How mimalloc behave when allocating a really large memory block?

I am now working with some database projects. I found that in the case of Clickhouse, instead of trusting the allocator, they do mmap/mremap on their own if they want to allocate a really large block of memory (bigger than 64MB, and can expand to several GiBs).

While I do not quite understand why this is needed (AFAIK, 64MiB is still a valid size class for snmalloc and mimalloc), I do observe that doing so will actually reduce the performance of large hashtables (tons of syscalls during rehashing). So I begin to wonder why mmap/mremap are favored by them? and how do modern allocators like mimalloc handle large allocation in the user space?

リポジトリのファイル、テスト、エントリポイントは指定されていません。まず、64MBを超えるアロケーションの mimalloc における処理を追跡し、ClickHouse の mmap/mremap アプローチと比較してください。戦略が異なる理由と、大規模なアロケーションがどのように動作するかを文書化できれば完了です。