performance degrade with large pages

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http://stackoverflow.com – I play with a java benchmark (SPECJbb) but observed some degrade with large pages (2M per page) enabled. Our server is NUMA arch based, has 80 cores (160 logical cpu when HT enabled) and 260G memory. The benchmark consumes about 200G memory, so I allocated 200G large in kernel large page pool.But the performance is 6%~8% decline. I think I should have some performance gain because this benchmark used 200G memory. My question is about, in what situation, using large page will degrade performace? Our OS is RHEL6.3 and I tried HotSpot JDK 1.6.38 and 1.6.37, both these two versions I found such (HowTos)