“A thread on the lkml began with a query about using O_DIRECT
when opening a file. An early white paper written by Andrea
Arcangeli to describe the O_DIRECT patch before it was merged into
the 2.4 kernel explains, “with O_DIRECT the kernel will do DMA
directly from/to the physical memory pointed [to] by the userspace
buffer passed as [a] parameter to the read/write syscalls. So there
will be no CPU and memory bandwidth spent in the copies between
userspace memory and kernel cache, and there will be no CPU time
spent in kernel in the management of the cache (like cache lookups,
per-page locks etc..).””