GeoServer rendering performance significantly affected by vector icon size

Question

I'm having the craziest GeoServer experience right now and I was hoping to get some input on what might be causing this behaviour.

I'm using Apache Bench to benchmark rendering performance for a layer via GeoServer's WMS interface and accidentally discovered that the vector icon size has a very significant effect on performance. The image I'm rendering:

• 25,000 point features
• Single rendering rule in SLD
• 1900x1200 pixels
• Request concurrency is 5
• GeoServer is 2.6.1
• JVM is Oracle Java 7 SE
• Not using native JAI

The SLD's single rendering rule looks like this:

<PointSymbolizer>
    <Graphic>
      <Mark>
        <WellKnownName>circle</WellKnownName>
        <Fill>
          <CssParameter name="fill">#FF0000</CssParameter>
          <CssParameter name="fill-opacity">1.0</CssParameter>
        </Fill>
      </Mark>
      <Size>
        <ogc:Literal>50</ogc:Literal>
      </Size>
    </Graphic>
</PointSymbolizer>

This chart shows how response time for the same request varies based on icon size

As this is a vector icon I had assumed that rendering any size had as much impact on performance as any other size, but it appears that beyond a certain size (perhaps 16px) performance begins to suffer and beyond 30px it drops dramatically.

This has obvious implications for how I author future SLDs but right now I am more interested in why this would happen. Any thoughts?

EDIT: With a little more investigation I have found that increasing the (scaled) size of a PNG icon gives a much more linear increase in rendering time and there is a size, for this particular layer, when PNG icons become faster than vector icons. I have also seen that the relationship between icon size and performance is more linear with a smaller number of features. As it looks like the number of features and icon size both interact to affect performance I can only assume this is related to memory usage, I'm just not sure exactly how.

Answer 1

You actually forgot to mention, nor included it in the graph, how you measure "icon size": 1) width/height on the X-axis, 2) pixel area of icon in number of pixels

I guess it is 1), based on regular icon sizes.

That probably explains your graph: the increase in icon area size as measured in number of pixels is quadratic, each doubling of the width, increases the area by a factor 4.

So the number of pixels that need to be processed / generated / rendered for your vector icons, increases considerably with the "size" / width of the icon, which seems roughly consistent with the results in your graph.