Support for >4MB largepages in Intel/AMD

[Nitin Singh]

Hello OpenJDK Community,

I've been immersing myself in understanding large pages within OpenJDK, especially for the Windows OS. Here's an observation I'd like to share and seek feedback on.

It appears that on Windows Server 2022 Datacenter (OS Version: 10.0.20348), when employing AMD or Intel processors, there's an implicit restriction that limits the use of large page sizes to 4MB. This constraint seems to originate from line 3199 in os_windows.cpp

https://github.com/openjdk/jdk/blob/2e340e855b760e381793107f2a4d74095bd40199/src/hotspot/os/windows/os_windows.cpp#L3199.

To probe this further, I tweaked the code to see what would happen without this restriction. The results were enlightening. I was able to successfully use an 8MB large page size, and my subsequent benchmarks post this change have been quite positive.

What heightens my curiosity is the knowledge that these processors support up to 1G of large pages (HugeTLB pages). Given the widespread support for these processors by various cloud providers, it does feel like we're missing out on potential optimizations for HotSpot running on Windows.

Before delving deeper, I'd love to get some historical context. Can anyone share insights about why this 4MB limit was imposed in the first place? Were there specific technical challenges or concerns that led to this decision?

I'm currently working with the Microsoft JDK version microsoft-jdk-17.0.8-windows-x64.

Thank you for shedding light on this matter. I deeply appreciate your guidance as I tread these waters.

Thanks,

Nitin Singh

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