http://*.com/questions/41216388/java-jvm-parameter-xms-doesnt-take-effect-immediately

It seems the GC does it in the method HeapRegion::setup_heap_region_size(uintx min_heap_size) from the c++ file \openjdk-7-fcs-src-b147-27_jun_2011\openjdk\hotspot\src\share\vm\gc_implementation\g1\heapRegion.cpp , and method parse_each_vm_init_arg from file \openjdk-7-fcs-src-b147- 27_jun_2011\openjdk\hotspot\src\share\vm\runtime\arguments.cpp , someone who familiar with JVM GC source code can help to do some analysis for it.