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author | Wu Fengguang <fengguang.wu@intel.com> | 2009-06-16 22:31:28 (GMT) |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2009-06-17 02:47:29 (GMT) |
commit | 70ac23cfa31f68289d4b720c6162b3929ab4de36 (patch) | |
tree | d610e506a9247a22ba54c379d8abb58958ada6da | |
parent | ef00e08e26dd5d84271ef706262506b82195e752 (diff) | |
download | linux-70ac23cfa31f68289d4b720c6162b3929ab4de36.tar.xz |
readahead: sequential mmap readahead
Auto-detect sequential mmap reads and do readahead for them.
The sequential mmap readahead will be triggered when
- sync readahead: it's a major fault and (prev_offset == offset-1);
- async readahead: minor fault on PG_readahead page with valid readahead state.
The benefits of doing readahead instead of read-around:
- less I/O wait thanks to async readahead
- double real I/O size and no more cache hits
The single stream case is improved a little.
For 100,000 sequential mmap reads:
user system cpu total
(1-1) plain -mm, 128KB readaround: 3.224 2.554 48.40% 11.838
(1-2) plain -mm, 256KB readaround: 3.170 2.392 46.20% 11.976
(2) patched -mm, 128KB readahead: 3.117 2.448 47.33% 11.607
The patched (2) has smallest total time, since it has no cache hit overheads
and less I/O block time(thanks to async readahead). Here the I/O size
makes no much difference, since there's only one single stream.
Note that (1-1)'s real I/O size is 64KB and (1-2)'s real I/O size is 128KB,
since the half of the read-around pages will be readahead cache hits.
This is going to make _real_ differences for _concurrent_ IO streams.
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r-- | mm/filemap.c | 3 |
1 files changed, 2 insertions, 1 deletions
diff --git a/mm/filemap.c b/mm/filemap.c index 8275364..99977f0 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1472,7 +1472,8 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma, if (VM_RandomReadHint(vma)) return; - if (VM_SequentialReadHint(vma)) { + if (VM_SequentialReadHint(vma) || + offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) { page_cache_sync_readahead(mapping, ra, file, offset, 1); return; } |