Linux: Accessing Files With O_DIRECT
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A thread on the lkml began with a query about using O_DIRECT when opening a file. An earlywhite paper written by Andrea Arcangeli [interview] 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..)."
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