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Date: Sat, 27 Dec 2014 15:31:26 +0200 From: Leon Pollak <leon.pollak@...il.com> To: linux-kernel@...r.kernel.org Subject: Old O_DIRECT story Hi, all. There was a discussion here: https://lkml.org/lkml/2007/1/10/231 Linus wrote in this discussion: "So don't use O_DIRECT. Use things like madvise() and posix_fadvise() instead" After the full week of tests, searches, discussions, I have impudence to turn to the community - has one tried to implement this approach? The situation is very simple: I have the incoming DMA stream using scatter/gather technique. the driver read() function provides the next ready DMA buffer descriptor with the virtual address pointer to the acquired data. I need to store this data to the disk partition as fast as possible, as the incoming stream is too very fast. According to tests, O_DIRECT/mapping is fast enough, while write() is not. I tried in all ways to implement this with mmap(), but it does not success, because I did not find a way to mmap() file as O_WRONLY. Mapping as O_RDWR makes kernel to pre-fill mapped memory with partition data. So, kernel and DMA actually compete on the RAM area to fill it - one with garbage, one with actual data. Kernel wins. So, how to implement Linus's advice? Leon -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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