[PATCH] readahead: enforce full sync mmap readahead size

[Wu Fengguang]

Now that we do readahead for sequential mmap reads, here is
a simple evaluation of the impacts, and one further optimization.

It's an NFS-root debian desktop system, readahead size = 60 pages.
The numbers are grabbed after a fresh boot into console.

approach        pgmajfault      RA miss ratio   mmap IO count   avg IO size(pages)
   A            383             31.6%           383             11
   B            225             32.4%           390             11
   C            224             32.6%           307             13

case A: mmap sync/async readahead disabled
case B: mmap sync/async readahead enabled, with enforced full async readahead size
case C: mmap sync/async readahead enabled, with enforced full sync/async readahead size
or:
A = vanilla 2.6.30-rc1
B = A plus mmap readahead
C = B plus this patch

The numbers show that
- there are good possibilities for random mmap reads to trigger readahead
- 'pgmajfault' is reduced by 1/3, due to the _async_ nature of readahead
- case C can further reduce IO count by 1/4
- readahead miss ratios are not quite affected

The theory is
- readahead is _good_ for clustered random reads, and can perform
  _better_ than readaround because they could be _async_.
- async readahead size is guaranteed to be larger than readaround
  size, and they are _async_, hence will mostly behave better
However for B
- sync readahead size could be smaller than readaround size, hence may
  make things worse by produce more smaller IOs
which will be fixed by this patch.

Final conclusion:
- mmap readahead reduced major faults by 1/3 and no obvious overheads;
- mmap io can be further reduced by 1/4 with this patch.

Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
---
 mm/filemap.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- mm.orig/mm/filemap.c
+++ mm/mm/filemap.c
@@ -1471,7 +1471,8 @@ static void do_sync_mmap_readahead(struc
 	if (VM_SequentialReadHint(vma) ||
 			offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) {
 		ra->flags |= RA_FLAG_MMAP;
-		page_cache_sync_readahead(mapping, ra, file, offset, 1);
+		page_cache_sync_readahead(mapping, ra, file, offset,
+					  ra->ra_pages);
 		return;
 	}
 
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