Most common operating systems provide a facility to directly DMA disk pages into a user space buffer (and vice versa) without involving the page cache. On Linux

Most common operating systems provide a facility to directly DMA disk pages into a user space buffer (and vice versa) without involving the page cache. On Linux, for example, this is what the O_DIRECT flag is for. It is similar to mmap() but with different rules; you lose the page table magic but gain explicit control the I/O behavior. There is nothing faster if you know how to wield it. Operating systems add this feature to support database engines but it isn't the only use case. Because a page cache is expected to support all possible processes, it can neither overfit simple access pattern cases, because then it would offer poor performance for common cases that are more complex, and it will never have enough context to offer excellent performance for complex access patterns because doing so would reduce performance for many common simple cases. Real page caches try to straddle both worlds; it is better to be mediocre for simple and complex cases than to be excellent at only simple or only complex cases. Bypassing the page cache allows processes to design their own page caches that are highly optimized for their specific requirements, should it be worthwhile to do so. A reason people might not want to do this is that the APIs are operating system specific.