Why does Tex/Latex not speed up in subsequent runs?

Question

I really wonder, why even recent systems of Tex/Latex do not use any caching to speed up later runs. Every time that I fix a single comma*, calling Latex costs me about the same amount of time, because it needs to load and convert every single picture file.

(* I know that even changing a tiny comma could affect the whole structure but of course, a well-written cache format could see the impact of that. Also, there might be situations where 100% correctness is not needed as long as it’s fast.)

Is there something in the language of Tex which makes this complicated or impossible to accomplish or is it just that in the original implementation of Tex, there was no need for this (because it would have been slow anyway on those large computers)?

But then on the other hand, why doesn’t this annoy other people so much that they’ve started a fork which has some sort of caching (or transparent conversion of Tex files to a format which is faster to parse)?

Is there anything I can do to speed up subsequent runs of Latex? Except from putting all the stuff into chapterXX.tex files and then commenting them out?

Answer 1

There are solutions such as preview-latex, which pre-compile stuff into a dedicated format file for speed purposes. You need to remember that TeX optimises pages on a local basis. There is no concpet at the engine level of material being fixed on a particular page, so you can't just "re-TeX one page".

Answer 2

If you're using pdftex, then you can use --draftmode on the command line for the first runs. This instructs pdftex not to generate a PDF.

Of course lots of things could be cached (like graphics information, for instance), but the way TeX works makes it hard to do. There is a rather complex initialization of TeX when it starts up, and one TeX run always means exactly one PDF written out. In order to do caching, you need to keep the data in memory (to be efficient).

You could use IPC and talk to a daemon to get the cached information. But that would involve lots programming. TeX is for normal purposes so blazingly fast, that this does not really gain a lot. But on the other hand, this is a good question, as I have seen LaTeX runs (on currend hardware) that run > 10 hours that would have benefited from caching.

Answer 3

Actually, the correct answer is (IMO): LaTeX already caches information in its output file (.aux, additional files for other packages). So if you add a comma, this information is reused and thus the typeset run is much faster then without this .aux file.

Answer 4

Let's try to understand how TeX works. What happens when you write the following?

tex.exe myfile.tex

TeX reads your file byte by byte. First of all, TeX converts each char to pair <category, ascii-code>. Each character has category code and ascii code. Category code means that the character is an opening brace ({) or entrance into the mathematical mode ($), symbol-macro (~, for example) or letter (A-Z,a-z).

If TeX gets chars with category code 11 (letters) or 12 (other symbols: digits, comma, period) TeX starts a paragraph. You want to cache all paragraphes.

Suppose you changed something in your document. How can TeX check that all paragraphes after your changes is the same? May be you changed the category of some char. Me be you changed the meaning of some macro. Or you have removed } somewhere and thus changed the current font.

To be sure that the parahraph is the same you must be sure that all charachters in the paragraph is the same, that all caharacter catogories is the same, the current font is the same, all math fonts is the same, and the value of some internal variables is the same (for example, \hsize, \vsize, \pretolerance, \tolerance, \hypenpenalty, exhyphenpenalty, \widowpenalty, \spaceskip, ..., ........)

You can be sure only that all paragraphes before your changes is the same. But in this case you must keep all states after each paragraph.

Your system SuperCachedTeX is very complicated. Isn't it?

Answer 5

Yet another answer, not strictly related:

You can use the LaTeX macro \include{...} and with \includeonly{} you can rerun your document for a subset only. But this is not caching, nor does it give you the complete document.

Answer 6

Tex does have a caching facility, named format files, and I think, pace Alexey's valuable summary of the problems representing Tex's state, it should be possible to use them to allow resumption of editing after any page eject.

The major issue is that pagebreaks will affect paragraphs or floats, and these may not occur at a particular point in the text, but may be occur in the execution of macros that were invoked dependent on the transient state passed to them when they were invoked.

So to make the idea of creating "breakpoints" work, one would need to hack Tex internals to dump additional information, beyond that normaally dumped in format files, and package them up with the state of the auxiliary files. Given what Joseph says about Tex fragment previewers, why would anyone bother hacking Tex to do this?