Messages in this thread Patch in this message | | | Date | Sat, 22 Dec 2007 09:31:50 +0800 | From | Fengguang Wu <> | Subject | [PATCH 3/9] readahead: auto detection of sequential mmap reads |
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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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