% language=us

\startcomponent about-calls

\environment about-environment

\startchapter[title={Going nuts}]

\startsection[title=Introduction]

This is not the first story about speed and it will probably not be the last one
either. This time we discuss a substantial speedup: upto 50\% with \LUAJITTEX.
So, if you don't want to read further at least know that this speedup came at the
cost of lots of testing and adapting code. Of course you could be one of those
users who doesn't care about that and it may also be that your documents don't
qualify at all.

Often when I see a kid playing a modern computer game, I wonder how it gets done:
all that high speed rendering, complex environments, shading, lightning,
inter||player communication, many frames per second, adapted story lines,
\unknown. Apart from clever programming, quite some of the work gets done by
multiple cores working together, but above all the graphics and physics
processors take much of the workload. The market has driven the development of
this hardware and with success. In this perspective it's not that much of a
surprise that complex \TEX\ jobs still take some time to get finished: all the
hard work has to be done by interpreted languages using rather traditional
hardware. Of course all kind of clever tricks make processors perform better than
years ago, but still: we don't get much help from specialized hardware. \footnote
{Apart from proper rendering on screen and printing on paper.} We're sort of
stuck: when I replaced my 6 year old laptop (when I buy one, I always buy the
fastest one possible) for a new one (so again a fast one) the gain in speed of
processing a document was less than twice. The many times faster graphic
capabilities are not of much help there, not is twice the amount of cores.

So, if we ever want to go much faster, we need to improve the software. The
reason for trying to speed up \MKIV\ has been mentioned before, but let's
summarize it here:

\startitemize

\startitem
    There was a time when users complained about the speed of \CONTEXT,
    especially compared to other macro packages. I'm not so sure if this is still
    a valid complaint, but I do my best to avoid bottlenecks and much time goes
    into testing efficiency.
\stopitem

\startitem
    Computers don't get that much faster, at least we don't see an impressive
    boost each year any more. We might even see a slowdown when battery live
    dominates: more cores at a lower speed seems to be a trend and that doesn't
    suit current \TEX\ engines well. Of course we assume that \TEX\ will be
    around for some time.
\stopitem

\startitem
    Especially in automated workflows where multiple products each demanding a
    couple of runs are produced speed pays back in terms of resources and
    response time. Of course the time invested in the speedup is never regained
    by ourselves, but we hope that users appreciate it.
\stopitem

\startitem
    The more we do in \LUA, read: the more demanding users get and the more
    functionality is enabled, the more we need to squeeze out of the processor.
    And we want to do more in \LUA\ in order to get better typeset results.
\stopitem

\startitem
    Although \LUA\ is pretty fast, future versions might be slower. So, the more
    efficient we are, the less we probably suffer from changes.
\stopitem

\startitem
    Using more complex scripts and fonts is so demanding that the number of pages
    per second drops dramatically. Personally I consider a rate of 15 pps with
    \LUATEX\ or 20 pps with \LUAJITTEX\ reasonable minima on my laptop. \footnote
    {A Dell 6700 laptop with Core i7 3840QM, 16 GB memory and SSD, running 64 bit
    Windows 8.}
\stopitem

\startitem
    Among the reasons why \LUAJIT\ jitting does not help us much is that (at
    least in \CONTEXT) we don't use that many core functions that qualify for
    jitting. Also, as runs are limited in time and much code kicks in only a few
    times the analysis and compilation doesn't pay back in runtime. So we cannot
    simply sit down and wait till matters improve.
\stopitem

\stopitemize

Luigi Scarso and I have been exploring several options, with \LUATEX\ as well as
\LUAJITTEX. We observed that the virtual machine in \LUAJITTEX\ is much faster so
that engine already gives a boots. The advertised jit feature can best be
disabled as it slows down a run noticeably. We played with \type {ffi} as well,
but there is additional overhead involved (\type {cdata}) as well as limited
support for userdata, so we can forget about that too. \footnote {As we've now
introduced getters we can construct a metatable at the \LUA\ end as that is what
\type {ffi} likes most. But even then, we don't expect much from it: the four
times slow down that experiments showed will not magically become a large gain.}
Nevertheless, the twice as fast virtual machine of \LUAJIT\ is a real blessing,
especially if you take into account that \CONTEXT\ spends quite some time in
\LUA. We're also looking forward to the announced improved garbage collector of
\LUAJIT.

In the end we started looking at \LUATEX\ itself. What can be gained there,
within the constraints of not having to completely redesign existing
(\CONTEXT) \LUA\ code? \footnote {In the end a substantial change was needed but
only in accessing node properties. The nice thing about C is that there macros
often provide a level of abstraction which means that a similar adaption of \TEX\
source code would be more convenient.}

\stopsection

\startsection[title={Two access models}]

Because the \CONTEXT\ code is reasonably well optimized already, the only option
is to look into \LUATEX\ itself. We had played with the \TEX||\LUA\ interface
already and came to the conclusion that some runtime could be gained there. On
the long run it adds up but it's not too impressive; these extensions are
awaiting integration. Tracing and bechmarking as well as some quick and dirty
patches demonstrated that there were two bottlenecks in accessing fields in
nodes: checking (comparing the metatables) and constructing results (userdata
with metatable).

In case you're infamiliar with the concept this is how nodes work. There is an
abstract object called node that is in \LUA\ qualified as user data. This object
contains a pointer to \TEX's node memory. \footnote {The traditional \TEX\ node
memory manager is used, but at some point we might change to regular C
(de)allocation. This might be slower but has some advantages too.} As it is real
user data (not so called light) it also carries a metatable. In the metatble
methods are defined and one of them is the indexer. So when you say this:

\starttyping
local nn = n.next
\stoptyping

given that \type {n} is a node (userdata) the \type {next} key is resolved up
using the \type {__index} metatable value, in our case a function. So, in fact,
there is no \type {next} field: it's kind of virtual. The index function that
gets the relevant data from node memory is a fast operation: after determining
the kind of node, the requested field is located. The return value can be a
number, for instance when we ask for \type {width}, which is also fast to return.
But it can also be a node, as is the case with \type {next}, an then we need to
allocate a new userdata object (memory management overhead) and a metatable has
to be associated. And that comes at a cost.

In a previous update we had already optimized the main \type {__index} function
but felt that some more was possible. For instance we can avoid the lookup of the
metatable for the returned node(s). And, if we don't use indexed access but a
instead a function for frequently accessed fields we can sometimes gain a bit too.

A logical next step was to avoid some checking, which is okay given that one pays
a bit attention to coding. So, we provided a special table with some accessors of
frequently used fields. We actually implemented this as a so called \quote {fast}
access model, and adapted part of the \CONTEXT\ code to this, as we wanted to see
if it made sense. We were able to gain 5 to 10\% which is nice but still not
impressive. In fact, we concluded that for the average run using fast was indeed
faster but not enough to justify rewriting code to the (often) less nice looking
faster access. A nice side effect of the recoding was that I can add more advanced
profiling.

But, in the process we ran into another possibility: use accessors exclusively
and avoiding userdata by passing around references to \TEX\ node memory directly.
As internally nodes can be represented by numbers, we ended up with numbers, but
future versions might use light userdata instead to carry pointers around. Light
userdata is cheap basic object with no garbage collection involved. We tagged
this method \quote {direct} and one can best treat the values that gets passed
around as abstract entities (in \MKIV\ we call this special view on nodes
\quote {nuts}).

So let's summarize this in code. Say that we want to know the next node of
\type {n}:

\starttyping
local nn = n.next
\stoptyping

Here \type {__index} will be resolved and the associated function be called. We
can avoid that lookup by applying the \type {__index} method directly (after all,
that one assumes a userdata node):

\starttyping
local getfield = getmetatable(n).__index

local nn = getfield(n,"next") -- userdata
\stoptyping

But this is not a recomended interface for regular users. A normal helper that
does checking is as about fast as the indexed method:

\starttyping
local getfield = node.getfield

local nn = getfield(n,"next") -- userdata
\stoptyping

So, we can use indexes as well as getters mixed and both perform more of less
equal. A dedicated getter is somewhat more efficient:

\starttyping
local getnext = node.getnext

local nn = getnext(n) -- userdata
\stoptyping

If we forget about checking, we can go fast, in fact the nicely interfaced \type
{__index} is the fast one.

\starttyping
local getfield = node.fast.getfield

local nn = getfield(n,"next") -- userdata
\stoptyping

Even more efficient is the following as that one knows already what to fetch:

\starttyping
local getnext = node.fast.getnext

local nn = getnext(n) -- userdata
\stoptyping

The next step, away from userdata was:

\starttyping
local getfield = node.direct.getfield

local nn = getfield(n,"next") -- abstraction
\stoptyping

and:

\starttyping
local getnext = node.direct.getnext

local nn = getnext(n) -- abstraction
\stoptyping

Because we considered three variants a bit too much and because \type {fast} was
only 5 to 10\% faster in extreme cases, we decided to drop that experimental code
and stick to providing accessors in the node namespace as well as direct variants
for critical cases.

Before you start thinking: \quote {should I rewrite all my code?} think twice!
First of all, \type {n.next} is quite fast and switching between the normal and
direct model also has some cost. So, unless you also adapt all your personal
helper code or provide two variants of each, it only makes sense to use direct
mode in critical situations. Userdata mode is much more convenient when
developing code and only when you have millions of access you can gain by direct
mode. And even then, if the time spent in \LUA\ is small compared to the time
spent in \TEX\ it might not even be noticeable. The main reason we made direct
variants is that it does pay of in \OPENTYPE\ font processing where complex
scripts can result in many millions of calls indeed. And that code will be set up
in such a way that it will use userdata by default and only in well controlled
case (like \MKIV) we will use direct mode. \footnote {When we are confident
that \type {direct} node code is stable we can consider going direct in generic
code as well, although we need to make sure that third party code keeps working.}

Another thing to keep in mind is that when you provide hooks for users you should
assume that they use the regular mode so you need to cast the plugins onto direct
mode then. Because the idea is that one should be able to swap normal functions
by direct ones (which of course is only possible when no indexes are used) all
relevant function in the \type {node} namespace are available in \type {direct}
as well. This means that the following code is rather neutral:

\starttyping
local x = node -- or: x = node.direct

for n in x.traverse(head) do
  if x.getid(n) == node.id("glyph") and x.getchar(n) == 0x123 then
    x.setfield(n,"char",0x456)
  end
end
\stoptyping

Of course one needs to make sure that \type {head} fits the model. For this you
can use the cast functions:

\starttyping
node.direct.todirect(node or direct)
node.direct.tonode(direct or node)
\stoptyping

These helpers are flexible enough to deal with either model. Aliasing the
functions to locals is of course more efficient when a large number of calls
happens (when you use \LUAJITTEX\ it will do some of that for you automatically).
Of course, normally we use a more natural variant, using an id traverser:

\starttyping
for n in node.traverse_id(head,node.id("glyph")) do
  if n.char == 0x123 then
    n.char = 0x456
  end
end
\stoptyping

This is not that much slower, especially when it's only ran once. Just count the
number of characters on a page (or in your document) and you will see that it's
hard to come up with that many calls. Of course, processing many pages of Arabic
using a mature font with many features enabled and contextual lookups, you do run
into quantities. Tens of features times tens of contextual lookup passes can add
up considerably. In Latin scripts you never reach such numbers, unless you use
fonts like Zapfino.

\stopsection

\startsection[title={The transition}]

After weeks of testing, rewriting, skyping, compiling and making decisions, we
reached a more or less stable situation. At that point we were faced with a
speedup that gave us a good feeling, but transition to the faster variant has a
few consequences.

\startitemize

\startitem We need to use an adapted code base: indexes are to be replaced by
function calls. This is a tedious job that can endanger stability so it has to be
done with care. \footnote {The reverse is easier, as converting getters and
setters to indexed is a rather simple conversion, while for instance changing
type {.next} into a \type {getnext} needs more checking because that key is not
unique to nodes.} \stopitem

\startitem When using an old engine with the new \MKIV\ code, this approach will
result in a somewhat slower run. Most users will probably accept a temporary
slowdown of 10\%, so we might take this intermediate step. \stopitem

\startitem When the regular getters and setters become available we get back to
normal. Keep in mind that these accessors do some checking on arguments so that
slows down to the level of using indexes. On the other hand, the dedicated ones
(like \type {getnext}) are more efficient so there we gain. \stopitem

\startitem As soon as direct becomes available we suddenly see a boost in speed.
In documents of average complexity this is 10-20\% and when we use more complex
scripts and fonts it can go up to 40\%. Here we assume that the macro package
spends at least 50\% of its time in \LUA. \stopitem

\stopitemize

If we take the extremes: traditional indexed on the one hand versus optimized
direct in \LUAJITTEX, a 50\% gain compared to the old methods is feasible.
Because we also retrofitted some fast code into the regular accessor, indexed
mode should also be somewhat faster compared to the older engine.

In addition to the already provide helpers in the \type {node} namespace, we
added the following:

\starttabulate[|Tl|p|]
\HL
\NC getnext    \NC this one is used a lot when analyzing and processing node lists \NC \NR
\NC getprev    \NC this one is used less often but fits in well (companion to \type {getnext}) \NC \NR
\NC getfield   \NC this is the general accessor, in userdata mode as fast as indexed \NC \NR
\HL
\NC getid      \NC one of the most frequent called getters when parsing node lists \NC \NR
\NC getsubtype \NC especially in fonts handling this getter gets used \NC \NR
\HL
\NC getfont    \NC especially in complex font handling this is a favourite \NC \NR
\NC getchar    \NC as is this one \NC \NR
\HL
\NC getlist    \NC we often want to recurse into hlists and vlists and this helps \NC \NR
\NC getleader  \NC and also often need to check if glue has leader specification (like list) \NC \NR
\HL
\NC setfield   \NC we have just one setter as setting is less critical \NC \NR
\HL
\stoptabulate

As \type {getfield} and \type {setfield} are just variants on indexed access, you
can also use them to access attributes. Just pass a number as key. In the \type
{direct} namespace, helpers like \type {insert_before} also deal with direct
nodes.

We currently only provide \type {setfield} because setting happens less than
getting. Of course you can construct nodelists at the \LUA\ end but it doesn't
add up that fast and indexed access is then probably as efficient. One reason why
setters are less an issue is that they don't return nodes so no userdata overhead
is involved. We could (and might) provide \type {setnext} and \type {setprev},
although, when you construct lists at the \LUA\ end you will probably use the
type {insert_after} helper anyway.

\stopsection

\startsection[title={Observations}]

So how do these variants perform? As we no longer have \type {fast} in the engine
that I use for this text, we can only check \type {getfield} where we can simulate
fast mode with calling the \type{__index} metamethod. In practice the \type
{getnext} helper will be somewhat faster because no key has to be checked,
although the \type {getfield} functions have been optimized according to the
frequencies of accessed keys already.

\starttabulate
\NC node[*]              \NC 0.516 \NC \NR
\NC node.fast.getfield   \NC 0.616 \NC \NR
\NC node.getfield        \NC 0.494 \NC \NR
\NC node.direct.getfield \NC 0.172 \NC \NR
\stoptabulate

Here we simulate a dumb 20 times node count of 200 paragraphs \type {tufte.tex}
with a little bit of overhead for wrapping in functions. \footnote {When
typesetting Arabic or using complex fonts we quickly get a tenfold.} We encounter
over three million nodes this way. We average a couple or runs.

\starttyping
local function check(current)
  local n = 0
  while current do
    n = n + 1
    current = getfield(current,"next") -- current = current.next
  end
  return n
end
\stoptyping

What we see here is that indexed access is quite okay given the amount of nodes,
but that direct is much faster. Of course we will never see that gain in practice
because much more happens than counting and because we also spend time in \TEX.
The 300\% speedup will eventually go down to one tenth of that.

Because \CONTEXT\ avoids node list processing when possible the baseline
performance is not influenced much.

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

With \LUATEX\ we get some 575 pages per second and with \LUAJITTEX\ more than 610
pages per second.

\starttyping
\setupbodyfont[pagella]

\edef\zapf{\cldcontext
  {context(io.loaddata(resolvers.findfile("zapf.tex")))}}

\starttext \dorecurse{1000}{\zapf\par} \stoptext
\stoptyping

For this test \LUATEX\ needs 3.9 seconds and runs at 54 pages per second, while
\LUAJITTEX\ needs only 2.3 seconds and gives us 93 pages per second.

Just for the record, if we run this:

\starttyping
\starttext
\stoptext
\stoptyping

a \LUATEX\ runs takes 0.229 seconds and a \LUAJITTEX\ run 0.178 seconds. This includes
initializing fonts. If we run just this:

\starttyping
\stoptext
\stoptyping

\LUATEX\ needs 0.199 seconds and \LUAJITTEX\ only 0.082 seconds. So, in the
meantime, we hardly spend any time on startup. Launching the binary and managing
the job with \type {mtxrun} calling \type {mtx-context} adds 0.160 seconds
overhead. Of course this is only true when you have already ran \CONTEXT\ once as
the operating system normally caches files (in our case format files and fonts).
This means that by now an edit|-|preview cycle is quite convenient. \footnote {I
use \SCITE\ with dedicated lexers as editor and currently \type {sumatrapdf} as
previewer.}

As a more practical test we used the current version of \type {fonts-mkiv} (166
pages, using all kind of font tricks and tracing), \type {about} (60 pages, quite
some traced math) and a torture test of Arabic text (61 pages dense text). The
following measurements are from 2013-07-05 after adapting some 50 files to the
new model. Keep in mind that the old binary can fake a fast getfield and setfield
but that the other getters are wrapped functions. The more we have, the slower it
gets. We used the mingw versions.

\starttabulate[|l|r|r|r|]
\HL
\NC version                                 \NC fonts \NC about \NC arabic \NC \NR
\HL
\NC old mingw, indexed plus some functions  \NC  8.9  \NC  3.2  \NC  20.3  \NC \NR
\NC old mingw, fake functions               \NC  9.9  \NC  3.5  \NC  27.4  \NC \NR
\HL
\NC new mingw, node functions               \NC  9.0  \NC  3.1  \NC  20.8  \NC \NR
\NC new mingw, indexed plus some functions  \NC  8.6  \NC  3.1  \NC  19.6  \NC \NR
\NC new mingw, direct functions             \NC  7.5  \NC  2.6  \NC  14.4  \NC \NR
\HL
\stoptabulate

The second row shows what happens when we use the adapted \CONTEXT\ code with an
older binary. We're slower. The last row is what we will have eventually. All
documents show a nice gain in speed and future extensions to \CONTEXT\ will no
longer have the same impact as before. This is because what we here see also
includes \TEX\ activity. The 300\% increase of speed of node access makes node
processing less influential. On the average we gain 25\% here and as on these
documents \LUAJITTEX\ gives us some 40\% gain on indexed access, it gives more
than 50\% on the direct function based variant.

In the fonts manual some 25 million getter accesses happen while the setters
don't exceed one million. I lost the tracing files but at some point the Arabic
test showed more than 100 millions accesses. So it's save to conclude that
setters are sort of neglectable. In the fonts manual the amount of accesses to
the previous node were less that 5000 while the id and next fields were the clear
winners and list and leader fields also scored high. Of course it all depends on
the kind of document and features used, but we think that the current set of
helpers is quite adequate. And because we decided to provide that for normal
nodes as well, there is no need to go direct for more simple cases.

Maybe in the future further tracing might show that adding getters for width,
height, depth and other properties of glyph, glue, kern, penalty, rule, hlist and
vlist nodes can be of help, but quite probably only in direct mode combined with
extensive list manipulations. We will definitely explore other getters but only
after the current set has proven to be useful.

\stopsection

\startsection[title={Nuts}]

So why going nuts and what are nuts? In Dutch \quote {node} sounds a bit like
\quote {noot} and translates back to \quote {nut}. And as in \CONTEXT\ I needed
word for these direct nodes they became \quote {nuts}. It also suits this
project: at some point we're going nuts because we could squeeze more out
of \LUAJITTEX, so we start looking at other options. And we're sure some folks
consider us being nuts anyway, because we spend time on speeding up. And adapting
the \LUATEX\ and \CONTEXT\ \MKIV\ code mid||summer is also kind of nuts.

At the \CONTEXT\ 2013 conference we will present this new magic and about that
time we've done enough tests to see if it works our well. The \LUATEX\ engine
will provide the new helpers but they will stay experimental for a while as one
never knows where we messed up.

I end with another measurement set. Every now and and then I play with a \LUA\
variant of the \TEX\ par builder. At some point it will show up on \MKIV\ but
first I want to abstract it a bit more and provide some hooks. In order to test
the performance I use the following tests:

% \testfeatureonce{1000}{\tufte \par}

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

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

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

We use a \type {\hbox} to determine the baseline performance. Then we break lines
using the built|-|in parbuilder. Next we do the same but now with the \LUA\
variant. \footnote {If we also enable protrusion and hz the \LUA\ variant suffers
less because it implements this more efficient.}

\starttabulate[|l|l|l|l|l|]
\HL
\NC                \NC \bf \rlap{luatex} \NC \NC \bf \rlap{luajittex} \NC \NC \NR
\HL
\NC                \NC \bf total \NC \bf linebreak \NC \bf total \NC \bf linebreak \NC \NR
\HL
\NC 223 pp nodes   \NC 5.67      \NC 2.25 flushing \NC 3.64      \NC 1.58 flushing \NC \NR
\HL
\NC hbox nodes     \NC 3.42      \NC               \NC 2.06      \NC               \NC \NR
\NC vbox nodes     \NC 3.63      \NC 0.21 baseline \NC 2.27      \NC 0.21 baseline \NC \NR
\NC vbox lua nodes \NC 7.38      \NC 3.96          \NC 3.95      \NC 1.89          \NC \NR
\HL
\NC 223 pp nuts    \NC 4.07      \NC 1.62 flushing \NC 2.36      \NC 1.11 flushing \NC \NR
\HL
\NC hbox nuts      \NC 2.45      \NC               \NC 1.25      \NC               \NC \NR
\NC vbox nuts      \NC 2.53      \NC 0.08 baseline \NC 1.30      \NC 0.05 baseline \NC \NR
\NC vbox lua nodes \NC 6.16      \NC 3.71          \NC 3.03      \NC 1.78          \NC \NR
\NC vbox lua nuts  \NC 5.45      \NC 3.00          \NC 2.47      \NC 1.22          \NC \NR
\HL
\stoptabulate

We see that on this test nuts have an advantage over nodes. In this case we
mostly measure simple font processing and there is no markup involved. Even a 223
page document with only simple paragraphs needs to be broken across pages,
wrapped in page ornaments and shipped out. The overhead tagged as \quote
{flushed} indicates how much extra time would have been involved in that. These
numbers demonstrate that with nuts the \LUA\ parbuilder is performing 10\% better
so we gain some. In a regular document only part of the processing involves
paragraph building so switching to a \LUA\ variant has no big impact anyway,
unless we have simple documents (like novels). When we bring hz into the picture
performance will drop (and users occasionally report this) but here we already
found out that this is mostly an implementation issue: the \LUA\ variant suffers
less so we will backport some of the improvements. \footnote {There are still
some aspects that can be approved. For instance these tests still checks lists
for \type {prev} fields, something that is not needed in future versions.}

\stopsection

\startsection[title={\LUA\ 5.3}]

When we were working on this the first working version of \LUA\ 5.3 was
announced. Apart from some minor changes that won't affect us, the most important
change is the introduction of integers deep down. On the one hand we can benefit
from this, given that we adapt the \TEX|-|\LUA\ interfaces a bit: the distinction
between \type {to_number} and \type {to_integer} for instance. And, numbers are
always somewhat special in \TEX\ as it relates to reproduction on different
architectures, also over time. There are some changes in conversion to string
(needs attention) and maybe at some time also in the automated casting from
strings to numbers (the last is no big deal for us).

On the one hand the integers might have a positive influence on performance
especially as scaled points are integers and because fonts use them too (maybe
there is some advantage in memory usage). But we also need a proper efficient
round function (or operator) then. I'm wondering if mixed integer and float usage
will be efficient, but on the the other hand we do not that many calculations so
the benefits might outperform the drawbacks.

We noticed that 5.2 was somewhat faster but that the experimental generational
garbage collecter makes runs slower. Let's hope that the garbage collector
performance doesn't degrade. But the relative gain of node versus direct will
probably stay.

Because we already have an experimental setup we will probably experiment a bit
with this in the future. Of course the question then is how \LUAJITTEX\ will work
out, because it is already not 5.2 compatible it has to be seen if it will
support the next level. At least in \CONTEXT\ \MKIV\ we can prepare ourselves as
we did with \LUA\ 5.2 so that we're ready when we follow up.

\stopsection

\stopchapter
