Use ujson for serializing GeoJSON and TopoJSON

[iandees]

For #139.

simplejson was merged into Python core in 2.6, so there's actually a slow-down using simplejson 3.6.4 (which is currently installed, but unused) or 3.10.0 (the latest version):

Coord	Format	Python 2.7.6 Built-in	simplejson 3.6.4	simplejson 3.10.0
1/1/1	json	358.7ms	385.3 (+7.42%)	387.1 (+7.93%)
1/1/1	topojson	232.0ms	252.9ms (+8.98%)	257.3 (+10.91%)
7/32/47	json	897.2	910.4ms (+1.46%)	927.5 (+3.37%)
7/32/47	topojson	567.7	586.6ms (+3.33%)	601.3 (+5.92%)
12/1050/1522	json	3238.7	3366.8ms (+3.95%)	3368.8 (+4.02%)
12/1050/1522	topojson	2128.4	2213.2ms (+3.98%)	2244.2 (+5.44%)
16/16812/24357	json	390.2	395.9ms (+1.45%)	412.5 (+5.70%)
16/16812/24357	topojson	296.6	294.3ms (-0.79%)	294.7 (-0.63%)

As a result, this pull request switches to use ujson, a faster (but less feature-rich) JSON library. It seems to shave 10-20% of time:

Coord	Format	Python 2.7.6 Built-in	ujson 1.35
1/1/1	json	358.7ms	292.7ms (-18.4%)
1/1/1	topojson	232.1ms	189.1ms (-18.5%)
7/32/47	json	897.3ms	780.9ms (-13%)
7/32/47	topojson	567.7ms	488.9ms (-13.9%)
12/1050/1522	json	3238.8ms	2935ms (-9.4%)
12/1050/1522	topojson	2128.5ms	1858.7ms (-12.7%)
16/16812/24357	json	390.3ms	334.3ms (-14.3%)
16/16812/24357	topojson	296.7ms	240.5ms (-18.9%)

ujson resulted in a < 2% difference for the mvt and mvtb formats.

[nvkelso]

Nice work! I was expecting a similar (or slightly smaller) improvement in MVT generation since it first has to calculate the JSON version. Does that mean most the savings here is JSON > String > file IO related changes?

[nvkelso]

Please also link up related PR for tileserver changes.

[iandees]

My measurements here are for the runtime of the _create_formatted_tile() function for each of the 4 formats, and it doesn't look like the mvt format does anything with JSON at all.

[iandees]

@nvkelso said:

Please also link up related PR for tileserver changes.

A tileserver PR would just be an adjustment of the tilequeue dependency, I think. We'd need to merge this PR and tag tilequeue first.

[nvkelso]

We declare simplejson in the dependencies for tileserver now (which seems weird to me, but @rmarianski said it was to play nicer with tilequeue?):

• https://github.com/tilezen/tileserver/blob/master/requirements.txt#L18

And we import json in __init__.py:

• https://github.com/tilezen/tileserver/blob/master/tileserver/__init__.py#L22

[nvkelso]

To observe the expected MVT format times improvement, I suspect you'd additionally need to measure the total time tileserver took to generate the example tiles per format (not just the _create_formatted_tile() function in tilequeue)?

[iandees]

Yep, it looks like tileserver does the JSON decoding stuff here.

[rmarianski]

We declare simplejson in the dependencies for tileserver now (which seems weird to me, but @rmarianski said it was to play nicer with tilequeue?):

It should be removed from all requirements files.

Yep, it looks like tileserver does the JSON decoding stuff here.

Some of that, if not all, was a vestige of when tileserver handled custom layer requests. There's an issue for cleaning that up.

[iandees]

I checked before and after ujson and it looks like the only difference is that the default output for ujson is to not include and whitespace between JSON syntax. For example, ujson outputs:

"landuse":{"type":"FeatureCollection","features":[]}}

whereas the built-in JSON serializer includes whitespace:

"landuse": {"type": "FeatureCollection", "features": []}}