walkmap/doc/dali-performance.md

Dali performance

Notes written while trying to characterise the performance problem in Dali.

Hypothesis one: it's the way I format the polygons that's the issue

Firstly, with both versions of stl->svg using the same version of facet->svg-poly, i.e. this one:

(defn- facet->svg-poly
  [facet]
  [:polygon
   {:points (s/join " " (map #(str (:x %) "," (:y %)) (:vertices facet)))}])

we get this performance using the smaller isle_of_man map:

walkmap.svg=> (def ^:dynamic *preferred-svg-render* :hiccup)
#'walkmap.svg/*preferred-svg-render*
walkmap.svg=> (time (def hiccup (binary-stl-file->svg "resources/isle_of_man.stl" "resources/isle_of_man.svg")))
20-05-25 09:21:43 mason INFO [walkmap.svg:82] - Generating SVG for  :hiccup  renderer
20-05-25 09:21:43 mason INFO [walkmap.svg:96] - Emitting SVG with  :hiccup  renderer
"Elapsed time: 86.904891 msecs"
#'walkmap.svg/hiccup
walkmap.svg=> (def ^:dynamic *preferred-svg-render* :dali)
#'walkmap.svg/*preferred-svg-render*
walkmap.svg=> (time (def dali (binary-stl-file->svg "resources/isle_of_man.stl" "resources/isle_of_man.svg")))
20-05-25 09:22:17 mason INFO [walkmap.svg:82] - Generating SVG for  :dali  renderer
20-05-25 09:22:17 mason INFO [walkmap.svg:96] - Emitting SVG with  :dali  renderer
"Elapsed time: 890.863814 msecs"
#'walkmap.svg/dali

If we switch the Dali render to use my original version of facet->svg-poly, i.e. this one:

(defn- dali-facet->svg-poly
  [facet]
  (vec
    (cons
      :polygon
      (map #(vec (list (:x %) (:y %))) (:vertices facet)))))

we get this performance:

walkmap.svg=> (def ^:dynamic *preferred-svg-render* :hiccup)
#'walkmap.svg/*preferred-svg-render*
walkmap.svg=> (time (def hiccup (binary-stl-file->svg "resources/isle_of_man.stl" "resources/isle_of_man.svg")))
20-05-25 09:35:33 mason INFO [walkmap.svg:82] - Generating SVG for  :hiccup  renderer
20-05-25 09:35:33 mason INFO [walkmap.svg:96] - Emitting SVG with  :hiccup  renderer
"Elapsed time: 84.09972 msecs"
#'walkmap.svg/hiccup
walkmap.svg=> (def ^:dynamic *preferred-svg-render* :dali)
#'walkmap.svg/*preferred-svg-render*
walkmap.svg=> (time (def dali (binary-stl-file->svg "resources/isle_of_man.stl" "resources/isle_of_man.svg")))
20-05-25 09:35:41 mason INFO [walkmap.svg:82] - Generating SVG for  :dali  renderer
20-05-25 09:35:41 mason INFO [walkmap.svg:96] - Emitting SVG with  :dali  renderer
"Elapsed time: 874.292007 msecs"
#'walkmap.svg/dali

No significant difference in performance.

If we generate but don't render, we get this:

walkmap.svg=> (def ^:dynamic *preferred-svg-render* :hiccup)
#'walkmap.svg/*preferred-svg-render*
walkmap.svg=> (time (def hiccup (binary-stl-file->svg "resources/isle_of_man.stl")))
20-05-25 09:37:44 mason INFO [walkmap.svg:82] - Generating SVG for  :hiccup  renderer
"Elapsed time: 52.614707 msecs"
#'walkmap.svg/hiccup
walkmap.svg=> (def ^:dynamic *preferred-svg-render* :dali)
#'walkmap.svg/*preferred-svg-render*
walkmap.svg=> (time (def dali (binary-stl-file->svg "resources/isle_of_man.stl")))
20-05-25 09:38:07 mason INFO [walkmap.svg:82] - Generating SVG for  :dali  renderer
"Elapsed time: 49.891043 msecs"
#'walkmap.svg/dali

This implies that the problem is not in the way polygons are formatted.

The difference between the two versions of facet->svg-poly is as follows:

New version, works with both Hiccup and Dali:

walkmap.svg=> (def stl (decode-binary-stl "resources/isle_of_man.stl"))
#'walkmap.svg/stl
walkmap.svg=> (def facet (first (:facets stl)))
#'walkmap.svg/facet
walkmap.svg=> (pprint facet)
{:normal {:x -0.0, :y 0.0, :z 1.0},
 :vertices
 [{:x 3.0, :y 1.0, :z 1.0}
  {:x 2.0, :y 3.0, :z 1.0}
  {:x 0.0, :y 0.0, :z 1.0}],
 :abc 0}
nil
walkmap.svg=> (pprint (facet->svg-poly facet))
[:polygon {:points "3.0,1.0 2.0,3.0 0.0,0.0"}]
nil

In other words, the new version constructs the :points attribute of the :polygon tag by string concatenation, and the renderer just needs to output it.

Older version, works with Dali only:

walkmap.svg=> (pprint (dali-facet->svg-poly facet))
[:polygon [3.0 1.0] [2.0 3.0] [0.0 0.0]]
nil

This means that the renderer is actually doing more work, since it has to compose the :points attribute itself; nevertheless there doesn't seem to be an increased time penalty.

Conclusion

It doesn't seem that formatting the polygons is the issue.

Hypothesis two: Dali renderer scales non-linearly with number of objects drawn

To test this, we need some otherwise-similar test files with different numbers of objects:

walkmap.svg=> (count (:facets stl))
4416
walkmap.svg=> (def small-stl (assoc stl :facets (take 400 (:facets stl))))
#'walkmap.svg/small-stl
walkmap.svg=> (count (:facets small-stl))
400
walkmap.svg=> (def large-stl (decode-binary-stl "../the-great-game/resources/maps/heightmap.stl"))
#'walkmap.svg/large-stl
walkmap.svg=> (count (:facets large-stl))
746585
walkmap.svg=> (def ^:dynamic *preferred-svg-render* :dali)
#'walkmap.svg/*preferred-svg-render*
walkmap.svg=> (time (dali.io/render-svg (stl->svg small-stl) "dali-small.svg"))
20-05-25 10:12:25 mason INFO [walkmap.svg:92] - Generating SVG for  :dali  renderer
"Elapsed time: 32.55506 msecs"
nil
walkmap.svg=> (def ^:dynamic *preferred-svg-render* :hiccup)
#'walkmap.svg/*preferred-svg-render*
walkmap.svg=> (time (spit "hiccup-small.svg" (hiccup.core/html (stl->svg small-stl))))
20-05-25 10:14:07 mason INFO [walkmap.svg:92] - Generating SVG for  :hiccup  renderer
"Elapsed time: 10.026369 msecs"

So we have

	Dali		Hiccup
# of facets	time (msecs)	objets/msec	time (msecs)	objets/msec	ratio (Dali/Hiccup)
-----------	----------------	-----------	------------	-----------	--------------------
400	32.55506	12.29	10.026369	39.89	3.35
4416	874.292007	5.05	84.09972	52.51	10.40
746585	29,695,695.61	0.03	16724.848222	44.64	1775.54

Conclusion

What we're seeing is that Hiccup renders more or less linearly by the number of objects (bear in mind that all of these objects are triangles, so essentially equally complex to render), whereas trhe performance of Dali degrades significantly as the number of objects increases.