Depends on your workload and how many TLB entries your CPU has for superpages. The Zen 2 TLB can hold tons (1000s) of 2MB superpages but relatively few (64) 1GB superpages. Older CPU models had worse capacity for 1GB and 2MB superpages. E.g., Haswell (2013) had only 32 entries for 2MB superpages and 4 entries for 1GB superpages (data).

In addition to the limited number of cache slots available for superpages (varies depending on cpu), remember that those can be invalidated (again, depending on cpu). If you're ping-ponging processes on a single CPU, you won't necessarily have what you need in the TLB.

> If you're ping-ponging processes on a single CPU, you won't necessarily have what you need in the TLB.

Is that really likely on a production database server?

Depends on the design. At a minimum you're ping-ponging between userland and kernel; but you might also be bouncing between a transport layer unwrapper, an authentication front-end, the database core, and a storage back-end.

Ideally, with io_uring(2)/DPDK/SR-IOV-NVMe, you can skip enough syscalls to drop their performance impact below 1%.

Spectre may have changed things, but for a long time a user->kernel switch didn't imply a full TLB flush.