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Hisashi Hifumi authored
I added blk_run_backing_dev on page_cache_async_readahead so readahead I/O is unpluged to improve throughput on especially RAID environment. The normal case is, if page N become uptodate at time T(N), then T(N) <= T(N+1) holds. With RAID (and NFS to some degree), there is no strict ordering, the data arrival time depends on runtime status of individual disks, which breaks that formula. So in do_generic_file_read(), just after submitting the async readahead IO request, the current page may well be uptodate, so the page won't be locked, and the block device won't be implicitly unplugged: if (PageReadahead(page)) page_cache_async_readahead() if (!PageUptodate(page)) goto page_not_up_to_date; //... page_not_up_to_date: lock_page_killable(page); Therefore explicit unplugging can help. Following is the test result with dd. #dd if=testdir/testfile of=/de...
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