% language=us

\startcomponent about-speed

\environment about-environment

\startchapter[title=Speed]

\startsection[title=Introduction]

In the \quote {mk} and \type {hybrid} progress reports I have spend some words
on speed. Why is speed this important?

In the early days of \CONTEXT\ I often had to process documents with thousands of
pages and hundreds of thousands of hyperlinks. You can imagine that this took a
while, especially when all kind of ornaments had to be added to the page:
backgrounds, buttons with their own backgrounds and offsets, hyperlink colors
dependent on their state, etc. Given that multiple runs were needed, this could
mean that you'd leave the machine running all night in order to get the final
document.

It was the time when computers got twice the speed with each iteration of
hardware, so I suppose that it would run substantially faster on my current
laptop, an old Dell M90 workhorse. Of course a recently added SSD drive adds a
boost as well. But still, processing such documents on a machine with a 8Mhz 286
processor and 640 megabytes of memory was close to impossible. But, when I
compare the speed of core duo M90 with for instance an M4600 with a i5 \CPU\
running the same clock speed as the M90, I see a factor 2 improvement at most. Of
course going for a extremely clocked desktop will be much faster, but we're no
longer seeing a tenfold speedup every few years. On the contrary: we see a shift
multiple cores, often running at a lower clock speed, with the assumption that
threaded applications are used. This scales perfectly for web services and
graphic manipulations but not so much for \TEX. If we want go faster, we need to
see where we can be more efficient within more or less frozen clock speeds.

Of course there are some developments that help us. First of all, for programs
like \TEX\ clever caching of files by the operating system helps a lot. Memory
still becomes faster and \CPU\ cached become larger too. For large documents with
lots of resources an SSD works out great. As \LUA\ uses floating point, speedup
in that area also help with \LUATEX. We use virtual machines for \TEX\ related
services and for some reason that works out quite well, as the underlying
operating system does lots of housekeeping in parallel. But, with all maxing out,
we finally end up at the software itself, and in \TEX\ this boils down to a core
of compiled code along with lots of macro expansions and interpret \LUA\ code.

In the end, the question remains what causes excessive runtimes. Is it the nature
of the \TEX\ expansion engine? Is it bad macro writing? Is there too much
overhead? If you notice how fast processing the \TEX\ book goes on modern
hardware it is clear that the core engine is not the problem. It's no big deal to
get 100 pages per second on documents that use relative a simple page builder and
have macros that lack a flexible user interface.

Take the following example:

\starttyping
\starttext
\dorecurse{1000}{test\page}
\stoptext
\stoptyping

We do nothing special here. We use the default Latin Modern fonts and process
single words. No burden is put on the pagebuilder either. This way we get on a
2.33 Ghz T7600 \CPU\ a performance of 185 pages per second. \footnote {In this
case the mingw version was used. A version using the native \WINDOWS\ compiler
runs somewhat faster, although this depends on the compiler options. \footnote
{We've noticed that sometimes the mingw binaries are faster than native binaries,
but sometimes they're slower.} With \LUAJITTEX\ the 185 pages per second become
becomes 195 on a 1000 page document.} The estimated \LUA\ overhead in this 1000
page document is some 1.5 to 2 seconds. The following table shows the performance
on such a test document with different page numbers in pps (reported pages per
second).

\starttabulate[|r|r|]
\HL
\NC \bf \# pages \NC \bf pps \NC \NR
\HL
\NC     1 \NC   2  \NC \NR
\NC    10 \NC  15 \NC \NR
\NC   100 \NC  90 \NC \NR
\NC  1000 \NC 185 \NC \NR
\NC 10000 \NC 215 \NC \NR
\HL
\stoptabulate

The startup time, measured on a zero page document, is 0.5 seconds. This includes
loading the format, loading the embedded \LUA\ scripts and initializing them,
initializing and loading the file database, locating and loading some runtime
files and loading the absolute minumum number of fonts: a regular and math Latin
Modern. A few years before this writing that was more than a second, and the gain
is due to a slightly faster \LUA\ interpreter as well as improvements in
\CONTEXT.

So why does this matter at all, if on a larger document the startup time can be
neglected? It does because when I have to implement a style for a project or are
developing some functionality a fast edit||run||preview cycle is a must, if only
because even a few second wait feels uncomfortable. On the other hand, when I
process a manual of say 150 pages, which uses some tricks to explain matters, I
don't care if the processing rate is between 5 and 15 pages per second, simply
because you get (done) what you asked for. It mostly has to do with feeling
comfortable.

There is one thing to keep in mind: such measurements can vary over time, as they
depend on several factors. Even in the trivial case we need to:

\startitemize[packed]
\startitem
    load macros and \LUA\ code
\stopitem
\startitem
    load additional files
\stopitem
\startitem
    initialize the system, think of fonts and languages
\stopitem
\startitem
    package the pages, which includes reverting to global document states
\stopitem
\startitem
    create the final output stream (\PDF)
\stopitem
\stopitemize

The simple one word per page test is not that slow, and normally for 1000 pages we
measure around 200 pps. However, due to some small speedups (that somehow add up)
in three months time I could gain a lot:

\starttabulate[|r|r|r|r|]
\HL
\NC \bf \# pages \NC \bf Januari \NC \bf April \NC \bf May\rlap{\quad(2013)} \NR
\HL
\NC     1 \NC   2 \NC   2 \NC   2 \NC \NR
\NC    10 \NC  15 \NC  17 \NC  17 \NC \NR
\NC   100 \NC  90 \NC 109 \NC 110 \NC \NR
\NC  1000 \NC 185 \NC 234 \NC 259 \NC \NR
\NC 10000 \NC 215 \NC 258 \NC 289 \NC \NR
\HL
\stoptabulate

Among the improvements in April were a faster output to the console (first
prototyped in \LUA, later done in the \LUATEX\ engine itself), and a couple of
low level \LUA\ optimizations. In May a dirty (maybe too tricky) global document
state restore trick has beeing introduced. Although these changes give nice speed
bump, they will mostly go unnoticed in more realistic documents. There we are
happy if we end up in the 20 pps range. So, in practice a more than 10 percent
speedup between Januari and April is just a dream. \footnote {If you wonder why I
still bother with such things: sometimes speedups are just a side effect of
trying to accomplish something else, like less verbose output in full tracing
mode.}

There are many cases where it does matter to squeeze out every second possible.
We run workflows where some six documents are generated from one source. If we
forget about the initial overhead of fetching the source from a remote server
\footnote {In the user interface we report the time it takes to fetch the source
so that the typesetter can't be blamed for delays.} gaining half a second per
document (if we start frech each needs two runs at least) means that the user
will see the first result one second faster and have them all in six less than
before. In that case it makes sense to identify bottlenecks in the more high
level mechanisms.

And this is why during the development of \CONTEXT\ and the transition from
\MKII\ to \MKIV\ quite some time has been spent on avoiding bottlenecks. And, at
this point we can safely conclude that, in spite of more advanced functionality,
the current version of \MKIV\ runs faster than the \MKII\ versions in most cases,
especially if you take the additional functionality into account (like \UNICODE\
input and fonts).

\stopsection

\startsection[title=The \TEX\ engine]

Writing inefficient macros is not that hard. If they are used only a few times,
for instance in setting up properties it plays no role. But if they're expanded
many times it may make a difference. Because use and development of \CONTEXT\
went hand in hand we always made sure that the overhead was kept at a minimum.

\startsubject[title=The parbuilder]

There are a couple of places where document processing in a traditional \TEX\
engine gets a performance hit. Let's start with the parbuilder. Although the
paragraph builder is quite fast it can responsible for a decent amount of runtime.
It is also a fact that the parbuilder of the engines derived from original \TEX\
are more complex. For instance, \OMEGA\ adds bidirectionality to the picture
which involves some extra checking as well as more nodes in the list. The \PDFTEX\
engine provides protrusion and expansions, and as that feature was primarily a
topic of research it was never optimized.

In \LUATEX\ the parbuilder is a mixture of the \PDFTEX\ and \OMEGA\ builders and
adapted to the fact that we have split the hyphenation, ligature building,
kerning and breaking a paragraph into lines. The protrusion and expansion code is
still there but already for a few years I have alternative code for \LUATEX\ that
simplifies the implementation and could in principle give a speed boost as well
but till now we never found time to adapt the engine. Take the following test code:

\ifdefined\tufte \else \let\tufte\relax \fi

\starttyping
\testfeatureonce{100}{\setbox0\hbox{\tufte \par}} \tufte \par
\stoptyping

In \MKIV\ we use \LUA\ for doing fonts so when we measure this bit we get the
used time for typesetting our \type {\tufte} quote without breaking it into
lines. A normal \LUATEX\ run needs 0.80 seconds and a \LUAJITTEX\ run takes 0.47
seconds. \footnote {All measurements are on a Dell M90 laptop running Windows 8.
I keep using this machine because it has a decent high res 4:3 screen. It's the
same machine Luigi Scarso and I used when experimenting with \LUAJITTEX.}

\starttyping
\testfeatureonce{100}{\setbox0\vbox{\tufte \par}} \tufte \par
\stoptyping

In this case \LUATEX\ needs 0.80 seconds and \LUAJITTEX\ needs 0.50 seconds and
as we break the list into lines, we can deduct that close to zero seconds are
needed to break 100 samples. This (often used) sample text has the interesting
property that it has many hyphenation points and always gives multiple hyphenated
lines. So, the parbuilder, if no protrusion and expansion are used, is real fast!

\starttyping
\startparbuilder[basic]
  \testfeatureonce{100}{\setbox0\vbox{\tufte \par}} \tufte \par
\stopparbuilder
\stoptyping

Here we kick in our \LUA\ version of the par builder. This takes 1.50 seconds for
\LUATEX\ and 0.90 seconds for \LUAJITTEX. So, \LUATEX\ needs 0.70 seconds to
break the quote into lines while \LUAJITTEX\ needs 0.43. If we stick to stock
\LUATEX, this means that a medium complex paragraph needs 0.007 seconds of \LUA\
time and this is not that is not a time to be worried about. Of course these
numbers are not that accurate but the measurements are consistent over multiple
runs for a specific combination of \LUATEX\ and \MKIV. On a more modern machine
it's probably also close to zero.

These measurements demonstrate that we should add some nuance to the assumption
that parbuilding takes time. For this we need to distinguish between traditional
\TEX\ and \LUATEX. In traditional \TEX\ you build an horizontal box or vertical
box. In \TEX\ speak these are called horizontal and vertical lists. The main text
flow is a special case and called the main vertical list, but in this perspective
you can consider it to be like a vertical box.

Each vertical box is split into lines. These lines are packed into horizontal
boxes. In traditional \TEX\ constructing a list starts with turning references to
characters into glyphs and ligatures. Kerns get inserted between characters if
the font requests that. When a vertical box is split into lines, discretionary
nodes get inserted (hyphenation) and when font expansion or protrusion is enabled
extra fonts with expanded dimensions get added.

So, in the case of vertical box, building the paragraph is not really
distinguished from ligaturing, kerning and hyphenation which means that the
timing of this process is somewhat fuzzy. Also, because after the lines are
identified some final packing of lines happens and the result gets added to a
vertical list.

In \LUATEX\ all these stages are split into hyphenation, ligature building,
kerning, line breaking and finalizing. When the callbacks are not enabled the
normal machinery kicks in but still the stages are clearly separated. In the case
of \CONTEXT\ the font ligaturing and kerning get preceded by so called node mode
font handling. This means that we have extra steps and there can be even more
steps before and afterwards. And, hyphenation always happens on the whole list,
contrary to traditional \TEX\ that interweaves this. Keep in mind that because we
can box and unbox and in that process add extra text the whole process can get
repeated several times for the same list. Of course already treated glyphs and
kerns are normally kept as they are.

So, because in  \LUATEX\ the process of splitting into lines is separated we can
safely conclude that it is real fast. Definitely compared to al the font related
steps. So, let's go back to the tests and let's do the following:

\starttyping
\testfeatureonce{1000}{\setbox0\hbox{\tufte}}

\testfeatureonce{1000}{\setbox0\vbox{\tufte}}

\startparbuilder[basic]
    \testfeatureonce{1000}{\setbox0\vbox{\tufte}}
\stopparbuilder
\stoptyping

We've put the text into a macro so that we don't have interference from reading
files. The test wrapper does the timing. The following measurements are somewhat
rough but repetition gives similar results. \footnote {Before and between runs
we do a garbage collection.}

\starttabulate[|c|c|c|c|c|]
\HL
\NC   \NC \bf engine \NC \bf method \NC \bf normal \NC \bf hz \NC \NR % comment
\HL
\NC 1 \NC luatex     \NC tex hbox   \NC ~9.64      \NC ~9.64  \NC \NR % baseline font feature processing, hyphenation etc: 9.74
\NC 2 \NC            \NC tex vbox   \NC ~9.84      \NC 10.16  \NC \NR % 0.20 linebreak / 0.52 with hz -> 0.32 hz overhead (150pct more)
\NC 3 \NC            \NC lua vbox   \NC 17.28      \NC 18.43  \NC \NR % 7.64 linebreak / 8.79 with hz -> 1.33 hz overhead ( 20pct more)
\HL
\NC 4 \NC luajittex  \NC tex hbox   \NC ~6.33      \NC ~6.33  \NC \NR % baseline font feature processing, hyphenation etc: 6.33
\NC 5 \NC            \NC tex vbox   \NC ~6.53      \NC ~6.81  \NC \NR % 0.20 linebreak / 0.48 with hz -> 0.28 hz overhead (expected 0.32)
\NC 6 \NC            \NC lua vbox   \NC 11.06      \NC 11.81  \NC \NR % 4.53 linebreak / 5.28 with hz -> 0.75 hz overhead
\HL
\stoptabulate

In line~1 we see the basline: hyphenation, processing fonts and hpacking takes
9.74 seconds. In the second line we see that breaking the 1000 paragraphs costs
some 0.20 seconds and when expansion is enabled an extra 12 seconds is needed.
This means that expansion takes 150\% more runtime. If we delegate the task to
\LUA\ we need 7.64 seconds for breaking into lines which can not be neglected
but is still ok given the fact that we break 1000 paragraphs. But, interesting
is to see that our alternative expansion routine only adds 1.33 seconds which is
less than 20\%. It must be said that the built|-|in method is not that efficient
by design if only because it started out differently as part of research.

When measured three months later, the numbers for regular \LUATEX\ (at that time
version 0.77) with the latest \CONTEXT\ were: 8.52, 8.72 and 15.40 seconds for
the normal run, which demonstrates that we should not draw too many conclusions
from such measurements. It's the overal picture that matters.

As with earlier timings, if we use \LUAJITTEX\ we see that the runtime of \LUA\
is much lower (due to the virtual machine). Of course we're still 20 times slower
than the built|-| in method but only 10 times slower when we use expansion. To put
these numbers in perspective: 5 seconds for 1000 paragraphs.

\starttyping
\setupbodyfont[dejavu]

\starttext
  \dontcomplain \dorecurse{1000}{\tufte\par}
\stoptext
\stoptyping

This results in 295 pages in the default layout and takes 17.8 seconds or 16.6
pages per second. Expansion is not enabled.

\starttext
\startparbuilder[basic]
    \dontcomplain \dorecurse{1000}{\tufte\par}
\stopparbuilder
\stoptext

That one takes 24.7 seconds and runs at 11.9 pages per second. This is indeed
slower but on a bit more modern machine I expect better results. We should also
realize that with Dejavu being a relative large font a difficult paragraph like
the tufte example gives overfull boxes which in turn is an indication that quite
some alternative breaks are tried.

When typeset with Latin Modern we don't get overfull boxes and interesting is
that the native method needs less time (15.9 seconds or 14.1 pages per second)
while the \LUA\ variant also runs a bit faster: 23.4 or 9.5 pages per second. The
number of pages is 223 because this font is smaller by design.

When we disable hyphenation the the Dejavu variant takes 16.5 (instead of 17.8)
seconds and 23.1 (instead of 24.7) seconds for \LUA, so this process is not that
demanding.

For typesetting so many paragraphs without anything special it makes no sense to
bother with using a \LUA\ based parbuilder. I must admit that I never had to typeset
novels so all my 300 page runs are much longer anyway. Anyway, when at some point
we introduce alternative parbuilding to \CONTEXT, the speed penalty is probably
acceptable.

Just to indicate that predictions are fuzzy: when we put a \type {\blank} between
the paragraphs we end up with 313 pages and the traditional method takes 18.3
while \LUA\ needs 23.6 seconds. One reason for this is that the whitespace is
also handled by \LUA\ and in the pagebuilder we do some finalizing, so we
suddenly get interference of other processes (as well as the garbage collector).
Again an indication that we should not bother too much about speed. I try to make
sure that the \LUA\ (as well as \TEX) code is reasonably efficient, so in
practice it's the document style that is a more important factor than the
parbuilder, it being the traditional one or the \LUA\ variant.

\stopsubject

\startsubject[title=Copying boxes]

As soon as in \CONTEXT\ you start enhancing the page with headers and footers and
backgrounds you will see that the pps rate drops. This is partly due to the fact
that suddenly quite some macro expansion takes place in order to check what needs
to happen (like font and color switches, offsets, overlays etc). But what has
more impact is that we might end up with copying boxes and that takes time. Also,
by wrapping and repackaging boxes, we add additional levels of recursion in
postprocessing code.

\stopsubject

\startsubject[title=Macro expansion]

Taco and I once calculated that \MKII\ spends some 4\% of the time in accessing
the hash table. This is a clear indication that quite some macro expansions goes
on. Due to the fact that when I rewrote \MKII\ into \MKIV\ I no longer had to
take memory and other limitations into account, the codebase looks quite
different. There we do have more expansion in the mechanism that deals with
settings but the body of macros is much smaller and less parameters are passed.
So, the overall performance is better.

\stopsubject

\startsubject[title=Fonts]

Using a font has several aspects. First you have to define an instance. Then, when
you use it for the first time, the font gets loaded from storage, initialized and
is passed to \TEX. All these steps are quite optimized. If we process the following
file:

\starttyping
\setupbodyfont[dejavu]

\starttext
    regular, {\it italic}, {\bf bold ({\bi italic})} and $m^a_th$
\stoptext
\stoptyping

we get reported:

\starttabulate[||T|]
\NC \type{loaded fonts}    \NC xits-math.otf xits-mathbold.otf \NC \NR
\NC                        \NC dejavuserif-bold.ttf dejavuserif-bolditalic.ttf \NC \NR
\NC                        \NC dejavuserif-italic.ttf dejavuserif.ttf \NC \NR
\NC \type{fonts load time} \NC 0.374 seconds \NR
\NC \type{runtime}         \NC 1.014 seconds, 0.986 pages/second \NC \NR
\stoptabulate

So, six fonts are loaded and because XITS is used we also preload the math bold
variant. Loading of text fonts is delayed but in order initialize math we need to
preload the math fonts.

If we don't define a bodyfont, a default set gets loaded: Latin Modern. In that
case we get:

\starttabulate[||T|]
\NC \type{loaded fonts}    \NC latinmodern-math.otf \NC \NR
\NC                        \NC lmroman10-bolditalic.otf lmroman12-bold.otf \NC \NR
\NC                        \NC lmroman12-italic.otf lmroman12-regular.otf \NC \NR
\NC \type{fonts load time} \NC 0.265 seconds \NR
\NC \type{runtime}         \NC 0.874 seconds, 1.144 pages/second \NC \NR
\stoptabulate

Before we had native \OPENTYPE\ Latin Modern math fonts, it took slightly longer
because we had to load many small \TYPEONE\ fonts and assemble a virtual math font.

As soon as you start mixing more fonts and/or load additional weights and styles
you will see these times increase. But if you use an already loaded font with
a different featureset or scaled differently, the burden is rather low. It is
safe to say that at this moment loading fonts is not a bottleneck.

Applying fonts can be more demanding. For instance if you typeset Arabic or
Devanagari the amount of node and font juggling definitely influences the total
runtime. As the code is rather optimized there is not much we can do about it.
It's the price that comes with flexibility. As far as I can tell getting the same
results with \PDFTEX\ (if possible at all) or \XETEX\ is not taking less time. If
you've split up your document in separate files you will seldom run more than a
dozen pages which is then still bearable.

If you are for instance typesetting a dictionary like document, it does not make
sense to do all font switches by switching body fonts. Just defining a couple of
font instances makes more sense and comes at no cost. Being already quite
efficient given the complexity you should not expect impressive speedups in this
area.

\stopsubject

\startsubject[title=Manipulations]

The main manipulation that I have to do is to process \XML\ into something
readable. Using the built||in parser and mapper already has some advantages
and if applied in the right way it's also rather efficient. The more you restrict
your queries, the better.

Text manipulations using \LUA\ are often quite fast and seldom the reason for
seeing slow processing. You can do lots of things at the \LUA\ end and still have
all the \CONTEXT\ magic by using the \type {context} namespace and function.

\stopsubject

\startsubject[title=Multipass]

You can try to save 1 second on a 20 second run but that is not that impressive
if you need to process the document three times in order to get your cross
references right. Okay you'd save 3 seconds but still to get result you needs
some 60 seconds (unless you already have run the document before). If you have a
predictable workflow you might know in advance that you only need two runs in
case you can enforce that with \type {--runs=2}. Furthermore you can try to
optimize the style by getting rid of redundant settings and inefficient font
switches. But no matter what we optimize, unless we have a document with no cross
references, sectioning and positioning, you often end up with the extra run,
although \CONTEXT\ tries to minimize the number of needed runs needed.

\stopsubject

\startsubject[title=Trial runs]

Some mechanisms, like extreme tables, need multiple passes and all but the last
one are tagged as trial runs. Because in many cases only dimensions matter, we
can disable some time consuming code in such case. For instance, at some point
Alan Braslau and I found out that the new chemical manual ran real slow, mainly
due to the tens of thousands of \METAPOST\ graphics. Adding support for trial
runs to the chemical structure macros gave a fourfold improvement. The manual is
still a slow|-|runner, but that is simply because it has so many runtime
generated graphics.

\stopsubject

\stopsection

\startsection[title=The \METAPOST\ library]

When the \METAPOST\ library got included we saw a drastic speedup in processing
document with lots of graphics. However, when \METAPOST\ got a different number
system (native, double and decimal) the changed memory model immediately lead to
a slow down. On one 150 page manual which a graphic on each page I saw the
\METAPOST\ runtime go up from about half a second upto more than 5 seconds. In
this case I was able to rewrite some core \METAFUN\ macro to better suit the new
model, but you might not be so lucky. So more careful coding is needed. Of course
if you only have a few graphics, you can just ignore the change.

\stopsection

\startsection[title=The \LUA\ interpreter]

Where the \TEX\ part of \LUATEX\ is compiled, the \LUA\ code gets interpreted,
converted into bytecode, and ran by the virtual machine. \LUA\ is by design quite
portable, which means that the virtual machine is not optimized for a specific
target. The \LUAJIT\ interpreter on the other hand is written in assembler and
available for only some platforms, but the virtual machine is about twice as
fast. The just||in||time part of \LUAJIT\ is not if much help and even can slow
down processing.

When we moved from \LUA~5.1 to 5.2 we found out that there was some speedup but
it's hard to say why. There has been changes in the way strings are dealt with
(\LUA\ hashes strings) and we use lots of strings, really lots. There has been
changes in the garbage collection and during a run lots of garbage needs to be
collected. There are some fundamental changes in so called environments and who
knows what impact that has.

If you ever tried to measure the performance of \LUA, you probably have noticed
that it is quite fast. This means that it makes no sense to optimize code that
gets visited only occasionally. But some of the \CONTEXT\ code gets exercised a
lot, for instance all code that deals with fonts. We use attributes a lot and
checking them is for good reason not the fastest code. But given the often
advanced functionality that it makes possible we're willing to pay the price.
It's also functionality that you seldom need all at the same time and for
straightforward text only documents all that code is never executed.

When writing \TEX\ or \LUA\ code I spent a lot of time making it as efficient as
possible in terms of performance and memory usage. The sole reason for that is
that we happen to process documents where a lot of functionality is combined, so
if many small speed||ups accumulate to a noticeable performance gain it's worth
the effort.

So, where does \LUA\ influence runtime? First of all we use \LUA\ do deal with all
in- and output as well as locating files in the \TEX\ directory structure. Because
that code is partly shared with the script manager (\type {mtxrun}) it is optimized
but some more is possible if needed. It is already not the most easy to read code,
so I don't want to introduce even more obscurity.

Quite some code deals with loading, preparing and caching fonts. That code is
mostly optimized for memory usage although speed is also okay. This code is only
called when a font is loaded for the first time (after an update). After that
loading is at matter of milliseconds. When a text gets typeset and when fonts are
processed in so called node mode, depending on the script and|/|or enabled
features, a substantial amount of time is spent in \LUA. There is still a bit
complex dealing with inserting kerns but future \LUATEX\ will carry kerning
in the glyph node so there we can gain some runtime.

If a page has 4000 characters and if font features as well as other manipulations
demand 10 runs over the text, we have 40.000 checks of nodes and potential
actions. Each involves an id check, maybe a subtype check, maybe some attribute
checking and possibly some action. So, if we have 200.000 (or more) function
calls to the per page \TEX\ end it might add up to a lot. Around the time that we
went to \LUA~5.2 and played with \LUAJITTEX, the node accessors have been sped
up. This gave indeed a measurable speedup but not on an average document, only on
the more extreme documents or features. Because the \MKIV\ \LUA\ code goes from
experimental to production to final, some improvements are made in the process
but there is not much to gain there. We just have to wait till computers get
faster, \CPU\ cache get bigger, branch prediction improves, floating point
calculations take less time, memory is speedy, and flash storage is the standard.

The \LUA\ code is plugged into the \TEX\ machinery via callbacks. For
instance each time a box is build several callbacks are triggered, even if it's
an empty box or just an extra wrapper. Take for instance this:

\starttyping
\hbox \bgroup
    \hskip \zeropoint
    \hbox \bgroup
        test
    \egroup
    \hskip \zeropoint
\egroup
\stoptyping

Of course you won't come up with this code as it doesn't do much good but macros
that you use can definitely produce this. For instance, the zero skip can be a
left and right margin that happen to be. For 10.000 iterations I measured 0.78
seconds while the text one takes 0.62 seconds:

\starttyping
\hbox \bgroup
    \hbox \bgroup
        test
    \egroup
\egroup
\stoptyping

Why is this? One reason is that a zero skip results in a node and the more nodes
we have the more memory (de)allocation takes place and the more nodes in the list
need to be checked. Of course the relative difference is less when we have more
text. So how can we improve this? The following variant, at the cost of some
testing takes just as much time.

\starttyping
\hbox \bgroup
    \hbox \bgroup
        \scratchdimen\zeropoint
        \ifdim\scratchdimen=\zeropoint\else\hskip\scratchdimen\fi
        test
        \ifdim\scratchdimen=\zeropoint\else\hskip\scratchdimen\fi
    \egroup
\egroup
\stoptyping

As does this one, but the longer the text, the slower it gets as one of the two
copies needs to be skipped.

\starttyping
\hbox \bgroup
    \hbox \bgroup
        \scratchdimen\zeropoint
        \ifdim\scratchdimen=\zeropoint
            test%
        \else
            \hskip\scratchdimen
            test%
            \hskip\scratchdimen
        \fi
    \egroup
\egroup
\stoptyping

Of course most speedup is gained when we don't package at all, so if we test
before we package but such an optimization is seldom realistic because much more
goes on and we cannot check for everything. Also, 10.000 is a lot while 0.10
seconds is something we can live with. By the way, compare the following

\starttyping
\hbox \bgroup
    \hskip\zeropoint
    test%
    \hskip\zeropoint
\egroup

\hbox \bgroup
    \kern\zeropoint
    test%
    \kern\zeropoint
\egroup
\stoptyping

The first variant is less efficient that the second one, because a skip
effectively is a glue node pointing to a specification node while a kern is just
a simple node with the width stored in it. \footnote {On the \LUATEX\ agenda is
moving the glue spec into the glue node.} I must admit that I seldom keep in mind
to use kerns instead of skips when possible if only because one needs to be sure
to be in the right mode, horizontal or vertical, so additional commands might be
needed.

\stopsection

\startsection[title=Macros]

Are macros a bottleneck? In practice not really. Of course we have optimized the
core \CONTEXT\ macros pretty well, but one reason for that is that we have a
rather extensive set of configuration and definition mechanisms that rely heavily
on inheritance. Where possible all that code is written in a way that macro
expansion won't hurt too much. because of this users themselves can be more
liberal in coding. There is a lot going on deep down and if you turn on tracing
macros you can get horrified. But, not all shown code paths are entered. During the
move (and rewrite) from \MKII\ to \MKIV\ quite some bottlenecks that result from
limitations of machines and memory have been removed and as a result the macro
expansion part is somewhat faster, which nicely compensates the fact that we
have a more advanced but slower inheritance subsystem. Readability of code and
speed are probably nicely balanced by now.

Once a macro is read in, its internal representation is pretty efficient. For
instance references to macro names are just pointers into a hash table. Of
course, when a macro is seen in your source, that name has to be looked up, but
that's a fast action. Using short names in the running text for instance really
doesn't speed up processing much. Switching font sets on the other hand does, as
then quite some checking happens and the related macros are pretty extensive.
However, once a font is loaded references to it a pretty fast. Just keep in mind
that if you define something inside a group, in most cases it got forgotten. So,
if you need something more often, just define it at the outer level.

\stopsection

\startsection[title=Optimizing code]

Optimizing only makes sense if used very often and called frequently or when the
problem to solve is demanding. An example of code that gets done often is page
building, where we pack together many layout elements. Font switches can also be
time consuming, if defined wrong. These can happen for instance for formulas,
marked words, cross references, margin notes, footnotes (often a complete
bodyfont switch), table cells, etc. Yet another is clever vertical spacing that
happens between structural elements. All these mechanisms are reasonably
optimized.

I can safely say that deep down \CONTEXT\ is no that inefficient, given what it
has to do. But when a style for instance does redundant or unnecessary massive
font switches you are wasting runtime. I dare to say that instead of trying to
speed up code (for instance by redefining macros) you can better spend the time
in making styles efficient. For instance having 10 \type {\blank}'s in a row
will work out rather well but takes time. If you know that a section head has no
raised or lowered text and no math, you can consider using \type {\definefont} to
define the right size (especially if it is a special size) instead of defining
an extra bodyfont size and switch to that as it includes setting up related sizes
and math.

It might sound like using \LUA\ for some tasks makes \CONTEXT\ slower, but this
is not true. Of course it's hard to prove because by now we also have more
advanced font support, cleaner math mechanisms, additional features especially in
especially structure related mechanisms, etc. There are also mechanisms that are
faster, for instance extreme tables (a follow up on natural tables) and mixed
column modes. Of course on the previously mentioned 300 page simple paragraphs
with simple Latin text the \PDFTEX\ engine is much faster than \LUATEX, also
because simple fonts are used. But for many of todays document this engine is no
longer an options. For instance in our \XML\ processing in multiple languages,
\LUATEX\ beats \PDFTEX. There is not that much to optimize left, so most speed up
has to come from faster machines. And this is not much different from the past:
processing 300 page document on a 4.7Mhz 8086 architecture was not much fun and
we're not even talking of advanced macros here. Faster machines made more clever
and user friendly systems possible but at the cost of runtime, to even if
machines have become many times faster, processing still takes time. On the other
hand, \CONTEXT\ will not become more complex than it is now, so from now on we
can benefit from faster \CPU's, memory and storage.

\stopsection

\stopchapter

\stopcomponent

