From: Wu Fengguang Date: Tue, 16 Jun 2009 22:31:28 +0000 (-0700) Subject: readahead: sequential mmap readahead X-Git-Tag: v2.6.31-rc1~299^2~170 X-Git-Url: http://git.samba.org/samba.git/?p=sfrench%2Fcifs-2.6.git;a=commitdiff_plain;h=70ac23cfa31f68289d4b720c6162b3929ab4de36 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 Signed-off-by: Wu Fengguang Cc: Ying Han Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- diff --git a/mm/filemap.c b/mm/filemap.c index 827536485599..99977f0a94e4 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; }