[NTG-context] Using ConTeXt-LMTX for modern Mathematically-Literate-Programming 1/2

[Hans Hagen]

On 11/30/2020 10:51 AM, Stephen Gaito wrote:
> Hello,
> 
> I am slowly working on a Mathematical problem requiring underlying
> computation.
> 
> As Mathematicians (myself included) are rather "conservative", I need
> to discuss each "chunk" of code with the full set of Mathematical
> notation.
> 
> A couple of years ago I started using ConTeXt-MKIV as a
> Mathematically-Literate-Programming tool by using its excellent Lua
> interface to capture the code and dump it to disk for external
> compilation.
> 
> I am now revisiting my original design and want to redo my tools using
> ConTeXt-LMTX.
> 
> I would *like* to be able to "stop" the ConTeXt typesetting at various
> points for differing purposes:
> 
> 1. After all macro expansions (and hence after *my* calls into Lua)
>     but before line/paragraph/page layout begins.

maybe something

\startmystuff

\stopmystuff

and then you can hook something into startmystuff and \stopmystuff

> 2. After line/paragraph/page layout but before PDF generation.

pdf is generated per page, if needed one can kick in a shipout overload

but keep in mind that multipass data is flushed as part of the shipout 
(because it is often location and order bound)

> 3. After all PDF generated (ie. a "normal" "full" ConTeXt run).
> 
> Stopping after all macro expansions would allow my code generation
> builds to proceed without the un-needed page setting or PDF generation.

hm, the problem is always in the 'state' of all kind of variables

> Stopping after the line/paragraph/page layout would allow multiple
> "faster(?)" ConTeXt runs while the "*.tuc" file converges to a complete
> set of page numbers and cross references (etc). Then, once the "*.tuc"
> file has converged, a full ConTeXt run with PDF output could be
> done.

not sure what you mean here ... what is fast? or: how slow is it now? 
what is the bottleneck? can you cache data that didn't change?

a large document is normally split up in sections that can be processed 
independent

\starttext
     \dorecurse{10000}{\samplefile{ward}\par}
\stoptext

runs on my 2013 laptop at over 65 pages per second

quite often performance is hit by inefficient styling and such .. it's 
no problem to bring a tex system a grinding halt

> I am very aware that *internally* ConTeXt is probably structured as a
> tight pipeline with each of the "traditional" TeX stages "Mouth",
> "Stomach", "page setting", PDF generation.... tightly "chained"...
> This means that there is no "one" place in the code where all macro
> expansions have completed but before the page setting "starts", or
> similarly, after the page setting has finished but before the PDF
> generation "starts".

yes and often something is left over for a next page so it's kind of fluid

> ----
> QUESTION: Is it possible to use the new LuaMetaTeX callbacks (found in
> chapter 10 of the "LuaMetaTEX Reference Manual") to "suppress" any
> further computation at various points in the ConTeXt pipeline?
> ----

sure, you can kick in handlers at various stages (assuming that you keep 
in mind where you kick them in as there is some order involved)

> For example, could I use one of the "*_linebreak_filter"s (or the
> "append_to_vlist_filter") to "return" an empty value and hence reduce
> further computation downstream in the pipeline?

you can but linebreak is not the most costly one, you probbaly want to 
intercept the list builder but when you do that you can as well do a 
\stoptext which prevents further reading of content (but i probably 
misunderstand)

> Could I use the "pre_output_filter" to "return" an empty value and
> hence "stop" PDF generation?

assuming a properky structured document forcing a \stoptext should work 
in most cases

> (I realize that these callbacks *are* a currently fast moving target. I
> am happy to follow their changes, equally I would be testing their
> usefulness and/or impact)

actually, the callbacks themselves hardly change, but the code plugged 
into them might occasionally (a lot of mkiv code is already quite old so 
i'm now looking at it and see if i can use some recent tricks)

> ALTERNATIVE QUESTION: Would it be possible to provide official
> ConTeXt-LMTX "modes" which suppress further computation at these points?

the question is: what do you want to suppress? best first identify the 
bottleneck and then figure out what can be skipped (as mentioned: 
multipass data can be made more independent I guess but it still demands 
some calculations and analyzing and it's that bit that takes the time)

> This alternative, while some more work for the writing of
> ConTeXt-LMTX, would ensure less direct external dependence on the
> LuaMetaTeX callbacks, but would almost certainly be welcomed by the
> ConTeXt community.

i need more info (also from others then) about what the reason, goal and 
possible gain is

- tex: the context code is quite efficient, and tex is quite fast, so 
there's little to gain there (but as said one can write slow macros that 
spoil that game)

- lua: on the average lua is fast but garbage collection can be of 
influence (i need to see code in order to be able to tell if there is a 
gain there); the lua code in context is quite ok but for instance 
messing with node lists will always come at a cost (crossing the c 
boundary and such)

- pdf: the backend code in luametatex is somewhat slower than in luatex 
but we're gaining there (because in related areas we can do things 
different, although there is new functionality that when used also comes 
  at a price); but as far as i can tell a luametatex run here is on the 
average some 20% faster than a luatex run so the pdf generation slowdown 
gets kind of obscured by it

> ----
> QUESTION: Are the "stages" I have identified major, computationally
> expensive, "steps" in the overall ConTeXt "computation"?
> ----

basic typesetting (hyphenation, font handling): takes a bit of time, 
extra features that you use add some too: some timings are reported 
after a run so you get an idea

par building: unles hz is used, quite fast

page building: fast but depending on what features are enables 
finalizing the page can take some time

expansion: pretty fast on the average

summary: try to identify where the bottlenecks are

you can run with

   \enabletrackers[pages.timing]

(put it on cont-loc.mkxl somewhere in in texmf-local) and get timings 
per page (i have that enabled on my machine)

Hans


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