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readahead: reduce unnecessary mmap_miss increases

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The original INT_MAX is too large, reduce it to

- avoid unnecessarily dirtying/bouncing the cache line

- restore mmap read-around faster on changed access pattern

Background: in the mosbench exim benchmark which does multi-threaded page
faults on shared struct file, the ra->mmap_miss updates are found to cause
excessive cache line bouncing on tmpfs.  The ra state updates are needless
for tmpfs because it actually disabled readahead totally
(shmem_backing_dev_info.ra_pages == 0).

Tested-by: Tim Chen <[email protected]>
Signed-off-by: Andi Kleen <[email protected]>
Signed-off-by: Wu Fengguang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
This commit is contained in:
Andi Kleen 2011-05-24 17:12:29 -07:00 • committed by Linus Torvalds
commit 207d04baa3
1 changed files with 2 additions and 1 deletions
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3
mm/filemap.c
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@ -1566,7 +1566,8 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
return;
}
if (ra->mmap_miss < INT_MAX)
/* Avoid banging the cache line if not needed */
if (ra->mmap_miss < MMAP_LOTSAMISS * 10)
ra->mmap_miss++;
/*