ikiwiki/doc/todo/should_optimise_pagespecs.mdwn

I think there is a problem in my "dependency graph". As an example, 
[here](http://poivron.org/~nil/misc/ikiwiki_buggy_index) is the index 
ikiwiki generated for [my site](http://poivron.org/~nil/misc/ikiwiki_buggy_index)
(note that the site changed since this index was generated).

Some **HUGE** dependencies appear, clearly non optimal, like

    depends = A｜ B | A | C | A | D | A | E | A | F | A | G | ....

or 

    depends= A | B | C | D | A | B | C | D | A | B | C | D | ....

Couldn't isolate the cause, but some sources for this problem may be:

* related to the img module
* easily observable in my sire because one of my pages includes 80 resized images

Other special things in my templates and site:

* a sidebar with \[[!include pages="notes/\*" template=foo]] while notes.mdwn has 
  a \[[!include pages="notes/*"]] and uses the sidebar; removed it, doesn't change
* a template (biblio.tmpl) calling the "img" plugin with a template parameter as the
  image filename; removed it, doesn't change
* some strange games with tags whose page calls a "map" directive to show other tags
  shile tags are also used in tagclouds (in the sidebar and in the main pages)
* ...

I observed these problems (same *kind*, I didn't check in details) on
 
* ikiwiki 2.00gpa1 + v5.8.4 + Debian 3.1
* ikiwiki 2.3 + v5.8.8 + Ubuntu 7.04

I can think about reducung the size of my wiki source and making it available online for analysis.

-- NicolasLimare

> As long as these dependencies don't grow over time (ie, when a page is
> edited and nothing changed that should add a dependency), I wouldn't
> worry about them. There are many things that can cause non-optimal
> dependencies to be recorded. For one thing, if you inline something, ikiwiki
> creates a dependency like:
> 
> (PageSpec) or (file1 or file2 or file3 ...)
> 
> Where fileN are all the files that the PageSpec currently matches. (This
> is ncessary to detect when a currently inlined file is deleted, and know
> the inlining page needs an update.) Now consider what it does if you have
> a single page with two inline statements, that inline the same set of
> stuff twice:
> 
> ((PageSpec) or (file1 or file2 or file3 ...) or (PageSpec) or (file1 or file2 or file3 ...)
>
> Clearly non-optimal, indeed.
> 
> Ikiwiki doesn't bother to simplify complex PageSpecs
> because it's difficult to do, and because all they use is some disk
> space. Consider what ikiwiki uses these dependencies for.
> All it wants to know is: does the PageSpec for this page it's considering
> rebuilding match any of the pages that have changed? Determining this is
> a simple operation -- the PageSpec is converted to perl code. The perl
> code is run.
> 
> So the total impact of an ugly dependency like this is:
> 
> 1. Some extra data read/written to disk.
> 2. Some extra space in memory.
> 3. A bit more data for the PageSpec translation code to handle. But that
>    code is quite fast.
> 4. Typically one extra function call when the generated perl code is run.
>    Ie, when the expression on the left-hand side fails, which typically
>    happens after one (inexpensive) function call, it has to check
>    the identical expression on the right hand side.
> 
> So this is at best a wishlist todo item, not a bug. A PageSpec simplifier
> (or improved `pagespec_merge()` function) could be written and improve
> ikiwiki's memory and disk usage, but would it actually speed it up any?
> We'd have to see the code to the simplifier to know.
> 
> --[[Joey]]

>> I've been looking at optimizing ikiwiki for a site using
>> [[plugins/contrib/album]] (which produces a lot of pages) and it seems
>> that checking which pages depend on which pages does take a significant
>> amount of time. The optimize-depends branch in my git repository
>> avoids using `pagespec_merge()` for this (indeed it's no longer used
>> at all), and instead represents dependencies as a list of pagespecs
>> rather than a single pagespec. This does turn out to be faster, although
>> not as much as I'd like. --[[smcv]]

>>> I just wanted to note that there is a whole long discussion of dependencies and pagespecs on the [[todo/tracking_bugs_with_dependencies]] page. -- [[Will]]

>>>> Yeah, I had a look at that (as the only other mention of `pagespec_merge`).
>>>> I think I might have solved some of the problems mentioned there,
>>>> actually - `pagespec_merge` no longer needs to exist in my branch (although
>>>> I haven't actually deleted it), because the "or" operation is now done in
>>>> the Perl code, rather than by merging pagespecs and translating. --[[smcv]]

[[!tag wishlist patch patch/core]]