Virtual memory is not exactly free. To work fast, modern CPUs require quite a lot of data to be available in the on-chip caches. TLB cache is one of these piece

Virtual memory is not exactly free. To work fast, modern CPUs require quite a lot of data to be available in the on-chip caches. TLB cache is one of these pieces of data. When you reserve many GB of address space for stacks of many threads, the code running on these threads is less likely to hit that TLB cache. I've heard that huge pages might help, unfortunately they're somewhat harder to use.