[PATCH 3/9] readahead: auto detection of sequential mmap reads

Fengguang Wu
2007-12-22T01:35:13Z

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Auto-detect sequential mmap reads and do sync/async 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.

It's a bit conservative to require valid readahead state for async readahead,
which means we don't do readahead for interleaved reads for now, but let's make
it safe for this initial try.

======
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

Some numbers on 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. 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 cache hits.

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---

---
mm/filemap.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- linux-2.6.24-rc5-mm1.orig/mm/filemap.c
+++ linux-2.6.24-rc5-mm1/mm/filemap.c
@@ -1318,7 +1318,8 @@ static void do_sync_mmap_readahead(struc
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;
}
@@ -1360,7 +1361,8 @@ static void do_async_mmap_readahead(stru
return;
if (ra->mmap_miss > 0)
ra->mmap_miss--;
- if (PageReadahead(page))
+ if (PageReadahead(page) &&
+ offset == ra->start + ra->size - ra->async_size)
page_cache_async_readahead(mapping, ra, file, page, offset, 1);
}

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