling report, decode_coding_utf_8 is 
at the top (Emacs function). Followed by malloc and produce_chars. Are 
you saying they are not involved in decoding strings?

> > Two: while I might be misreading the profile, simply omitting the
> > decoding function call (and using the returned strings as-is) halves the
> > execution time.
>
> You cannot use the returned string as is, you must make a Lisp string
> out of it.

Yes. There's a patch that makes a Lisp string without string additional 
decoding, and it makes the runtime 2x faster. It's been posted in this 
report. Would you like me to point at it?

> In any case, even if decoding takes 50% of the time we spend in Emacs
> code, it is still not significant enough to justify the un-safety of
> using a string that we didn't decode, because if that string ever
> includes raw bytes, Emacs will surely crash.

What was the point of us doing the exercise, and me wasting time on it, 
if you're set on not changing anything at all?

We've already had the performance numbers from before.

Given the encouragement, I really expected you to choose *some* path 
toward improvement. Either forgoing conversion (maybe adding an 
additional test suite, for cases you're worrying about), or doing some 
kind of faster validation instead of full conversion (where we don't 
allocate new strings but just check existing ones for validity), or, you 
know, optimizing everything everywhere.

Considering libjansson manages to do both JSON parsing and string 
conversion in ~the same time make_specified_string only does string 
conversion on the returned strings, it most likely follows that 
make_specified_string could be made faster.