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WAR STORY: When turning to the profiler turns out to be a good call

Some of you might have seen the little bit of news circulating around about my new project. It's called Elsa (stands for Emacs Lisp Static Analyzer), which is exactly what you would expect: you throw it some Elisp code and it analyzes the types, dead code, style and all sorts of other rules (stars are the limit!).

I've been working on this "secretly" in public (the repo was on Github from day 1.) for about a year and very early on I've made a couple decisions that I now thought doomed me to failure. These were:

For this kind of a problem where we are walking some recursive structure and then doing some "arithmetics" on hierarchies of types, classes and methods are a very natural choice. You can write generic implementations and then "pattern match" on specific classes to provide more elaborate strategies.

These two were notoriously known for having very bad performance in the past, but nonetheless I decided that the simplicity of the implementation has higher priority for me now and that in the worst case I can transform it into a manual-dispatch-conds-and-defuns later.

EIEIO in particular was also known to be quite undebuggable. They did a good job on it with the 25 release and the error messages are now pretty helpful. It's no worse than the rest of Emacs (M-x faint-praise).

When I finally finished the reader module which turns the source file into a tree of EIEIO objects, to my horror I found out that it was indeed slow. Way slow. I was using smartparens.el as a benchmark: it's a single lisp file package of about 10000 lines with enough complexity to inhibit most of the features of Elisp.

It took 42 seconds to analyze. That is not great when you want to use it with something like flycheck. So I'm pretty much thinking I'm screwed at this point and begin thinking about how to get rid of the generics, because obviously that must be the issue, right?

Oh, how wrong I was. Finally in a stroke of genius I fired up an emacs -nw -q, loaded all the necessary things and did M-x profiler-start.

Then I run the analysis in that instance, get the report and get my mind blown to pieces: the culprit was the little innocent looking function line-number-at-pos. The reader spent about 90% of all the time inside that function.

What happened? Every time I read any form I need to remember the line and column it was encountered at for error reporting to the user later. So every time I read anything (a symbol, a list, a keyword, a string) I would call line-number-at-pos with the token's start position to get the line number.

What this function does is it jumps to the first line of the file, then regexp searches for \n until it reaches the point from which it was called.

So you can imagine, in your source code on line 8500, a form like

(very innocent looking form at line 8500)

would trigger 8 calls to line-number-at-pos which would in turn do 68000 regexp searches for \n. The total number of these went way up in the lower hundreds of millions, somewhere around the marks of 400 to 500 million calls. Now that's a lot of completely useless computation.

So what I did to fix this? At the beginning of the analysis I run through all the lines and place a text property elsa-line on the first character with the line number.

(goto-char (point-min))
(let ((line 1))
  (put-text-property (point) (1+ (point)) 'elsa-line line)
  (while (= (forward-line) 0)
    (cl-incf line)
    (put-text-property (point) (min
                                (buffer-size)
                                (1+ (point))) 'elsa-line line)))

Then to get the line number all I need to do is

(get-text-property (line-beginning-position) 'elsa-line)

Because the text properties use efficient data structures this access is really really fast (something like O(log n) in length of buffer).

With this little change, the analysis time dropped to 5 seconds, almost a factor of 10! So the moral of the story is, as has been repeated to death, when in trouble always bring out the big guns: profilers are our friends! No way in the world I would've guessed this function was responsible (I actually started changing a bunch of code into defsubst and inlining code before I figured that's completely bonkers).

On a closing note, I'm looking for people to help out on the project, there is a lot of things for people of all skill ranging from beginners to seasoned hackers. Check out issues marked help wanted or good first issue and get involved! If anything's unclear go ahead and ask questions.


Last updated at: 2018-08-12 12:48
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