Hi Joris,
yes, the evaluation of rules is already highly optimized.
1) It uses an index to filter those rules which cannot match.
So, for a way which doesn't have a building tag a rule like
building=* & is_in(...) ....
is simply ignored unless there is a rule which might set the building tag.
2) It uses caches to remember the results of repeated evaluations. Assume that
you have
highway=residential & surface=paved & (smoothness=bad | smoothness =horrible)
[...]
highway=residential & surface=paved & (smoothness=intermediate) [...]
highway=residential & surface=paved [...]
With a highway that has the tags highway=residential and surface=paved but no
smoothness tag the first two rules will not match but they are probably
evaluated. Since the 2nd and 3rd rule contain the same expression
highway=residential & surface=paved mkgmap remembers this result as long as no
tags are changed.
2a) A BAD idea regarding performance would be this:
highway=residential & surface=paved {set paved_residential=true}
paved_residential=true & (smoothness=bad | smoothness =horrible) [...]
paved_residential=true & (smoothness=intermediate) [...]
paved_residential=true [...]
as this requires more memory and more evaluations. It might make the rules
easier to understand though.
2b) Up to now using if () doesn't improve performance. It is just a means to
improve readability and the better alternative to the above BAD approach.
So, using
if (highway=residential & surface=paved) then
(smoothness=bad | smoothness =horrible) [...]
(smoothness=intermediate) [...]
[...]
end
can be easier to understand, but currently mkgmap translates it back to the
longer rules shown in 2) and thus performance should be equal.
In general, the evaluation of tags is very fast compared with the evaluation of
e.g. a is_in(...) style function.
Gerd
________________________________________
Von: mkgmap-dev <[email protected]> im Auftrag von Joris
Bo <[email protected]>
Gesendet: Samstag, 30. Mai 2020 10:35
An: Development list for mkgmap
Betreff: [mkgmap-dev] performance best practice
Hi
I have a question about improving performance
(I’m very happy already, I think is absolutely fast enough, its just that I
could improve by doing things differently)
I assume the is_in() function requires extra calculations and lookups which may
cost time as well calculating unnecessary rules
(That could already be a wrong assumption)
Maybe an expert could just say what would be better in general. No need to
prove or test. I tried myself some things which actually did not make a big
difference.
For example I moved the rules for buildings to the top of the style and
expected serious improvements, because buildings are the majority of polygons
in the PBF
But I gained only a little bit (3-5%? max) and even one out of 4 runs it was
even a bit slower. So mkgmap must be very optimized already was my conclusion
Question 1) ‘unnecessary’ processing rules
Answer A
Walk through 100 rules about highways when it’s a waterway or railway and just
wait for rule 101
Answer B
Introduce an extra IF, which causes extra
calculation itself but skips the 100 rules
IF (highway = *) Then
Highway 1,2,3,4,5,6,7,8-100
END
Waterway = river [0x18 resolution 22]
Question 2) Help mkgmap to avoid calculation of possible expensive
calculations? Or is it already smart enough to not start the IS_IN() if “name =
*” failed already
Answer A
waterway = river & name = * & is_in(natural,
sea, any) = true [0x18 resolution 22]
Answer B
If (waterway = river) then
name = * & is_in(natural, sea,
any) = true [0x18 resolution 22]
End if
Thx,
Joris Bo
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