From 7ffc59b4d0bdfa00e882339f85b8a969bb7021e2 Mon Sep 17 00:00:00 2001 From: Wu Fengguang Date: Tue, 16 Jun 2009 15:31:38 -0700 Subject: [PATCH] readahead: enforce full sync mmap readahead size 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 Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/filemap.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/mm/filemap.c b/mm/filemap.c index 2e9bcc2e7977..6846a902f5cf 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -1471,7 +1471,8 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma, if (VM_SequentialReadHint(vma) || offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) { - page_cache_sync_readahead(mapping, ra, file, offset, 1); + page_cache_sync_readahead(mapping, ra, file, offset, + ra->ra_pages); return; } -- 2.34.1