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由 Wu Fengguang 提交于
Previously the mmap sequential readahead is triggered by updating ra->prev_pos on each page fault and compare it with current page offset. It costs dirtying the cache line on each _minor_ page fault. So remove the ra->prev_pos recording, and instead tag PG_readahead to trigger the possible sequential readahead. It's not only more simple, but also will work more reliably and reduce cache line bouncing on concurrent page faults on shared struct file. In the mosbench exim benchmark which does multi-threaded page faults on shared struct file, the ra->mmap_miss and ra->prev_pos updates are found to cause excessive cache line bouncing on tmpfs, which actually disabled readahead totally (shmem_backing_dev_info.ra_pages == 0). So remove the ra->prev_pos recording, and instead tag PG_readahead to trigger the possible sequential readahead. It's not only more simple, but also will work more reliably on concurrent reads on shared struct file. Signed-off-by: NWu Fengguang <fengguang.wu@intel.com> Tested-by: NTim Chen <tim.c.chen@intel.com> Reported-by: NAndi Kleen <ak@linux.intel.com> Signed-off-by: NAndrew Morton <akpm@linux-foundation.org> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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