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The tomcat server the WMS service is running on should have sufficient memory. When many concurrent users are using the WMS service the CPU requirements will increase as well since all images have to be rendered at the same time on the same server. It is recommended to start at least with 2GByte of memory with the -Xmx2G JVM parameter.
Since the WMS service is stateless, it can be scaled both vertically (more cpu and memory) and horizontally (more tomcat instances)..
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