VMCaches B-tree implementation allocating too many pages with huge pages

[fluffychaos]

I am currently working on huge pages support for vmcache and during this process I noticed that too many pages get allocated when using 2MB pages.

The changes I did:

• change pageSize from 4096 to 2097152
• include MAP_HUGETLB | MAP_HUGE_2MB in the mmap flags in the non-exmap case (I currently only try to get huge pages working outside of exmap)
• change all indices and sizes in BTreeNodeHeader, BTreeNode and BTree from u16 to u32 to avoid overflows when addressing in the full 2MB range
• changed virtAllocSize from 1ul << 16 to max(pageSize, 1ul << 16) to align the mmap allocation size to the page size
• the madvice call for virtMem was made conditional depending on pageSize being 4096 to avoid it when using huge pages

How I ran vmcache:

• THREADS=1
• DATASIZE=1
• VIRTGB=6
• PHYSGB=4
• BLOCK pointing to a 16GB file
• All other environment variables for vmcache on default => TPC-C got used
• 32GB of RAM
• A Ryzen 7 1800X CPU
• 4096 2MB pages allocated via sysctl

The problem symptoms:

• I get the message VIRTGB too low
• BTreeNode::mergeNodes never gets called
• The last node to split is full, triggering a call to BTree::trySplit

Does anyone have an idea for what the cause could be? Apart from the indices and sizes the implementation looks generic over the page size to me.