A page fault is not always a disk read
Treating all faults as slow conflates the cheap and the expensive, which is why the split between minor and major matters when you tune memory-heavy services. Minor faults map a page A minor page fault happens when the page is already resident in RAM but the process does not yet have it mapped. The kernel only has to wire up a page-table entry — pointing the virtual address at physical memory that is already there. Common cases: first touching a freshly allocated zeroed region, mapping a shared library that another process already loaded, or a page that was swapped in once and sits in memory. Minor faults are cheap: a few hundred nanoseconds, no disk, no IO. They happen constantly in normal-running processes and are not, by themselves, a performance problem. ...